Hervé Lebris Pr

Agrocampus Ouest

Menard Frédéric

IRD / Rapporteur Yves Dr

Cherel Dr

Jean Frédéric

UBO / Président Mcf

Bertrand Arnaud

Dr

Anne Lorrain

Trophic dynamics in the northern Humboldt Current system: insights from stable isotopes and stomach content analyses

Keywords: Northern Humboldt Current system, trophic ecology, oxygen minimum zone, anchoveta Engraulis ringens, sardine Sardinops sagax, jumbo squid Dosidicus gigas, squat lobster Pleuroncodes monodon, stable isotope analysis, stomach content euphausiids, Humboldt Current system, oxygen, Peruvian anchovy, spatiotemporal variability, trophic ecology

In this study we use available historical data since 1954 to get new insight on ontogenetic and spatiotemporal variability in Peruvian anchovy or anchoveta Engraulis ringens diet. Whatever the period, E. ringens foraged mainly on macrozooplankton and the importance of euphausiids in E. ringens diet appears directly related to euphausiids abundance. This bottom-up effect is also observed at smaller scale since the euphausiids fraction increased with E. ringens size and euphausiids accessibility. Selecting the largest prey, the euphausiids, provide an energetic advantage for E. ringens in its ecosystem where oxygen depletion imposes strong metabolic constrain to pelagic fish. Finally this study is another illustration of E. ringens plasticity which allows it to cope with its highly variable environment.

Abstract

The northern Humboldt Current system (NHCS) off Peru is one of the most productive world marine regions. It represents less than 0.1% of the world ocean surface but presently sustains about 10% of the world fish catch, with the Peruvian anchovy or anchoveta Engraulis ringens as emblematic fish resource. Compared with other eastern boundary upwelling systems, the higher fish productivity of the NHCS cannot be explained by a corresponding higher primary productivity. On another hand, the NHCS is the region where El Niño, and climate variability in general, is most notable. Also, surface oxygenated waters overlie an intense and extremely shallow Oxygen Minimum Zone (OMZ). In this context, the main objective of this study is to better understand the trophic flows in the NHCS using both stomach content and stable isotope analyses. The study focuses on a variety of organisms from low trophic levels such as zooplankton to top predators (seabirds and fur seals). The approach combines both long-term and specific studies on emblematic species such as anchoveta, and sardine Sardinops sagax and a more inclusive analysis considering the 'global' food web in the recent years (2008 -2012) using stable isotope analysis.

Revisiting anchovy and sardine we show that whereas phytoplankton largely dominated anchoveta and sardine diets in terms of numerical abundance, the carbon content of prey items indicated that zooplankton was by far the most important dietary component. Indeed for anchovy euphausiids contributed 67.5% of dietary carbon, followed by copepods (26.3%). Selecting the largest prey, the euphausiids, provide an energetic advantage for anchoveta in its ecosystem where oxygen depletion imposes strong metabolic constrain to pelagic fish. Sardine feed on smaller zooplankton than do anchoveta, with sardine diet consisting of smaller copepods and fewer euphausiids than anchoveta diet. Hence, trophic competition between sardine and anchovy in the northern Humboldt Current system is minimized by their partitioning of the zooplankton food resource based on prey size, as has been reported in other systems. These results suggest an ecological role for pelagic fish that challenges previous understanding of their position in the foodweb (zooplanktophagous instead of phytophagous), the functioning and the trophic models of the NHCS.

Finally to obtain a more comprehensive vision of the relative trophic position of NHCS main components we used stable isotope analyses. For that purpose we analyzed the δ 13 C and δ 15 N stable isotope values of thirteen taxonomic categories collected off Peru from 2008 -2011, i.e., zooplankton, fish, squids and air-breathing top predators. The δ 15 N isotope signature was strongly impacted by the species, the body length and the latitude. Along the Peruvian coast, the OMZ get more intense and shallow south of ~7.5ºS impacting the baseline nitrogen stable isotopes. Employing a linear mixed-effects modelling approach taking into account the latitudinal and body length effects, we provide a new vision of the relative trophic position of key ecosystem components. Also we confirm stomach content-based results on anchoveta Engraulis ringens and highlight the potential remarkable importance of an often neglected ecosystem component, the squat lobster Pleuroncodes monodon. Indeed, our results support the hypothesis according to which this species forage to some extent on fish eggs and larvae and can thus predate on the first life stages of exploited species. However, the δ 13 C values of these two species suggest that anchoveta and squat lobster do not exactly share the same habitat. This would potentially reduce some direct competition and/or predation.

List of figures

1.1 Fish catch versus primary productivity for the four main eastern boundary coastal upwelling ecosystems for the years 1998-2005. Source: [START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF] 1.2 Sea surface properties and oceanic circulation scheme. (a) Sea-Surface Salinity (SSS, color shading) and surface circulation. (b) Sea-Surface Temperature (SST, colour shading in °C) and subsurface circulation. 1.3 Distribution of the main water masses in the HCS: Cold coastal water (CCW), Surface subtropical water (SSW), Surface equatorial water (SEW), Surface tropical water (STW). Source: Ayón et al. (2008a).

1.4

Upper plot: OMZ thickness and upper boundary in the Eastern South Pacific. Thickness is colour-coded according to the colour bar on the right-hand side of the figure; units are in m. The upper boundary of the OMZ is shown in black contour lines with 50 m intervals; Source: [START_REF] Fuenzalida | Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean[END_REF]. Lower plot: Meridional variation of the upper OMZ averaged between the coast and 200 km offshore during a survey performed in 2005 (black solid line) and corresponding ± standard deviation (grey shaded area). Source: [START_REF] Bertrand | A c o u s t i c O b s e r v a t i o n o f L i v i n g O r g a n i s m s Reveals the Upper Limit of the Oxygen Minimum Zone[END_REF].

1.5 First ''ecosystem-based" diagram for the Northern Humboldt Current system. Source: [START_REF] Vogt | Road to survival[END_REF].

1.6

Major interaction in the pelagic ecosystem off Peru. Source: [START_REF] Muck | Major trends in the pelagic ecosystem off Peru and their implications for management[END_REF].

1.7

Elements to be considered in a simulation model of the Peruvian upwelling ecosystem. Source: [START_REF] Csirke | Informe del Taller de trabajo sobre modelos para la predicción del rendimiento en el sistema de afloramiento peruano IMARPE[END_REF].

1.8 Trophic flows average in the Northern Humboldt Current System, from the periods 1964-1971 and 1973-1981. Flows of wet mass are in units of 103 kg km -2 yr -1 . Flows leave boxes on the upper half and enter them on the lower half. The width of the line indicates the order of magnitude of trophic flows. Biomass of detritus is a rough estimate. Source: [START_REF] Jarre | Two approaches for modelling fish stock interactions in the Peruvian upwelling ecosystem[END_REF] and [START_REF] Jarre-Teichmann | The potential role of mass-balance models for the management of upwelling ecosystems[END_REF].

1.9 Schematic representation of the food web in the Peruvian upwelling system without (a) and with (b) the industrial fishery for anchoveta included in the model. The numbers correspond to the proportion of productivity available at one trophic level consumed by the next trophic level at any given time. Source [START_REF] Jahncke | Trends in carbon flux to seabirds in the Peruvian upwelling system: effects of wind and fisheries on population regulation[END_REF].

1.10 Conceptual model of (a) coastal and (b) open ocean pelagic ecosystems. Near coasts the nutrients are abundant, dense, colonial, centric diatom blooms form. The primary production can be transferred rapidly to small plentiful fish and the food chain is short and efficient. In the open ocean, under low nutrient input, photosynthetic picoplanktonic organisms dominate. The consumers of picoplankton are also very small and have growth rates similar to their prey. Scarce nutrients are recycled and retained in the upper layer. Modified from Chavez et al. (2002).

1.11 Illustration of trophic enrichment in δ 13 C and δ 15 N from primary producers (plants and diatoms), to herbivores, to predators for A) terrestrial ecosystems and B) marine ecosystems. Panel A also shows differences in δ 13 C between food webs based on C3 (black symbols) and C4 plants (gray symbols). Values (mean ± SE) were adapted from the literature. Source: Ben-David and Flaherty (2012).

1.12 Mean (±1 SE) δ 13 C versus mean (±1 SE) δ 15 N for various fish (solid symbols) and invertebrates (open symbols) of the Newfoundland and Labrador continental shelf food web. The abscissa represents a continuum of feeding types ranging from highly pelagic (more negative, or depleted in 13 C) to highly benthic (more positive, or enriched in 13 C).

The ordinate should represent a continuum of trophic positions. Source: [START_REF] Sherwood | Stable isotope analysis of some representative fish and invertebrates of the Newfoundland and Labrador continental shelf food web[END_REF].

1.13 Isotopic changes during nitrate turnover in the open ocean. Source: [START_REF] Gruber | The dynamics of the marine nitrogen cycle and its influence on atmospheric CO 2[END_REF].

1.14 Conceptual representation of the calculation used to estimate trophic position of consumers (relative to assumed trophic position 2.3 for benthic or pelagic invertebrates) from the Newfoundland and Labrador continental shelf food web. The two stars have the same δ 15 N signature (i.e. 15.5‰), but very different trophic positions; the white star represents a near-shore, benthic consumer that is much closer to the baseline than the black star (an offshore, pelagic consumer). Calculation assumes classical trophic fractionation factors of 3.4‰ for δ 15 N and 0.8‰ for δ 13 C. Hence, consumers are offset to the right of their appropriate baseline δ 13 C signature, which is actually the δ 13 C signature of their prey (i.e. we assumed trophic fractionation of δ 13 C only between consumers and their prey). Source: [START_REF] Sherwood | Stable isotope analysis of some representative fish and invertebrates of the Newfoundland and Labrador continental shelf food web[END_REF].

2.1

The locations of trawls (grey dots) from which anchovetas were collected for trophodynamic analysis. The black solid line indicates the 200 m isobaths.

2.2

Percentage contribution of phytoplankton (green), copepods (red), euphausiids (orange) and other zooplankters (grey) by number (total num) and to dietary carbon (total carb) for the total set of 21,203 anchoveta stomach contents. Also shown are the percentage contribution to dietary carbon by prey type for each of the 23 surveys from which anchoveta stomach contents were analyzed; surveys are identified along thex-axis, with the first two numbers corresponding to the year, the following two numbers the start month of the survey, and the last two numbers the end month of the survey.

2.3

Scatter plot (grey dots) and cubic spline smoother fits (black solid lines) of GAM models (the black dashed lines show the 95% confidence limits) based on anchoveta stomach fullness according to time of day, latitude, distance from the coast (DC) and sea surface temperature (°C) for the complete set of 21,203 stomach contents (top panel) and for each of the 23 surveys from which stomach contents were analyzed (lower panels); surveys are identified along they-axis, with the first two numbers corresponding to the year, the following two numbers the start month of the survey, and the last two numbers the end month of the survey. The lefty-axis shows stomach fullness (% WW) and scaling along the righty-axes is relative and corresponds to the spline smoother that was fitted on the data such that ay-value of zero is the mean effect of the variables on the response.

3.1 Figure 3.1 Per cent composition of prey groups (■ diatoms, ■ other phytoplankters, ■ calanoid copepods, ■ other copepods, ■ euphausiids and, ■ other zooplankters) to Engraulis ringens dietary carbon for the periods: (a) 1954-1969[START_REF] Rojas De Mendiola | Some observations on the feeding of the Peruvian anchoveta Engraulis ringens J. in two regions of the Peruvian coast[END_REF]) and (b) 1996[START_REF] Levin | Oxygen minimum zone benthos: adaptation and community response to hypoxia[END_REF](Espinoza & Bertrand, 2008).

3.2

Frequency of occurrence (grey bars -left y-axis) of euphausiids in Engraulis ringens stomachs, and euphausiids dominance from Ayón et al. (2011) in zooplankton net samples (additive model results; black line -right y-axis). No bar indicates absence of data.

3.3

Percent contribution of prey groups ( diatoms, other phytoplankters, calanoid copepods, other copepods, euphausiids, other zooplankters) to dietary carbon of Engraulis ringens by size-class (a), degree of latitude (b), distance to the shelf break (in km, zero corresponds to the shelf break, values are negative inshore and positive offshore) (c) and diel period (d) from years 1996-2003.

3.4

Classification and regression tree of Engraulis ringens d i e t a c c o r d i n g t o t h e spatiotemporal factors from years 1996-2003: distance of the shelf break (dsb), latitude (lat), size-class (size), diel period (period). Each terminal node of the tree is characterized by a dominant prey. Below a histogram shows the predicted prey distribution (carbon proportion) for the taxonomic groups, given the explanatory variables: euphausiids (Euph), calanoid copepods (Cala), other copepods (Oth.Cop.), other zooplankton (Oth.zoo), diatoms (Diat) and other phytoplankton (Oth.phyt).

4.1 (a) Percentage contribution of diatoms, dinoflagellates, other phytoplankters, copepods, euphausiids, gastropods and other zooplankters, by number (Total Nb.) and by contribution to dietary carbon (Total Carb.) for (a) the pooled sardine stomach content data (two left histograms), and for each of the 6 surveys from which sardine stomach contents were analyzed off Peru (surveys are identified along the x-axis, with the first two numbers corresponding to the year, the following two numbers the start month of the survey, and the last two numbers the end month of the survey), and (b) by number (%N) and by contribution to dietary carbon (%C) for Sardinops sagax in the Southern Benguela current system (SBCS), Sardina pilchardus in the Northern Canary current system (NCCS), and S. sagax in the Northern Humboldt Current system (NHCS).

4.2

Average percentage contribution to dietary carbon (%C) by prey category for Engraulis ringens and Sardinops sagax in the Northern Humboldt Current system, S. sagax in the Southern Benguela system, and Sardina pilchardus in the Northern Canary Current system.

4.3

Average percentage contribution to dietary carbon (%C) by prey size class (mm) for Sardinops sagax in the Southern Benguela system, Sardina pilchardus in the Northern Canary system (Iberian coast), and S. sagax (sard) and Engraulis ringens (anch) in the Northern Humboldt system.

5.1

Sampling station per year, red=2008, green=2009, blue=2010, black=2011; the blue line indicate the 200 m isobath.

5.2 δ 13 C vs. δ 15 N biplot for zooplankton, nekton and top predators collected in the NHCS: (a) depicting isotopic niche space (b) δ 13 C according to the distance from the coast (km), (c) δ 15 N according to the distance from the coast (km). Values are x±SD (denoted in Table 1).

5.3

Body length -based variability of δ 15 N values (red=2008; green=2009; blue=2010; black=2011).

5.4

Latitudinal variability of δ 15 N values. (red=2008; green=2009; blue=2010; black=2011).

5.5

Cross shore (distance from the coast) variability of δ 15 N values. (red=2008; green=2009; blue=2010; black=2011).

5.6

Predicted δ 15 N values from the two LME models: north and south of 7.5ºS. For each species the latitudinal range of prediction covers the one where individuals were sampled.

5.7

TP variability with body length of the most important species at 7.5°S and 11.7ºS.

5.8

Predicted TP at 11.7ºS (Vinciguerria, mackerel, squat lobster, anchoveta, jumbo squid, jack mackerel, Peruvian booby, guanay, fur seal) and 7.5ºS (euphausiids, myctophid, Vinciguerria, anchoveta, jumbo squid, mackerel, hake and squat lobster).

6.1

Conceptual schematic of how physical forcing may lead to environments that favour either sardine or anchovy. Source: van der Lingen et al. (2006).

6.2 (A) Anchovy and sardine biomass estimated using virtual population analysis (VPA). Source: Ñiquen et al. (2000), R. Guevara pers. com. (B) Dominance of euphausiids and small zooplankton from generalized additive model (GAM) results. Source: Ayón et al. (2011).

6.3

Conceptual model of decadal changes in anchovy and sardine populations in the southeastern tropical Pacific. A: Schematic of the temporal evolution of large plankton and anchovy (blue solid line), and oxygen, small plankton and sardine (red solid line) between 1960 and 2010. B: Energetic costs of feeding on dominant plankton sizespectra for anchovy and sardine according to the scenarios from A. C: Schematic of the available habitat for anchovy (blue shaded area) and sardine (red shaded area). Source: [START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF].

6.4

Anchovy and squat lobsters.

6.5

GAMs of muscle δ13C and δ15N values as a function of latitude, mantle length (ML) and distance to shelf break (DSB). Source: Argüelles et al. (2012).

6.6 Conceptual model and cross-shore profiles of oxygen and organisms distribution. The lower panel shows the mean (spline smooth) cross-shore profiles of dissolved oxygen saturation in % (grey 639 dashed line), depth of the 2 ml.l-1 isoline in m (black solid line) and the acoustic-estimated biomass of anchovy (blue solid line) and the jumbo squid acoustic-estimated biomass (red solid line). Oxygen and anchovy data come from [START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF]; jumbo squid data come from IMARPE, unpublished data. Note that the oxygen data cover the range 7ºS to 18ºS. The upper part shows the crossshore distribution of jumbo squid along its ontogenetic cycle. The colours in the arrow represent the schematic range of distribution and proportional abundance of the three main prey groups i.e., the other cephalopoda, euphausiids and mesopelagic fish. Source: Alegre et al. (2014).

List of Tables

1.1

Main characteristics of the water masses presents in the Humboldt Current System (source: Bertrand et al. 2004).

2.1

Survey code, start and end date, the number of trawls, and the number, length range (in cm) of anchoveta collected for stomach content analysis during this study.

2.2

Type and size of zooplankton recorded from anchoveta stomach contents; length was estimated by IMARPE except where indicated.

2.3

Relationships employed to calculate dry weight and carbon content of the different categories of phytoplankton and zooplankton. Carbon contents not in μg were transformed in μg for comparison; PL: prosome length (μm); TL: total length (mm); DW: dry weight (μg); WW: wet weight (μg).

3.1

Year, start and end date, number of trawls, total number of stomachs with prey contents, and length range (cm) of Engraulis ringens during 1996-2003.

3.2

Factors used to pool the data.

3.3

Results of non-parametric ANOVA (Kruskal-Wallis test); n=1938 (the 13,430 stomachs pooled by size class).

4.1 Survey characteristics.

5.1 Data description.

5.2

Predicted δ15N (‰) values and trophic position (TP) at 7.5°S and 11.7°S.

5.3

Trophic position estimates of some Engraulis spp. around the world.

Résumé en Français

Le nord du système du Courant de Humboldt (NHCS), le long des côtes péruviennes, est l'une des régions océanique les plus productives au monde. Il représente moins de 0.1% de la surface des océans mondiaux mais contribue actuellement à plus de 10% des captures mondiales en poissons, avec l'anchois Engraulis ringens comme espèce emblématique. Comparé aux autres systèmes d'upwelling de bord Est, la forte productivité en poissons ne peut être expliquée par une productivité primaire plus élevée. Par contre, le NCHS est la région où El Niño, et la variabilité climatique en général est la plus notable. D'autre part, les eaux de surface oxygénées recouvrent une zone de minimum d'oxygène extrêmement intense et superficielle. L'objectif principal de ce travail est de mieux comprendre les relations trophiques au sein du NHCS en combinant l'analyse de contenus stomacaux et d'isotopes stables. Cette étude se focalise sur une variété d'organismes allant des bas niveaux trophiques comme le zooplancton aux prédateurs supérieurs (oiseaux et les otaries à fourrure). Elle combine des études de contenus stomacaux ponctuelles et sur le long terme d'espèces clés telles que l'anchois et la sardine Sardinops sagax et une analyse plus globale, basée sur l'utilisation d'isotopes stables et considérant l'ensemble du réseau trophique dans les années récentes (2008 -2012).

Les analyses des contenus stomacaux d'anchois et de sardine ont permis de revisiter l'écologie de ces espèces. En effet, bien que le phytoplancton domine largement les contenus stomacaux en termes d'abondance numérique, le zooplancton est de loin la composante alimentaire la plus importante pour ces deux espèces en termes de carbone. Dans le cas de l'anchois, les euphausiacés contribuent à plus de 67.5% du carbone ingéré, suivis par les copépodes (26.3%). Sélectionner les proies les plus grandes telles que les euphausiacés procure un avantage énergétique pour l'anchois dans cet écosystème où les carences en oxygène imposent de fortes contraintes métaboliques aux poissons pélagiques. La sardine se nourrit de zooplancton plus petit que l'anchois (copépodes plus petits et moins d'euphausiacés

CHAPTER 1 General Introduction

Eastern boundary upwelling systems (EBUS), i.e., Benguela, California, Canary and the Humboldt Current systems support ~20% of worldwide fish harvests [START_REF] Fréon | Eastern Boundary Upwelling Ecosystems: Integrative and comparative approaches[END_REF]. The landings are mostly based on two heavily exploited fish resources, anchovy (the genus Engraulis) and sardine (the genera Sardinops and Sardina). The northern Humboldt Current system (NHCS) off Peru produces more fish per unit area than any other oceanic region. It represents less than 0.1% of the world ocean surface but presently sustains about 10% of the world fish catch, with the Peruvian anchovy or anchoveta Engraulis ringens as emblematic fish resource [START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF]. Compared with other EBUS, the higher fish productivity of the NHCS cannot be explained by a corresponding higher primary productivity (Fig. 1.1) (Chavez et al., 2008). On another hand, the NHCS is the region where El Niño, and climate variability in general, is most notable [START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF]. Also, as a result of sinking and decay of primary production and poor ventilation, surface oxygenated waters overlie an intense and extremely shallow Oxygen Minimum Zone (OMZ). The OMZ affects global nutrient budgets, as nitrate instead of oxygen is used by bacteria as a terminal electron acceptor [START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF]. A series of hypothesis based on the characteristics of the NHCS were proposed to explain the secret of its productivity but the ultimate reasons are still not clear.

In this introduction we will briefly present the main characteristics of the NHCS and explain why trophic dynamics are important processes that should be considered to better understand ecosystem functioning. We will then expose the main objectives of this thesis before presenting a short summary of each chapter.

Circulation and dynamics in the Northern Humboldt Current system

The NHCS is mostly wind-driven by the South Pacific subtropical anticyclone [START_REF] Strub | Coastal ocean circulation off Western South America[END_REF] forcing the South Pacific gyre (see Figure 1.2 for a schematic representation of the circulation in the NHCS). Near the Peruvian coast, the dominant alongshore equatorward winds and cyclonic wind-stress curl produce offshore Ekman transport of surface water generating an intense upwelling characterized by an alongshore narrow strip of cold (Fig. 1.2b, color shading) and highly productive water and a current system composed of equatorward surface and mainly poleward subsurface flows. Upwelling of cool waters brings phytoplankton nutrients to the surface, dramatically increasing biological productivity. Upwelling intensity is generally high during most of the year with a maximum during austral winter [START_REF] Gunther | A report on oceanographic investigations in the Peru Coastal Current[END_REF][START_REF] Strub | Coastal ocean circulation off Western South America[END_REF], which results in year-round high nutrient concentrations in the surface waters [START_REF] Pennington | Primary production in the eastern tropical Pacific: a review[END_REF][START_REF] Echevin | The seasonal cycle of surface chlorophyll in the Peru upwelling system: a modelling study[END_REF]. The Peruvian coastal upwelling region is thus characterised by extremely high rates of primary productivity [START_REF] Thiede | Sedimentary record of ancient coastal upwelling[END_REF][START_REF] Berger | Productivity of the Ocean: Present and Past[END_REF][START_REF] Pennington | Primary production in the eastern tropical Pacific: a review[END_REF] even if lower than in the Canaria and Benguela Current systems [START_REF] Carr | Estimation of potential productivity in Eastern Boundary Currents using remote sensing[END_REF][START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF]. Note that in surface waters along the shelf region, nutrient concentrations, especially of nitrate, phosphate and iron, are very high and are not limiting for blooming diatoms [START_REF] Bruland | Iron, macronutrients and diatom blooms in the Peru upwelling regime: brown and blue waters of Peru[END_REF][START_REF] Franz | Dynamics and stoichiometry of nutrients and phytoplankton in waters influenced by the oxygen minimum zone in the eastern tropical Pacific[END_REF]. Large diatoms tend to dominate in phytoplankton biomass [START_REF] Rojas De Mendiola | Seasonal phytoplankton distribution along the Peruvian coast[END_REF][START_REF] Wilkerson | Biomass and productivity in Monterey Bay, California: contribution of the large phytoplankton[END_REF]. However, the dinoflagellates are other key phytoplankton component. They are associated with warm events such as El Niño [START_REF] Rojas De Mendiola | Red tide along the Peruvian coast[END_REF][START_REF] Delgado | El fitoplancton frente a la costa peruana durante El Niño[END_REF]. Also these organisms are motile and capable to migrate vertically [START_REF] Forward | Effects of temperature on swimming speed of the dinoflagellate Gymnodinium splendens[END_REF] to take up nitrate below the thermocline [START_REF] Dortch | Dark uptake of nitrate and nitrate reductasa activity of a red-tide population off Peru[END_REF][START_REF] Franz | Dynamics and stoichiometry of nutrients and phytoplankton in waters influenced by the oxygen minimum zone in the eastern tropical Pacific[END_REF]. Therefore, if stratification increases but a shallow thermocline/nutricline persists, coastal dinoflagellates will increase by swimming to shallow depths to take up nutrients at night and return to the surface to photosynthesize during the day [START_REF] Chavez | Marine primary production in relation to climate variability and change[END_REF]. [START_REF] Chaigneau | Near-coastal circulation in the Northern Humboldt Current System: from shipboard ADCP data[END_REF].

Because of the constant upwelling of subsurface cold water, a wide region off the Peruvian coast is dominated by relatively cold, highly productive water mass, the Cold Coastal Water (CCW). The oligotrophic Subtropical Surface Water (SSW) is located offshore the CCW. The Surface Tropical Water (STW) is found north of the equator and characterized by higher temperatures and lower salinities than the other water masses present off Peru. The Surface Equatorial Water (SEW) is found between the CCW and the STW (Fig. 1. [START_REF] Trophic Dynamics Van Der Lingen | Climate Change and Small Pelagic Fish[END_REF]. The main characteristics of the mater masses can be found in Table 1.1. The Peruvian upwelling system is subjected to bottom-up forcing at intraseasonal (Bertrand et al., 2008b) interannual [START_REF] Barber | Biological consequences of El Niño[END_REF], multidecadal [START_REF] Chavez | From anchovies to sardines and back; multidecadal change in the Pacific Ocean[END_REF][START_REF] Alheit | Regime shifts in the Humboldt Current ecosystem[END_REF]Ayón et al., 2008b) and centennial time scales [START_REF] Gutiérrez | Rapid reorganization in ocean biogeochemistry off Peru towards the end of the Little Ice Age[END_REF]Siffedine et al., 2008;[START_REF] Valdés | 250 years of sardine and anchovy scale deposition record in Mejillones Bay, northern Chile[END_REF]Salvatteci, 2013).

The most remarkable expressions of the interannual variability of the Humboldt Current system are the warm and cold phases of the El Niño Southern Oscillation (ENSO), El Niño and La Niña, respectively. During El Niño conditions the thermocline deepens, which makes the coastal upwelling 'inefficient' in terms of nutrient enrichment [START_REF] Barber | Biological consequences of El Niño[END_REF]. Conversely, during La Niña conditions the thermocline rises, which allow coastal upwelling to bring cold and nutrient-rich water toward the surface. As a consequence, the cold coastal water (CCW) mass expands. These two sets of conditions also affect the spatial organization of living organisms by modifying the volume of the favourable pelagic habitat, or the surface of the oxygenated bottom areas, due to the along-coast propagation of the KWs (Bertrand et al., 2008b;[START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF].

In the short term, the ENSO-related changes in the physical and biological conditions could have some negative effects, like reducing the abundance of the anchoveta and its predators [START_REF] Arntz | El Niño: experimento climático de la naturaleza[END_REF]. However, in the long term, it might contribute to maintaining the high fish production of the system by favouring fast-growing fish species like anchovy, which take advantage of the low predation pressure and rapidly increase their population and dominate the system [START_REF] Bakun | The marine ecosystem off Peru: What are the secrets of its fishery productivity and what might its future hold?[END_REF]. Still, each El Niño is different and has different effect on the ecosystem. Strong El Niño Events such as the events of 1982-83 and 1997-98 dramatically impact the coastal ecosystem, while others do not (Bertrand et al., 2004a). Recent studies showed that the extraordinary warm events such as the 1982-83 and 1997-98 El Niño are distinct from the others (e.g. 1992-93, 2002-03, 2004-05, 2009-10), which correspond to "El Niño Modoki" or "Central Pacific" [START_REF] Takahashi | ENSO regimes: Reinterpreting the canonical and Modoki El Niño[END_REF]. Indeed when Central Pacific El Niño occurs, the conditions can even be slightly cooler off Peru [START_REF] Dewitte | Vertical structure variability and equatorial waves during central Pacific and eastern Pacific El Niños in a coupled general circulation model[END_REF], far from the archetypal El Niño effects on the Peruvian ecosystem [START_REF] Barber | Biological consequences of El Niño[END_REF]Alheit and Niquen, 2004;Bertrand et al., 2004a).

Oxygen minimum zone

Oxygen depletion is widespread in the world oceans [START_REF] Kamykowski | Hypoxia in the world ocean as recorded in the historical data set[END_REF], occurring as permanent, seasonal and episodic features. Where upwelling is most intense in the world ocean there is high phytoplankton production. This material sinks and is decomposed in midwater, consuming dissolved oxygen. When high oxygen demand occurs in combination with low ventilation, a massive midwater oxygen minimum develops [START_REF] Wyrtki | The oxygen minima in relation to ocean circulation[END_REF]. Since it combines high productivity and low ventilation, a prominent feature of the NHCS is the presence of a permanent oxygen minimum zone (OMZ, Fig. 1.4). OMZ are defined as regions where oxygen concentrations are <0.5 ml.l-1 (or about 7.5% saturation; <22 μM) [START_REF] Levin | Oxygen minimum zone benthos: adaptation and community response to hypoxia[END_REF]. In the NHCS, the OMZ is thickest (>600 m) between 5 and 13ºS and to about 1 000 km offshore [START_REF] Fuenzalida | Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean[END_REF]. Because of the coastal upwelling, which lifts the coastal hypoxic layer closer to the sea surface [START_REF] Fuenzalida | Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean[END_REF], the OMZ upper boundary is shallowest at the coast (25-50 m) off Peru and northern Chile (e.g., [START_REF] Morales | Anchovy larval distribution in the coastal zone off northern Chile: the effect of low dissolved oxygen concentration and of a cold-warm sequence (1990-1995)[END_REF][START_REF] Strub | Coastal ocean circulation off Western South America[END_REF]. These OMZs contribute to 25-75% of oceanic N2O production [START_REF] Nevison | Quantifying the Nitrous Oxide Source from Coastal Upwelling[END_REF], a potent greenhouse gas, which influences the Earth's heat budget and depletes stratospheric ozone [START_REF] Prather | Chapter 2.2: Radiative Forcing of Climate Change, in Climate Change[END_REF]]. OMZs are generally the site of intense denitrification [START_REF] Naqvi | Increased marine production of N 2 O due to intensifying anoxia on the Indian continental shelf[END_REF]. Actually in OMZ, both intense denitrification and anaerobic ammonium oxidation (anammox) occur [START_REF] Kuypers | Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation[END_REF][START_REF] Lam | Revising the nitrogen cycle in the Peruvian oxygen minimum zone[END_REF][START_REF] Lam | Microbial Nitrogen Cycling Processes in Oxygen Minimum Zones[END_REF], which has strong impact on nitrogen cycling and organic matter remineralisation, subsequently limiting the biological production [START_REF] Lam | Revising the nitrogen cycle in the Peruvian oxygen minimum zone[END_REF].

In response to global warming and direct anthropogenic influences, OMZs of the World Ocean are expanding [START_REF] Oschlies | Simulated 21st century's increase in oceanic suboxia by CO2-enhanced biotic carbon export[END_REF][START_REF] Stramma | Expanding oxygen minimum zones in the tropical oceans[END_REF]. The upper limit of OMZs is rising and consequently, the vertical extent of the well-oxygenated surface layer shrinks, constraining the vertical habitat of epipelagic organisms. Indeed, the OMZ strongly influences the distribution and mortality of marine organisms [START_REF] Levin | Oxygen minimum zone benthos: adaptation and community response to hypoxia[END_REF][START_REF] Prince | Hypoxia-based habitat compression of tropical pelagic fishes[END_REF]Diaz and Rosenberg, 2008;[START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF]. Although a few species of zooplankton (e.g. the euphausiid Euphausia mucronata), mesopelagic fish, and squids (e.g. the jumbo squid Dosidicus gigas) have adapted their metabolism to temporarily (through diel vertical migration) or permanently inhabit OMZs, most marine species limit their distribution to the surface oxygenated layer. Intensification of oxygen-poor and acidic conditions could severely impact marine communities e.g. by (i) shrinking the available habitat, (ii) diminishing the capacity of plankton to develop calcium carbonate skeletons, (iii) eliminating species from metazoans to fish predators or (iv) hampering the spawning success of fish resources [START_REF] Levin | Oxygen minimum zone benthos: adaptation and community response to hypoxia[END_REF][START_REF] Orr | Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms[END_REF]Diaz and Rosenberg, 2008;[START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF].

The marine ecosystem with more fish production

As previously indicated, the NHCS is the "heavyweight champion of the world" in terms of fish productivity although it generates considerably less primary production than the Northern Benguela Current System [START_REF] Bakun | The marine ecosystem off Peru: What are the secrets of its fishery productivity and what might its future hold?[END_REF].

Different hypotheses have been proposed to explain this paradox:

(i) The proximity to the equator, unique among the EBUSs, allows strong upwelling with relatively weak winds so a weak turbulence [START_REF] Parrish | Comparative climatology of selected environmental processes in relation to eastern boundary current pelagic fish reproduction[END_REF], which may increase the occurrence of the optimal environmental windows for fish reproduction [START_REF] Cury | Optimal environmental window and pelagic fish recruitment success in upwelling areas[END_REF]); (ii) Differences in the trophic structure within EBUSs [START_REF] Carr | Production regimes in four Eastern Boundary Current systems[END_REF]; (iii) The strong influence of ENSO inter-annual variability might 're-set' the ecosystem and could favour fast-growing populations like small pelagic fishes in the NHCS [START_REF] Bakun | The marine ecosystem off Peru: What are the secrets of its fishery productivity and what might its future hold?[END_REF]; (iv) In opposition with other EBUS, the main s p a w n i n g p e r i o d s m a t c h t h e s e a s o n o f maximal shelf retention of ichthyoplankton and food concentration in the NHCS for both anchovy and sardine [START_REF] Brochier | Investigating the 'northern Humboldt paradox' from model comparisons of small pelagic fish reproductive strategies in eastern boundary upwelling ecosystems[END_REF].

Despite numerous studies the paradox is still not fully understood and trophic dynamics play probably an important role. Indeed, trophic dynamics greatly influence population dynamics, species coexistence and the organization of communities [START_REF] Pimm | Food webs[END_REF][START_REF] Polis | Food Webs: Integration of patterns and dynamics[END_REF]. Species are linked through trophic relationships which denote transfers of energy and nutrients [START_REF] Odum | The Strategy of Ecosystem Development[END_REF][START_REF] Holt | Biodiversity and ecosystem functioning: the role of trophic interactions and the importance of system openness[END_REF][START_REF] Link | Does food web theory work for marine ecosystems?[END_REF]. Such interactions are commonly called trophic structure or "food web" or "food chain", and is referred to the way in which organisms use food resources [START_REF] Shackell | Decline in top predator body size and changing climate alter trophic structure in an oceanic ecosystem[END_REF]. Of this way, a healthy marine ecosystem has a food web with highly interconnected linkages, which can denote the complexity of the ecosystem [START_REF] Menge | Indirect effects in marine rocky intertidal interaction webs: patterns and importance[END_REF]. Thus, the trophic dynamics goals is defining nodes (predator and prey), understand these and the relation with the environment [START_REF] Latour | Toward ecosystem-based fisheries management: strategies for multispecies modeling and associated data requirements[END_REF][START_REF] Link | Does food web theory work for marine ecosystems?[END_REF][START_REF] Pikitch | Ecosystem-based fishery management[END_REF].

A brief history of trophic studies in the NHCS

The search for a quantitative understanding of the dynamics of interactions between the biotic and abiotic components of marine ecosystems, and their effects on the dynamics of fish populations constitutes the foundation of modern fisheries oceanography [START_REF] Dower | Improving fisheries oceanography in the future[END_REF].

The foundation for fisheries management on an e c o s y s t e m -b a s i s m u s t l i e i n a p p r o p r i a t e modelling of the ecosystems. Furthermore, as stated by [START_REF] Denman | Can we see the future?[END_REF], "it is almost axiomatic to state that confidence in forecast will increase with the increased use of observations". Therefore a model (qualitative or q u a n t i t a t i v e ) i s c o n s t r u c t e d a c c o r d i n g t o t h e a v a i l a b l e knowledge and hypotheses. So models changes with time when new information are available.

In this sense the vision of the trophic dynamics evolved with time in the NHCS so our vision of ecosystem functioning and dynamics.

The first ecosystem perspective of the NHCS was published by Vogt (1948) (Fig. 1.5). The vision was mostly land-based with the seabirds occupying a central role and other land-related organisms (from condor to house fly!) also playing an important position. Indeed at that time, fish were virtually not exploited. On the contrary, seabirds excrement (guano -derived of an Inca word: guanay, the common name of the cormorant Phalacrocorax bougainvillii in Peru) was extracted to be used as fertilizer and generated an important role in the Peruvian economy, mainly at 19th century and beginning of the 20th [START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF]. The guano was exported worldwide and in particular in Europe. The importance of guano justified the development of the first marine biology scientific research entity in Peru, the Marine Biology Laboratory (Laboratorio de Biología Marina) of the Compañia Administradora de Guano, which aim was to study and preserve the guanay [START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF].

During the 1940s a fishery developed on Pacific Eastern bonito (Sarda chiliensis) and tuna (primarily Thunnus albacore) due to a high demand of the liver oil of these species in the US market during World War II and later the Korean War. The later increased demand for guano and coincidentally the strong El Niño of 1957-1958 led to a dramatic decrease in seabird populations [START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF]. Around 1955 the anchoveta fishery started. The catches were first used as fertilizer and then for producing fish meal.

To accompany the development of the anchovy fishery, the Peruvian government and United Nations established in 1960 the Instituto de Recursos Marinos (IREMAR), which began under the leaderships of FAO. Then, in 1964, was created the Instituto del Mar del Peru (IMARPE), fusioning IREMAR with the Consejo de Investigaciones Hidrobiológicas (CIH). The anchoveta fishery continued to grow during the 1960s to a peak harvest of 12 million tonnes per year in 1970 (actually probably 18 million tonnes when considering for unregistered catches, see [START_REF] Castillo | Estimation of unregistered Peruvian anchoveta (Engraulis ringens) in official catch statistics, 1951-1982[END_REF] accounting for 20% of the world catch, and the anchoveta population collapsed in early 1970s due to the combination of the El Niño 1972, the beginning of an unfavourable decadal climatic regime, and overfishing. Source: [START_REF] Vogt | Road to survival[END_REF].

Just before the anchoveta collapse, [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF] published a seminal article considering that anchoveta feed directly on phytoplankton, and this very short and efficient food chain explain the large population of this species (and then is transferred to the top predators. This paper had a considerable impact during decades.

The anchoveta crash led to develop intense scientific studies and an important advance was synthesized by IMARPE in conjunction with ICLARM (now World Fish Center) and the Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) during the mid 1980s [START_REF] Pauly | The Peruvian anchoveta and its upwelling ecosystem: three decades of change[END_REF]Pauly et al., 1989a,b).The anchoveta became the centre of the system (Fig. 1.6) but important links and feedbacks between climate, ocean circulation, biogeochemical cycles, trophic webs and fish production were also taken into account (Fig. 1.7).

Figure 1.6: Major interaction in the pelagic ecosystem off Peru. Source: [START_REF] Muck | Major trends in the pelagic ecosystem off Peru and their implications for management[END_REF].

Figure 1.7: Elements to be considered in a simulation model of the Peruvian upwelling ecosystem. Source: [START_REF] Csirke | Informe del Taller de trabajo sobre modelos para la predicción del rendimiento en el sistema de afloramiento peruano IMARPE[END_REF].

In these syntheses and the works later developed from this basis, the anchoveta was considered as mostly filter-feeder and phytoplanktivorous in agreement with [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF]. In this way Jarre-Teichmann (1998) focused on different biological regimes in the 1960s and 1980s with emphasis to set the basis for more sophisticated modelling approaches of the species interactions and how the fisheries impact the ecosystem functioning (Fig. 1.8). 1964-1971 and 1973-1981. Flows of wet mass are in units of 10 3 kg.km -2 .yr -1 . Flows leave boxes on the upper half and enter them on the lower half. The width of the line indicates the order of magnitude of trophic flows. Biomass of detritus is a rough estimate. Source: [START_REF] Jarre | Two approaches for modelling fish stock interactions in the Peruvian upwelling ecosystem[END_REF] and [START_REF] Jarre-Teichmann | The potential role of mass-balance models for the management of upwelling ecosystems[END_REF].

Figure 1.9: Schematic representation of the food web in the Peruvian upwelling system without (a) and with (b) the industrial fishery for anchoveta included in the model. The numbers correspond to the proportion of productivity available at one trophic level consumed by the next trophic level at any given time. Source [START_REF] Jahncke | Trends in carbon flux to seabirds in the Peruvian upwelling system: effects of wind and fisheries on population regulation[END_REF].

Following the efforts to model the trophic dynamics of the NHCS, [START_REF] Jahncke | Trends in carbon flux to seabirds in the Peruvian upwelling system: effects of wind and fisheries on population regulation[END_REF] constructed a model from environment to seabirds covering the period 1925-2000 (Fig. 1.9). The model indicates that the growth of seabird populations from 1925 to 1955 was likely a response to increased productivity of the Peruvian upwelling system and that the subsequent drastic decline in seabird abundance was likely due to competition for food with the fishery, which caught 85% of the anchovies, which otherwise would have been available for the seabirds. In this model also the main energetic flux towards anchoveta pass directly through phytoplankton (61.6% of the carbon come directly from phytoplankton, while 61.6% of 23.4%, i.e. 14.4% come from zooplankton; see Fig. 9). [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF]. This coastal food chain is short and efficient, but also leaky in the sense that a relatively large fraction of production is often exported [START_REF] Muller-Karger | The importance of continental margins in the global carbon cycle[END_REF] either by sinking, and supporting a rich benthic fauna or, alternately, resulting in anoxia [START_REF] Margalef | Life-forms of phytoplankton as survival alternatives in an unstable environment[END_REF], or by horizontal advection away from the coast [START_REF] Olivieri | A model of plankton dynamics for the coastal upwelling system of Monterey Bay, California[END_REF][START_REF] Pennington | The northern and central California Coastal Upwelling System[END_REF]. In the oligotrophic open ocean, small, photosynthetic picoplankton dominate, and the grazers are also small and have growth rates similar to their prey. This small predator-prey is characterized as a complex, low-nutrient input system that exports little of its production (e.g., non-leaky) [START_REF] Azam | The ecological role of water-column microbes in the sea[END_REF][START_REF] Pomeroy | The Ocean´s food web, a changing paradigm[END_REF]. This oceanic food chain has low nutrient inputs, efficient internal recycling of nutrients, and need multiple trophic transfers to transform picoplankton production to living marine resources, resulting in much fewer higher-trophic-level or fisheries resources.

As previously indicated, in all these work, the phytoplankton was the base of the diet of forage fish. Indeed, even rather recent works in the NHCS has concluded that anchoveta depends mainly on phytoplankton (Alamo et al., 1996a(Alamo et al., ,b, 1997a,b;,b;[START_REF] Alamo | Variaciones alimentarias en Engraulis ringens y otros recursos pelágicos durante invierno-primavera de 1997[END_REF]Espinoza et al., 1998aEspinoza et al., ,b, 1999Espinoza et al., , 2000)).

What can be the limit of the historical studies?

In theoretical sense, food-web diagrams summarising predator-prey relationships, feeding strategies, and energetic transfer are time-consuming to construct and are often subjective in their resolution and scope [START_REF] Paine | Food webs: road maps of interactions or grist for theoretical development[END_REF]. They are considered as informative by linking species and ecosystem-level characteristics (e.g., [START_REF] Stevens | Patterns of functional diversity across an extensive environmental gradient: vertebrate consumers, hidden treatments and latitudinal trends[END_REF][START_REF] Downing | Relative effects of species composition and richness on ecosystems properties in ponds[END_REF][START_REF] Micheli | Low functional redundancy in coastal marine assemblages[END_REF] but have some limitations and may be limited in their predictive power [START_REF] Petchey | Functional diversity (FD), species richness and community composition[END_REF][START_REF] Petchey | How do different measures of functional diversity perform?[END_REF][START_REF] Wright | Conventional functional classification schemes underestimate the relationship with ecosystem functioning[END_REF]. Indeed, many factors stem our ability to observe the relevant taxonomic, spatial, and temporal variations in trophic interactions and these constructions are subjective and potentially strongly biased [START_REF] Paine | Food webs: road maps of interactions or grist for theoretical development[END_REF][START_REF] Petchey | How do different measures of functional diversity perform?[END_REF].

Further, these diagrams typically hold all trophic links to be of equal importance, which makes them ineffectual for tracking energy or mass flow through ecological communities [START_REF] Paine | Food webs: road maps of interactions or grist for theoretical development[END_REF][START_REF] Hairston | Cause-effect relationships in energy flow, trophic structure, and interspecific interactions[END_REF][START_REF] Polis | Food web complexity and community dynamics[END_REF][START_REF] Persson | Trophic cascades: abiding heterogeneity and the trophic level concept at the end of the road[END_REF][START_REF] Vander Zanden | Primary consumer δ 13 C and δ 15 N and the trophic position of aquatic consumers[END_REF]. Many trophic interactions cannot be visually observed, and gut analysis is misleading if the gut contents are not assimilated and we should not underestimate the importance of subtle differences among species that manifest only under altered conditions or over long time periods [START_REF] Duffy | Biodiversity and ecosystem function: the consumer connection[END_REF] In the NHCS, the methods employed to study trophic functioning were not uniform. The diet of the main species (anchoveta and sardine) were based on frequency of occurrence, volume and semi-quantitative methods which have inherent biases and are considered inadequate and subjective to study trophic relationships of planktivorous fishes [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF]. For example, methods based on frequency of occurrence, overemphasize small prey over big ones, due to great differences of size range between phytoplankton and zooplankton (10-6-10-5 mm for diatoms to 10 mm for zooplankton). Furthermore, the diet of their fish predators (e.g. hake Merluccius gayi peruanus or jack mackerel Trachurus murphyi) were based on frequency of occurrence or prey weight [START_REF] Konchina | The feeding niche of the hake, Merluccius gayi (Merluccidae), and the jack mackerel, Trachurus symmetricus (Carangidae), in the trophic system of the Peruvian coastal upwelling[END_REF], limiting the possibility to compare the diet between groups.

The difficulties afore mentioned plead for a methodology allowing to clarify trophic relationships and distinguish actual trophic links from those potentially possible. The most classic method to study the trophic ecology is the stomach content analysis since it provides high details on ingested prey. However, this technique is time consuming and not suitable if fast results are needed [START_REF] Olson | Food-web inferences of stable isotope spatial patterns in copepods and yellowfin tuna in the pelagic eastern Pacific Ocean[END_REF]. Furthermore, it only represents a snapshot of the diet [START_REF] Estrada | Diet and trophic position of Atlantic bluefin tuna (Thunnus thynnus) inferred from stable carbon and nitrogen isotope analysis[END_REF] even if it is still particularly suitable in long-term studies to detect changes in the ecosystem and the only technique if information is needed at the species level.

To overcome some limitation, stomach-based analyses can be combined with carbon and nitrogen stable isotope measurements [START_REF] Deniro | Influence of diet on the distribution of carbon isotopes in animals[END_REF][START_REF] Kling | Stable isotopes and planktonic trophic structure in Arctic lakes[END_REF]. The strength of stable-isotope analyses is that they measure actual assimilation integrated over the time scale of tissue turnover in the organism. The use of stable isotopes analysis (SIA) to study trophic dynamics is attractive because it is relatively inexpensive, stable isotopes are easy to measure, and their use allows the quantification of ecological concepts (e.g. food chain length), which can then be tested statistically or experimentally [START_REF] Post | Using stable isotopes to estimate trophic position: models, methods, and assumptions[END_REF].

What are stable isotopes?

Most elements exist in two or more forms, known as isotopes. Isotopes have the same number of protons but differ in their number of neutrons, resulting in different masses. The lighter form is generally the more common one [START_REF] Hobson | Tracking animal migration with stable isotopes[END_REF]. This variation in the relative abundance of stable isotopes results from tiny mass differences that cause the isotopes to act differently in chemical reactions and physical processes. The lighter isotope generally forms weaker bonds than the heavier one and tends to react faster. The change in isotopic abundance is called fractionation (Karasov and Martínez del Rio, 2007). Different environments are often characterized by predictable isotopic signatures [START_REF] West | Stable isotopes as one of nature's ecological recorders[END_REF]. Stable isotopes are measured as isotopic deviations from international standards and are expressed as delta (δ) values as parts per thousand [START_REF] Werner | Referencing strategies and techniques in stable isotope ratio analysis[END_REF]. These values are calculated as follows:

δ X = [(Rsample/Rstandard) -1] x 1000
where X is the element (such as 13 C or 15 N), and R is the corresponding isotope ratio ( 13 C/ 12 C or 15 N/ 14 N). The quotient of the ratios in the sample relative to the standard is the δ value. Therefore, an increase in the δ values denotes an increase in the amount of the heavier isotope component; while a decrease in the values denotes a decrease in the heavy isotope content [START_REF] Peterson | Stable isotopes in ecosystems[END_REF].

The SIA has been accepted broadly as an important tool to examine animal migration and movement [START_REF] Hobson | Tracing origins and migration of wildlife using stable isotopes: a review[END_REF][START_REF] Rubenstein | From birds to butterflies: animal movement patterns and stable isotopes[END_REF]; resource partitioning [START_REF] Young | Resource partitioning by species but not sex in sympatric boobies in the central Pacific Ocean[END_REF]; ecophysiological processes [START_REF] Gannes | Natural abundance variations in stable isotopes and their potential uses in animal physiological ecology[END_REF]; and ecosystem fluxes of carbon and nitrogen [START_REF] Peterson | Stable isotopes in ecosystems[END_REF]. When used in trophic ecology studies, it serves to reconstruct diets [START_REF] Tieszen | Fractionation and turnover of stable carbon isotopes in animal tissues: implications for δ 13 C analysis of diet[END_REF], to assign species to trophic positions in food webs [START_REF] Minagawa | Stepwise enrichment of 15N along food chains: further evidence and the relation between δ15N and animal age[END_REF][START_REF] Fry | Stable isotope diagrams of freshwater food webs[END_REF][START_REF] Post | Using stable isotopes to estimate trophic position: models, methods, and assumptions[END_REF], to elucidate patterns of resource acquisition and allocation [START_REF] Cherel | Isotopic discrimination between food and blood and feathers of captive penguins: implications for dietary studies in the wild[END_REF], and to characterize niche properties [START_REF] Bearhop | Determining trophic niche width: a novel approach using stable isotope analysis[END_REF][START_REF] Newsome | A niche for isotopic ecology[END_REF]. The central observation of SIA in trophic ecology is "You are what you eat...plus a few per mille" (DeNiro and Epstein, 1978) (Fig. 1.11). The abscissa represents a continuum of feeding types ranging from highly pelagic (more negative, or depleted in 13 C) to highly benthic (more positive, or enriched in 13 C). The ordinate should represent a continuum of trophic positions. Source: [START_REF] Sherwood | Stable isotope analysis of some representative fish and invertebrates of the Newfoundland and Labrador continental shelf food web[END_REF].

The stable isotope ratios of carbon ( 13 C/ 12 C, δ 13 C) and nitrogen ( 15 N/ 14 N, δ 15 N) have been used extensively to trace pathways of organic matter among organisms, because the stable isotopic signature of a consumer reflects that of its diet [START_REF] Mccutchan | Variation in trophic shift for stable isotope ratios of carbon, nitrogen, and sulfur[END_REF]. Consumer tissues are stepwise-enriched in 15 N relative to their food and consequently δ 15 N measurements serve as indicators of a consumer's trophic position [START_REF] Mccutchan | Variation in trophic shift for stable isotope ratios of carbon, nitrogen, and sulfur[END_REF][START_REF] Vanderklift | Sources of variation in consumer-diet δ 15 N enrichment: a meta-analysis[END_REF]. In contrast, δ 13 C values vary little (no or slight increase) along the food chain and are mainly used to determine primary sources in a trophic network [START_REF] Mccutchan | Variation in trophic shift for stable isotope ratios of carbon, nitrogen, and sulfur[END_REF] (Fig. 1.12). However, the carbon and nitrogen isotopic composition of organisms does not only depend on what they eat but also where they eat, because the isotopic composition at the base of the food web (isotopic baseline, i.e., phytoplankton) can greatly vary and has been shown to propagate up to the food web (see below).

SIA present a series of advantages...

-They have the potential to provide estimates of long-term average trophic position and feeding habits and can therefore detect changes in the trophic functioning of the ecosystem. -They are cheapest and timeless consuming compared to stomach content analyses.

-They are particularly fruitful to study spatial and ontogenetic changes in the diet of organisms. -They can provide a continuous measure of trophic position that integrates the assimilation of energy or mass flow through all the different trophic pathways leading to an organism. Stable isotopes have the potential to simultaneously capture complex interactions, including trophic omnivory, and to track energy or mass flow through ecological communities [START_REF] Peterson | Stable isotopes in ecosystems[END_REF]Kling et al., 1992, Cabana and[START_REF] Cabana | Comparison of aquatic food chains using nitrogen isotopes[END_REF].

... but also limitations

-They do not allow the level of precision in prey item identification as the stomach content analysis. -The SIA depends of the turnover rate i.e. how quickly the isotope signature in a body tissue changes following a dietary change [START_REF] Buchheister | Turnover and fractionation of carbon and nitrogen stable isotopes in tissues of a migratory coastal predator, summer flounder (Paralicthys dentatus)[END_REF]. Other process linked to turnover rates is the replacement or conversion of existing tissue using material synthesized from recent diet (i.e., metabolism) [START_REF] Hesslein | Replacement of sulfur, carbon, and nitrogen in tissue of growing broad whitefish (Coregonus nasus) in response to change in diet traced by δ 34 S, δ 13 C, and δ 15 N[END_REF]. Hence, the blood which is more metabolically active tends to respond more quickly to dietary changes [START_REF] Tieszen | Fractionation and turnover of stable carbon isotopes in animal tissues: implications for δ 13 C analysis of diet[END_REF][START_REF] Hobson | Assessing avian diets using stable isotopes. I: turnover of 13 C in tissues[END_REF], whereas other tissues such as muscle can require over one year to equilibrate to a constant diet [START_REF] Hesslein | Replacement of sulfur, carbon, and nitrogen in tissue of growing broad whitefish (Coregonus nasus) in response to change in diet traced by δ 34 S, δ 13 C, and δ 15 N[END_REF][START_REF] Macavoy | Isotopic turnover in aquatic predators: quantifying the exploitation of migratory prey[END_REF]. -When comparing among ecosystems, the δ 15 N and δ 13 C of an organism alone provides little information about its absolute trophic position or ultimate source of carbon, because there is substantial variation among ecosystems in the δ 15 N and δ 13 C at the base of the food web from which organisms draw their nitrogen and carbon (δ 15 Nbase, δ 13 C base ; baseline, e.g. [START_REF] Rounick | Stable carbon isotopes and carbon flow in ecosystems[END_REF][START_REF] Zohary | Seasonality of stable carbon isotopes within the pelagic food web of Lake Kinneret[END_REF][START_REF] Cabana | Comparison of aquatic food chains using nitrogen isotopes[END_REF][START_REF] Macleod | Effects of light intensity, water velocity, and species composition on carbon and nitrogen stable isotope ratios in periphyton[END_REF][START_REF] Kitchell | Nutrient cycling at the landscape scale: the role of diel foraging migrations by geese at the Bosque del Apache National Wildlife Refuge, New Mexico[END_REF][START_REF] Vander Zanden | Primary consumer δ 13 C and δ 15 N and the trophic position of aquatic consumers[END_REF]. Without suitable estimates of the baseline in each system, there is no way to determine if variation in the baseline of an organism reflects changes in food web structure and carbon flow, or just a variation in the baseline isotopic values. Information on the isotopic baseline is thus required to estimate an organism's trophic position (see paragraph below). These isotopic baselines can be estimated from primary producers such as particulate organic matter (POM) or primary consumers (trophic position TP ~ 2) such as zooplankton (e.g., copepods, [START_REF] Olson | Food-web inferences of stable isotope spatial patterns in copepods and yellowfin tuna in the pelagic eastern Pacific Ocean[END_REF].

What is driving the baseline isotopic variations?

A number of studies have suggested that variations in carbon isotope composition among marine animals may be explained, in part, by differences in the isotope composition of marine plants between nearshore and offshore ecosystems [START_REF] Michener | Stable isotope ratios as tracers in marine aquatic food webs[END_REF][START_REF] Hobson | Preservation of blood and tissue samples for stable-carbon and stable-nitrogen isotope analysis[END_REF]. Factors contributing to nearshore-offshore differences in the isotopic composition of organic carbon at the base of marine food webs include differences in (1) the isotopic composition and concentration of dissolved CO 2 [START_REF] Rau | The relationship between δ 13 C of organic matter and [CO 2 (aq)] in ocean surface water: Data from a JGOFS site in the northeast Atlantic Ocean and a model[END_REF][START_REF] Goericke | Variations of marine plankton δ 13 C with latitude, temperature, and dissolved CO 2 in the world ocean[END_REF][START_REF] Hinga | Carbon isotope fractionation by marine phytoplankton in culture: The effects of CO 2 concentration, pH, temperature, and species[END_REF]), ( 2) the species composition and rate of growth of phytoplankton [START_REF] Fry | Diatom sources of 13 C-rich carbon in marine food webs[END_REF]Pancost et al. 1997), and (3) the input of carbon from benthic macrophytes, which are significantly enriched in 13 C relative to phytoplankton [START_REF] Michener | Stable isotope ratios as tracers in marine aquatic food webs[END_REF]France1995). In areas of coastal upwelling, these factors lead to the production of relatively 13 C enriched organic carbon at the base of the food web. In offshore regions, where nutrients are more limited, phytoplankton growth rates are lower, and macrophytes are absent, organic carbon at the base of the food web is more enriched in 12 C. If nearshore-offshore carbon isotope gradients are generated by the effects of differential growth rates of phytoplankton, then differences of 2-3‰ would be expected. Isotopic gradients supported by high nearshore production of benthic macrophytes could be much larger, perhaps 5-6‰ [START_REF] France | Carbon-13 enrichment in benthic compared to planktonic algae: food web implications[END_REF]. Systematic spatial variations in δ 15 N values at the base of marine food webs can also occur in the oceans due to variations in the dominant dissolved N species present, demand for those nitrogenous nutrients by primary producers and N biogeochemical cycling including N inputs from N 2 fixation and atmospheric deposition and N losses due to denitrification and annamox (e.g., Dugdale andGoering 1967, Altabet 2001;[START_REF] Sigman | Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin[END_REF]. Nitrate, the predominant form of dissolved nitrogen in the open ocean, has an average stable isotope value of approximately 5‰ (e.g., Sigman et al., 2000) (Fig. 1.13). This average value is determined by the interplay of nitrogen fixation on the one and denitrification (here referring to the microbial respiration of nitrate via N 2 O to N 2 ) and anammox (bacterial comproportionation of nitrite and ammonium to N 2 ) on the other side. Most biological turnover processes discriminate the heavier versus the lighter isotope, leading to a slightly faster turnover of light 14 N in comparison to 15 N. Dinitrogen fixation has a small isotope effect [START_REF] Delwiche | Nitrogen isotope fractionation in soils and microbial reactions[END_REF], and forms light organic matter with an isotope value of ~ -2 to +2‰, which, upon complete remineralization and oxidation, lowers the average δ 15 N NO3 value in the open ocean (e.g., [START_REF] Montoya | Nitrogen-fixation and nitrogen isotope abundances in zooplankton of the oligotrophic North Atlantic[END_REF][START_REF] Emeis | Impacts of past climate variability on marine ecosystems: lessons from sediment records[END_REF]. On the other hand, denitrification in anoxic water bodies has a large isotope effect of ~-25 ‰ (e.g., [START_REF] Brandes | Abiotic nitrogen reduction on the early Earth[END_REF][START_REF] Voss | Nitrogen isotope patterns in the oxygen-defficient waters of the Eastern Tropical North Pacific Ocean[END_REF]. To date, little is known about the isotope fractionation of anammox in the water column, but it is generally assumed that anammox, just like denitrification, coincides with considerable, possibly similar, isotope fractionation [START_REF] Naqvi | Coastal versus open-ocean denitrification in the Arabian Sea[END_REF]. The most important implication is locally that denitrification and supposingly anammox, can produce NO 3 and organic matter strongly enriched in 15 N (up to 20‰). These 15 N enrichment in NO 3 -are clearly associated with O 2 minimum zone (OMZ). As expected in regions with significant volumes of suboxic water such as the Eastern Tropical S. Pacific and the Arabian Sea, elevated δ 15 N are found [START_REF] Schäfer | Seasonal variability of δ 15 N in settling particles in the Arabian Sea and its palaeogeochemical significance[END_REF].

These geographical variations in primary producer stable isotope values, or nitrogen (N) and carbon (C) isoscapes, have been shown to propagate up the food web and to be reflected in the isotope value of predators [START_REF] Cherel | Geographical variation in carbon stable isotope signatures of marine predators: a tool to investigate their foraging areas in the Southern Ocean[END_REF][START_REF] Popp | Insight into the Trophic Ecology of Yellowfin Tuna, Thunnus albacares, from Compound-Specific Nitrogen Isotope Analysis of Proteinaceous Amino Acids[END_REF][START_REF] Olson | Food-web inferences of stable isotope spatial patterns in copepods and yellowfin tuna in the pelagic eastern Pacific Ocean[END_REF][START_REF] Graham | Using isoscapes to trace movements and foraging behaviour of top predators in oceanic ecosystems[END_REF][START_REF] Dale | Nursery habitat use and foraging ecology of the brown stingray Dasyatis lata from stomach contents, bulk and amino acid stable isotopes[END_REF]. Marine isoscapes can therefore be used to delineate foraging habitats and to infer movements of marine consumers, and have been increasingly applied for this purpose across diverse predator groups, including seabirds, cephalopods, pinnipeds, rays and tunas (e.g., [START_REF] Lee | Effects of feeding height and distance from protective cover on foraging strategies in birds[END_REF][START_REF] Newsome | A niche for isotopic ecology[END_REF][START_REF] Ménard | Isotopic evidence of distinct feeding ecologies and movement patterns in two migratory predators (yellowfin tuna and swordfish) of the western Indian Ocean[END_REF][START_REF] Olson | Food-web inferences of stable isotope spatial patterns in copepods and yellowfin tuna in the pelagic eastern Pacific Ocean[END_REF][START_REF] Dale | Nursery habitat use and foraging ecology of the brown stingray Dasyatis lata from stomach contents, bulk and amino acid stable isotopes[END_REF][START_REF] Navarro | Trophic niche of squids: Insights from isotopic data in marine systems worldwide[END_REF].

In addition to incorporating the primary producer δ 15 N baseline across their foraging range, predator δ 15 N values are also influenced by their diet and foraging ecology as δ 15 N values increase with each trophic level [START_REF] Post | Using stable isotopes to estimate trophic position: models, methods, and assumptions[END_REF][START_REF] Vanderklift | Sources of variation in consumer-diet δ 15 N enrichment: a meta-analysis[END_REF]. The complexity of regional food webs, which also influences food chain length, can differ and in turn, affect the relative trophic position of the same predator between regions, i.e., the average trophic level, relative to primary producers, at which that organism feeds [START_REF] Vander Zanden | Global patterns of aquatic food chain length[END_REF]Jenning et al. 2008). Accurate and reliable assessment of isotopic baselines is therefore critical to estimating a consumer's trophic position and concomitantly food chain length.

The most direct approach to dissecting the effects of foraging location (e.g., nearshore vs offshore, latitude) and trophic level on the isotopic composition of studied organisms would then be to compare isotope compositions of the organism to those from a proxy of the baseline (i.e., phytoplankton).

Trophic position estimations

Trophic position of a consumer can be calculated using the following equation from [START_REF] Post | Using stable isotopes to estimate trophic position: models, methods, and assumptions[END_REF]:

TP consumer = (δ 15 N consumer -δ 15 N base )/ TEF + TP base
Where TPconsumer is the trophic position of the consumer; δ 15 N consumer is the stable nitrogen signature of the consumer; δ 15 N base is the stable nitrogen signature of the dietary baseline; TEF is the trophic enrichment factor between a consumer and its prey; and, TP baseline is the trophic position of the baseline. Isotopic baseline is generally assessed through isotopic composition of a primary producer (TP = 1 such as POM) or a primary consumer (e.g., mussels or zooplankton, TP = 2). POM does not only reprensent primary producers and can be a mixture of several organisms, a TP of 1.5 is then often assumed instead of 1. Accordingly, mussels or zooplankton are not always strict primary consumers as they can be omnivorous. TP for these organisms vary in publications between 2 and 2.5. The most commonly TEF value used is 3.4 [START_REF] Minagawa | Stepwise enrichment of 15N along food chains: further evidence and the relation between δ15N and animal age[END_REF], but there is substantial variations around this value and other studies for example suggested the use a TEF of 2.4 for marine fish [START_REF] Vanderklift | Sources of variation in consumer-diet δ 15 N enrichment: a meta-analysis[END_REF][START_REF] Olson | Food-web inferences of stable isotope spatial patterns in copepods and yellowfin tuna in the pelagic eastern Pacific Ocean[END_REF].

Specific case of oxygen minimum zones such as in the Humboldt ecosystem

As explained earlier, the Humboldt ecosystem off Peru is characterised by permanent or seasonal upwellings and by a strong OMZ that changes in intensity and depth of occurrence with latitude. Under such conditions, strong denitrification is supposed to occur, leading to high baseline δ 15 N values. For decades, oceanic N-loss has been attributed entirely to heterotrophic denitrification. However, recent studied showed that anammox is the main N-loss process in the Peruvian OMZs (Lam et al. 2 0 0 9 ; K a l v e l a g e et al. 2 0 1 3 ) . T h e n i t r o g e n f r a c t i o n a t i o n o f anammox, is still being investigated but is supposed to be the same as denitrification.

Hence baseline δ 15 N in this region may reflect both, water-column N-loss (denitrification and anammox) increasing the δ 15 N of upwelled NO 3 -, as well as its subsequent partial utilization in surface waters causing δ 15 N in organic matter to be lower than upwelled δ 15 N NO3 -values [START_REF] Mollier-Vogel | Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency[END_REF]. Organic matter δ 15 N is then expected to vary spatially and temporally with a) subsurface OMZ intensity and b) surface gradients in the relative nitrate utilization, respectively.

The intensification of the oxygen deficient condition to the south of Peru south of 8°S [START_REF] Fuenzalida | Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean[END_REF] would then contribute to latitudinal gradients in the δ 15 N baseline values, with highest denitrification and therefore highest δ 15 N values in the south. Indeed, a North to South gradient of δ 15 N values has been observed along the Peruvian margin [START_REF] Mollier-Vogel | Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency[END_REF]. The surface sediment δ 15 N data along the margin can be separated into a northern (1°N to 8°S) and southern (8°S to 18°S) group (Fig. 3a). In the north, sediment δ 15 N values are relatively low and uniform, ranging only between 4 to 5‰. In the southern part, values generally increase southward but reveal a larger range between 4.5 and 13‰.

Furthermore, variations of the size of the continental shelf over the north south gradient can lead to onshore offshore differences in both nitrogen and carbon isotopes.

Given these potential spatial variations in the stable isotopes values of the baseline off Peru, it is therefore essential to assess these variations to correctly interpret stable isotopes variations in the entire food web.

THESIS CHAPTERS

Taking into account this context, the main objective of this study is to better understand the trophic flows in the NHCS using both stomach content and stable isotope analyses. The study focuses on a variety of organisms from low trophic levels such as zooplankton to top predators, in an environment strongly influenced by persistent upwelling and OMZ. This study combines both long-term and specific studies on emblematic species such as anchoveta (Chapters 2 and 3), and sardine (Chapter 4), followed by a more inclusive approach considering the 'global' food web in the recent years (2008 -2012) using stable isotope analysis (Chapter 5), and ends with general conclusion (Chapter 6):

CHAPTER 2

The first step to reach a comprehensive vision of the trophodynamics in the NHCS was to focus on the most important pelagic fish of the system, the anchoveta. For that purpose we analyse 21,203 anchoveta stomach contents sampled during 23 acoustic surveys over the period 1996-2003. We show that whereas phytoplankton largely dominated anchoveta diet in terms of numerical abundance and comprised >99% of ingested prey items, the carbon content of prey items indicated that zooplankton was by far the most important dietary component, with euphausiids contributing 67.5% of dietary carbon followed by copepods (26.3%). Then we use generalized additive models to examine stomach fullness variability relative to the diel cycle, the distance from the coast, sea surface temperature, and latitude. Results show that anchoveta feed mainly during daytime, although night-time feeding also made a substantial contribution to total food consumption. Stomach fullness also varied with latitude, distance from the coast, and temperature, but with substantial variability indicating a high degree of plasticity in anchoveta feeding behaviour. The results suggest an ecological role for anchoveta that challenges current understanding of its position in the foodweb, the functioning and the trophic models of the NHCS (Espinoza and Bertrand, 2008).

CHAPTER 3

To complete the study of anchovy trophodynamics and get new insight on ontogenetic and spatiotemporal variability in anchoveta diet we use available historical SC data since 1954. We show that whatever the period, anchoveta forage mainly on macrozooplankton. The importance of euphausiids in anchoveta diet appears directly related to euphausiids abundance. This bottom-up effect is also observed at smaller scale since the euphausiids fraction increases with anchoveta size and euphausiids accessibility. Selecting the largest prey, the euphausiids, provide an energetic advantage for anchoveta in its ecosystem where oxygen depletion imposes strong metabolic constrain to pelagic fish. Finally this study is another illustration of anchoveta plasticity which allows it to cope with its highly variable environment [START_REF] Espinoza | Ontogenetic and spatiotemporal variability in anchoveta (Engraulis ringens) diet off Peru[END_REF].

CHAPTER 4

After anchoveta, sardine is an important pelagic fish species of the NHCS. During the last decades, sardine Sardinops sagax was abundant from the mid 1970s to the late 1990s; then sardine collapsed in early 2000s. Trophodynamics and prey-size structure can be one of the factors explaining fish population variability (van der Lingen et al., 2009). It is thus important to improve knowledge on sardine diet. For that purpose we describe the diet of sardinetrhough stomach contents in the NHCS, and compare our results with the diet of anchoveta Engraulis ringens off Peru and with the diets of sardines from the southern Benguela (also S. sagax) and the northern Canary (Sardina pilchardus) upwelling systems. The diet of sardine off Peru is based primarily on zooplankton, similar to that observed for anchoveta but with several important differences. Firstly, sardine feed on smaller zooplankton than do anchoveta, with sardine diet consisting of smaller copepods and fewer euphausiids than anchoveta diet. Secondly, whilst phytoplankton represents <2% of sardine dietary carbon, this fraction is dominated by dinoflagellates, whereas diatoms are the dominant phytoplankton consumed by anchoveta. Hence, trophic competition between sardine and anchovy in the northern Humboldt Current system is minimized by their partitioning of the zooplankton food resource based on prey size, as has been reported in other systems. Whereas sardine in the NHCS feed on smaller zooplankton than do anchovy in that system, sardine in the NHCS forage on larger prey and obtain a substantial portion of their dietary carbon from euphausiids compared to sardine from the northern Canary and southern Benguela Current systems (Espinoza et al., 2009).

CHAPTER 5

Finally to obtain a more comprehensive vision of the relative trophic position of NHCS main components, we used stable iotope analyses from zooplankton to top predators in an environment strongly influenced by persistent upwelling and OMZ. For that purpose we analyzed the δ 13 C and δ 15 N stable isotope values of thirteen taxonomic categories collected off Peru from 2008 -2011, i.e., zooplankton, fish, squids and air-breathing top predators. The δ 15 N isotope signature was strongly impacted by the species, the body length and the latitude. Along the Peruvian coast, the OMZ get more intense and shallow south of ~7.5ºS impacting the baseline nitrogen stable isotopes. Also, the sampling design was unbalanced in terms of body length and geographical position. To compare δ 15 N values and being able to estimate the trophic position (TP) of each species we employed a linear mixed-effects modelling approach taking into account the latitudinal and body length effects. We provide a new vision of the relative trophic position of key ecosystem components. Also we confirm previous stomach content-based results on anchoveta Engraulis ringens TP and highlight the potential remarkable importance of an often neglected ecosystem component, the squat lobster Pleuroncodes monodon. Indeed, our results support the hypothesis according to which this species forage to some extent on fish eggs and larvae and can thus predate on the first life stages of exploited species. However, the δ 13 C values of these two species suggest that anchoveta and squat lobster do not exactly share the same habitat, with anchoveta having a larger and more offshore foraging range. This would potentially reduce some direct competition and/or predation.

CHAPTER 6

The manuscript ends with a section of general conclusions where on the base of the results we discuss how these findings allows for an improved vision of the functioning of the NHCS and what are the key next steps. To do so we will use information from studies (van der Lingen et al., 2009;Ayón et al., 2011;Argüelles et al., 2012, Alegre et al., 2013, 2014) related with this dissertation.

Abstract:

The Peruvian anchovy or anchoveta (Engraulis ringens) forages on plankton and is a main prey for marine mammals, seabirds, fish, and fishers, and is therefore a key element of the food web in the Humboldt Current system (HCS). Here, we present results from the analysis of 21,203 anchoveta stomach contents sampled during 23 acoustic surveys over the period 1996-2003. Prey items were identified to the genus level, and the relative dietary importance of different prey was assessed by determination of their carbon content. Variability in stomach fullness was examined relative to the diel cycle, the distance from the coast, sea surface temperature, and latitude, using generalized additive models (GAMs). Whereas phytoplankton largely dominated anchoveta diets in terms of numerical abundance and comprised >99% of ingested prey items, the carbon content of prey items indicated that zooplankton was by far the most important dietary component, with euphausiids contributing 67.5% of dietary carbon followed by copepods (26.3%). Stomach fullness data showed that anchoveta feed mainly during daytime between 07h00 and 18h00, although night-time feeding also made a substantial contribution to total food consumption. Stomach fullness also varied with latitude, distance from the coast, and temperature, but with substantial variability indicating a high degree of plasticity in anchoveta feeding behaviour. The results suggest an ecological role for anchoveta that challenges current understanding of its position in the foodweb, the functioning of the HCS, and trophic models of the HCS.

Key words: Anchoveta Engraulis ringens, Carbon content, Diel feeding behaviour, Trophodynamics, Peru Humboldt Current system, Plankton

Introduction

The Peruvian anchovy or anchoveta Engraulis ringens, is ecologically and economically the most important pelagic fish species in the Humboldt Current system (HCS). Anchoveta is the major prey of the principal top predators including marine mammals, seabirds, fish and fishers, and more than 250 million tons of anchoveta have been harvested by the purse seine fishery since the 1950s. Anchoveta forage on plankton and is a key element of the marine food web in the HCS and have been the subject of many studies (e.g. the books edited by [START_REF] Pauly | The Peruvian anchoveta and its upwelling ecosystem: three decades of change[END_REF]Pauly et al., 1989a).

The first trophodynamic studies on anchoveta in Peru concluded that anchoveta subsisted mainly on phytoplankton [START_REF] Rojas | Estudios preliminarios del contenido estomacal de las anchovetas[END_REF][START_REF] Rojas De Mendiola | The food of the Peruvian anchovy[END_REF], and the ability of clupeoids like anchoveta to feed at low trophic levels (directly on primary producers) was suggested as the reason such large populations, biomasses and fisheries could be sustained in upwelling systems [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF]. Later studies suggested that in addition to filter-feeding on phytoplankton, anchoveta could also particulate feed on zooplankton [START_REF] Rojas De Mendiola | Stomach contents of anchoveta (Engraulis ringens)[END_REF][START_REF] Alamo | Stomach contents of anchoveta (Engraulis ringens), 1974-1982[END_REF], and zooplankton was sometimes considered equally important as phytoplankton in anchoveta diets [START_REF] Alamo | Stomach contents of anchoveta (Engraulis ringens), 1974-1982[END_REF]Pauly et al., 1989b;[START_REF] Jahncke | Trends in carbon flux to seabirds in the Peruvian upwelling system: effects of wind and fisheries on population regulation[END_REF]. With the exception of [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF], who suggested that anchoveta preferentially consume zooplankton, all other recent work in the HCS has concluded that anchoveta depends mainly on phytoplankton (Alamo et al., 1996a(Alamo et al., ,b, 1997a,b;,b;[START_REF] Alamo | Variaciones alimentarias en Engraulis ringens y otros recursos pelágicos durante invierno-primavera de 1997[END_REF]Espinoza et al., 1998aEspinoza et al., ,b, 1999Espinoza et al., , 2000)). However, these studies were based on counts of anchoveta prey, a method considered to be inadequate for estimating dietary importance [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF][START_REF] Konchina | On methods of determining the trophic status of species in ichthyocenes[END_REF].

In contrast, methods based on prey weight (e.g. gravimetric) or on nutritional value (e.g. carbon content, caloric or energetic value) may be more ecologically relevant [START_REF] Hyslop | Stomach contents analysis -a review of methods and their application[END_REF][START_REF] Koslow | Feeding selectivity of schools of northern anchovy, Engraulis mordax, in the southern California[END_REF][START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF][START_REF] Konchina | On methods of determining the trophic status of species in ichthyocenes[END_REF][START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF][START_REF] Van Der Lingen | Chapter 7, Trophic dynamics[END_REF]. In other upwelling systems, these latter methods indicate that zooplankton, rather than phytoplankton, support clupeoid populations (e.g. [START_REF] Koslow | Feeding selectivity of schools of northern anchovy, Engraulis mordax, in the southern California[END_REF][START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF][START_REF] James | A comparison of field based studies on the trophic ecology of Engraulis capensis and E. mordax[END_REF][START_REF] Chiappa-Carrara | Estudio del régimen y hábitos alimentarios de la anchoveta Engraulis mordax Girard (Pises: Engraulidae), en Baja California, Mexico[END_REF][START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF]. [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF] results highlighting the significance of zooplankton in the diet of anchoveta were based on gravimetric analysis of prey importance, but his study was based on a very small sample size (n= 65 fish). Here we revisit Peruvian anchoveta diet and feeding behaviour in Peru using a database which contains information on the stomach contents of 21,203 anchoveta sampled along the Peruvian coast (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003). In particular we assess the relative importance of different prey types to anchoveta using a method which estimates the carbon content of prey items. We also describe variations in anchoveta stomach fullness in relation to the diel cycle, latitude, distance to the coast, and sea surface temperature, using generalized additive models.

Our results confirm [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF] finding that Peruvian anchoveta subsist primarily on zooplankton, and suggest an ecological role f o r a n c h o v e t a t h a t c h a l l e n g e s c u r r e n t understanding of the functioning of the HCS.

Materials and methods

Sampling

Data were collected during 23 IMARPE (Instituto del Mar del Perú) acoustic surveys between 1996 and 2003 with the aim of estimating pelagic fish abundance in the Peruvian EEZ (Table 2.1). Fish were collected by pelagic trawling conducted throughout the survey area (Fig. 2.1), and a sub-sample of 10-50 anchoveta was randomly collected from each trawl. At sea, individual anchoveta were measured (total length) to the nearest 0.5 cm and weighed to the nearest 0.1 g, and the cardiac and pyloric sections of their stomachs were extracted fixed in 5% formalin. In the laboratory, stomach contents were extracted and weighed (wet weight, WW) to the nearest 0.01 g for samples collected between 1996 and the summer of 2001, and to the nearest 0.001 g thereafter. Stomach fullness was calculated by dividing individual stomach content weight by fish wet weight and was expressed as a percentage of fish wet weight [START_REF] Hureau | Biologie compare de quelques poissons antarctiques (Nototheniidae)[END_REF]. The stomach contents of individual fish were then pooled into 2 cm size classes for each sub-sample, and pooled stomach contents were filtered through a 125 μm mesh. The filtered material was diluted to 100 mL using filtered seawater and a 0.1 mL sub-sample was examined under the compound microscope to identify and count phytoplankton prey. The material that remained on the mesh was diluted to 100 mL using filtered seawater and a 10 mL sub-sample was examined using a stereoscopic microscope to identify (to genus) and count zooplankton prey. All prey counts were then standardized to number per 100 mL, except for anchoveta eggs, for which the whole sample was examined.

Estimation of prey volume, dry weight and carbon content

Different procedures were followed to estimate the carbon content of phytoplankton and zooplankton and hence determine their relative dietary importance. We did not measure the size of any prey items but instead used information from the literature to estimate prey size; the lack of available references on size and volume for planktonic organisms off Peru led us to use a wide range of references published for other ecosystems. Phytoplankton size was converted to phytoplankton cell volume using references describing the organism's geometric shape and providing equations relating shape to volume, with the coefficients for each equation being obtained from the literature or from estimations made by IMARPE (Annex 1). To transform volume into carbon content we used equations given in [START_REF] Menden-Deuer | Carbon to volume relationships for dinoflagellates, diatoms, and other protist plankton[END_REF] and [START_REF] Verity | Relationships between lorica volume, carbon, nitrogen, and ATP content of tintinnids in Narragansett Bay[END_REF] (Table 2.3). For zooplankton, prosome or total organism length was estimated from the literature or from some direct measurements made by IMARPE on zooplankton from stomach content samples (Table 2.2). Zooplankton dry weight and carbon content were calculated using equations given by [START_REF] Alexandrov | Caloric content of the Black Sea invertebrates: 1 Zooplankton and meiobenthos[END_REF], [START_REF] Deibel | Feeding mechanism and house of the appendicularian Oikopleura vanhoeffeni[END_REF], [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF], van der Lingen (2002), [START_REF] Parsons | Biological Oceanographic Processes[END_REF] and Sameoto (1971) (Table 2.4). All prey carbon content values were standardized to μg C, and were expressed as a percentage of all fish examined in each cruise.

Data analysis

For each survey from which anchoveta were collected for trophodynamic analysis we sought potential relationships between stomach fullness and the diel cycle, latitude, distance to the coast, and sea surface temperature. As the relationships are likely to be nonlinear and multivariate, a generalized additive modelling (GAM) approach was used (Hastie and Tibshirani, 1990) using S-Plus software (Insightful Corporation, Seattle, WA, USA). Cubic spline smoothers were used to estimate these nonparametric functions, and separate GAM models were performed for each survey, and also for all the surveys combined.

Results

A total of 21,203 anchoveta (E. ringens) ranging from 3 to 18 cm total length were analysed (Table 2.1). In total, 132 prey taxa were identified: 38 diatoms, 16 dinoflagelates, 2 silicoflagelates, 1 phytoflagelate, 4 microflagellates, 9 tintinnids, 34 copepods, and 28 other items (Tables 2.2 and Annex 1). Mean stomach fullness was 0.68% of fish WW, and varied between 0.29% WW in February-March 1999 and 1.23% WW in August-September 1998 (Fig. 2.2).

Dietary composition

Numerically, phytoplankton dominated anchoveta diet and comprised 99.52% of all ingested prey; copepods represented only 0.07%, euphausiids 0.003%, and other prey items accounted for only 0.40% of the total number of prey (Fig. 2.2). This view of anchoveta diet changes dramatically when prey carbon content is considered (Fig. 2.2); zooplankton was by far the most important component and contributed 98.0% of dietary carbon, whilst phytoplankton contributed only 2.0%. Among zooplankters, euphausiids dominated (contributing 67.5% of dietary carbon), followed by copepods (26.3%), and other zooplankton (4.2%). Other groups were much less important in terms of their carbon contribution, including Gastropoda (0.76%), Polychaeta (0.75%), lantern fish (Vinciguerria lucetia and myctophids) (0.45%), anchoveta eggs (0.40%), Amphipoda (0.40%), Bivalvia (0.31%), zoea (0.29%), megalopa (0.29%), and the red shrimp Pleuroncodes monodon ( 0 . 0 1 % ) , b u t t h e i r p r e s e n c e i l l u s t r a t e s t h e o m n i v o r o u s f e e d i n g behaviour of anchoveta. Our results vary depending on the parameters and relationships used to estimate prey carbon content (Tables 2.2, 2.3 and Annex 1). To test the robustness of our results we re-calculated dietary carbon after increasing twofold the relative volume (and thus carbon content) of phytoplankton items and decreasing the relative size of main zooplankton items by a third. Despite these changes our results were similar to those described above; even in the most 'extreme' case where phytoplankton was increased and zooplankton decreased by the maximum amounts the contribution by phytoplankton to anchoveta dietary carbon only reached 6% of the total, which appears to confirm the robustness of our results.

Whereas the relative contribution to anchoveta dietary carbon made by different prey types varied between surveys, no seasonal trend could be identified (Fig.

Stomach fullness dynamics

GAM computed on the combined data from all surveys (Fig. 2.3) show that the main ingestion period for anchoveta occurred during daytime, between 07h00 and 18h00, local time, and this general pattern was observed during most of the surveys, although there were several exceptions (Fig. 2.3). For instance, during four surveys (990809, 000607, 010204, 0208) the feeding period began earlier at about 02h00 and terminated in the early afternoon, whereas during two other surveys (980305, 030809) feeding occurred mainly at night. In some instances (surveys 961112 and 970910) anchoveta showed a longer (>16 h) feeding period with no clear evacuation period.

In addition to showing a diel pattern, stomach fullness also varied with latitude (Fig. 2.3), with higher values encountered off both Northern (<6°S) and Southern (>13°S) Peru although some surveys presented very different patterns. Indeed the global trend does not seem robust and strong variability in the relationship between stomach fullness and latitude occurred during the time series; for example, high stomach fullness values were observed in the central part of Peru in 960204, 960809, 991112 and 000607 (Fig. 2.

3).

Stomach fullness increased with distance from the coast and reached maximal values at distances >120 km (Fig. 2.3), but again this general trend obscures strong variability between surveys. When anchoveta were distributed close to the coast, high fullness levels could be observed close to shore, in particular during the 1997-1998 El Niño, but this was also seen in 1999, 2000 or 2001.

Anchoveta stomach fullness was related to temperature, with a minimum observed at 22°C (Fig. 2.3). However, the range of temperatures varied strongly depending on the timing of a particular survey, suggesting that this general trend is also not robust. In fact, whereas the general trend was observed for 13 surveys it was not observed in six others (960204, 970910, 990809, 020203, 010204, 010708) during which fullness increased with temperature, in two surveys (980305, 0208) where dome-shaped relationships between SST and stomach fullness were observed, in two other surveys (960809, 030203) where U-shaped patterns between SST and stomach fullness were observed, nor in one survey (980809) where a bimodal pattern was observed.

Discussion

Dietary composition

Our analysis of the stomach content composition of 21,203 anchoveta illustrates its omnivorous foraging character; this species feeds on both phytoplankton and zooplankton and has a large diversity of prey (132 taxa were identified at the genus level). As shown by [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF], the size range of anchoveta prey varies by several orders of magnitude, from tens of micrometers (microflagellates) to tens of millimetres (fish, e.g. V. lucetia). When considering only prey numbers, anchoveta diet is dominated by phytoplankton which comprise 99.5% of all prey and the zooplankton fraction appears negligible (Fig. 2.2). However when the carbon content of prey items is considered, zooplankton becomes by far the most important component and contributes 98.0% of dietary carbon, with a strong dominance of euphausiids (67.5%) followed by copepods (26.3%). These results depend on the parameters and relationships used to estimate prey carbon content (Tables 2.2, 2.3 and Annex 1); the analysis seems robust but the exact proportion of specific taxa should be considered with caution.

The phytoplankton carbon fraction was dominated by the large solitary diatoms Coscinodiscus and Ditylum, and the chain forming Thalassiosira. Similar dominance has been reported for anchovy (E. encrasicolus; formerly E. capensis) in the Benguela Current system, where the most important phytoplanktonic items were the large solitary diatoms Rhizosolenia, Pleurosigma and Coscinodiscus ( J a m e s , 1 9 8 7 ) . P r e v i o u s s t u d i e s p e r f o r m e d o f f P e r u a n d b a s e d o n numerical or frequency of occurrence analysis also highlighted the importance of diatoms in anchoveta diet, in particular nine genera of the Coscinodiscineae: Coscinosdiscus, Melosyra, Cyclotella, Skeletonema, Thalassiosira, Stephanopyxis, Coscinoscira, Planktoniella, and Asterolampra [START_REF] Rojas De Mendiola | The food of the Peruvian anchovy[END_REF], 1971, 1989;[START_REF] Alamo | Stomach contents of anchoveta (Engraulis ringens), 1974-1982[END_REF]Pauly et al., 1989b). Other taxa such as the diatoms Chaetoceros a n d Asterionellopsis, and the dinoflagellate Protoperidinium, were also considered important to anchoveta diet (Alamo et al., 1996a(Alamo et al., ,b, 1997a,b;,b;[START_REF] Alamo | Variaciones alimentarias en Engraulis ringens y otros recursos pelágicos durante invierno-primavera de 1997[END_REF]Espinoza et al., 1998aEspinoza et al., ,b, 1999Espinoza et al., , 2000)).

Zooplankton dietary carbon was dominated by euphausiids and copepods. The most important copepods were Eucalanus, Calanus and Centropages, in partial agreement with [START_REF] Rojas De Mendiola | Some observations on the feeding of the Peruvian anchoveta Engraulis ringens J. in two regions of the Peruvian coast[END_REF], 1989) and Pauly et al. (1989b) [START_REF] Blaskovic | Hábitos alimentarios de la anchoveta frente al litoral peruano durante la primavera 1998. Crucero BIC José Olaya Balandra 9811-12[END_REF]Espinoza et al., 1998aEspinoza et al., ,b, 1999Espinoza et al., , 2000)).

In spite of the zooplankton dominance in anchoveta diet described above, the contribution made by different prey items varied strongly during the study period (Fig. 2.2), illustrating the anchoveta's opportunistic feeding ability. For instance, the phytoplankton fraction was high (up to 40% of carbon content) in 1996, but thereafter was never higher than 5%, irrespective of the overall environmental condition (i.e. El Niño 1997-1998or La Niña 1999). It is difficult to relate these changes in the proportion of dietary phytoplankton to specific conditions. The proportion of anchoveta dietary carbon derived from copepods increased from November-December 1996 (2.0%) to a maximum in November-December 1998 (86.7%) and February-March 1999 (77.1%), after the 1997-1998 El Niño; the copepod fraction also reached high levels (>40%) in June-July 2000, October-November 2000 and August 2002, making the extraction of any seasonal pattern difficult.

The anchoveta: a predator

Our results demonstrate that anchoveta is a predator foraging at a higher trophic level than has generally been recognised (e.g. by Pauly et al., 1989b;[START_REF] Rojas De Mendiola | Stomach contents of anchoveta (Engraulis ringens)[END_REF], with its main input of carbon coming from zooplankters such as euphausiids and large copepods. Our results differ from the conventional dogma on clupeoid feeding ecology in the HCS, in which diatoms are considered as the major food source. Such feeding low on the food chain has been invoked to explain how such large anchoveta populations can be supported in this system [START_REF] Rojas De Mendiola | Some observations on the feeding of the Peruvian anchoveta Engraulis ringens J. in two regions of the Peruvian coast[END_REF]. These conclusions now appear incorrect as they are based on counts of very small (phytoplankton) prey who's caloric (carbon) contribution to anchoveta diet is often trivial [START_REF] Cushing | Upper trophic levels in upwelling areas[END_REF][START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF]. [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF] demonstrated that assessing fish diet from the number of prey items is unsatisfactory due to the preponderance of minute food items and the great difference on size range between phytoplankton and zooplankton (10 -3 mm for diatoms to 10 mm for zooplankton), which makes the use of carbon or calorie equivalents for assessing the importance of dietary components essential. The determination of prey carbon content helps in determining the true importance of a food type [START_REF] James | Are clupeid microphagists herbivorous or omnivorous? A review of the diets of some commercially important clupeids[END_REF], and this method has been used to highlight the importance of zooplankton over phytoplankton for E. mordax off California [START_REF] Koslow | Feeding selectivity of schools of northern anchovy, Engraulis mordax, in the southern California[END_REF][START_REF] Chiappa-Carrara | Estudio del régimen y hábitos alimentarios de la anchoveta Engraulis mordax Girard (Pises: Engraulidae), en Baja California, Mexico[END_REF]), E. encrasicolus and Sardinops sagax in the Benguela Current upwelling system [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF][START_REF] Van Der Lingen | Diet of sardine Sardinops sagax in the southern Benguela upwelling ecosystem[END_REF][START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF], and Sardina pilchardus in the north-eastern Atlantic [START_REF] Garrido | Diet and feeding intensity of sardine Sardina pilchardus: correlation with satellitederived chlorophyll data[END_REF]; see van der Lingen et al. (2006,2009), for recent reviews of clupeoid trophic ecology. In the Peruvian case, [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF] was the only scientist to conclude that anchoveta preferentially consumes zooplankton, and that most anchoveta biomass is supported by copepods and euphausiids. Finally, in Central Chile, using isotope analysis, [START_REF] Hückstädt | Stable isotope analysis reveals pelagic foraging by the Southern sea lion in central Chile[END_REF] estimated the trophic level of anchoveta to 3.63, which confirm the dominance of zooplankton in anchoveta diet.

Anchoveta is therefore a 'predator' who select the largest available prey from the phytoplankton and zooplankton communities (Tables 2.2, Annexes 1 and 2). This is shown for zooplankton with the selection of euphausiids and the large copepods Eucalanus and Calanus, and for phytoplankton with the selection of the large diatom Coscinodiscus. Similar selectivity was observed for other anchovy species (e.g. [START_REF] Koslow | Feeding selectivity of schools of northern anchovy, Engraulis mordax, in the southern California[END_REF][START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF][START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF].

Stomach fullness dynamics

As previously observed for E. encrasicolus [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF], E. ringens displays an extremely high degree of opportunism in fulfilling its dietary requirements and can shift between prey types according to their availability (Fig. 2.2). This flexible feeding behavior was confirmed by the GAM models (Fig. 2.3) which illustrated the high variability between surveys when relating stomach fullness to time, latitude, distance to the coast, or SST. In the highly variable HCS anchoveta have had to evolve adaptive strategies in space and time (Bertrand et al., 2004a), including its spawning behaviour (e.g. [START_REF] Buitrón | Aspectos reproductivos de la anchoveta peruana durante el período 1992-2000[END_REF], its mode of spatial distribution (e.g. Bertrand et al., 2004bBertrand et al., , 2008a;;[START_REF] Gutiérrez | Anchovy (Engraulis ringens) and sardine (Sardinops sagax) spatial dynamics and aggregation patterns in the Humboldt Current ecosystem, Peru, from 1983-2003[END_REF], or its feeding behaviour as illustrated by this study.

Such variability precludes the determination of robust patterns, and indeed results differ strongly between surveys and therefore from one dataset to another. Our study was based on more than 20,000 stomach content samples collected during an 8 year period which is, to our knowledge, the largest database on anchoveta diet, and the results we obtained from all surveys combined (Fig. 2.3) can therefore be considered as representative. However, except in the case of their diel feeding behaviour, we think that the mean signal in anchoveta stomach fullness is not a good representation of the biological reality at any one moment. Instead, we consider that the most important point arising from our stomach fullness analysis does not reside in the mean patterns but in the variability from one survey to the other (Fig. 2.3). This variability is another illustration of anchoveta plasticity.

The general trend of diel temporal variation in stomach fullness was the most consistent, and the main feeding period as determined using GAMs ranged between 07h00 and 18h00 (Fig. 2.3). This general pattern was observed for most surveys, although night-time feeding behaviour made substantial contributions to total ingestion (moon cycle could play a role).

Feeding duration also varied from one survey to another, as also shown by [START_REF] Espinoza | Cambios en la dieta de la anchoveta Engraulis ringens y su influencia en la dinámica de alimentación[END_REF]. The primarily diurnal feeding behaviour we observed differs from results of other studies of anchoveta feeding periodicity based on a large database (5245 individuals) from samples collected over the period [START_REF] Rojas | Estudios preliminarios del contenido estomacal de las anchovetas[END_REF]-1982(Pauly et al., 1989b;;[START_REF] Jarre | Two approaches for modelling fish stock interactions in the Peruvian upwelling ecosystem[END_REF], which indicated that most feeding occurred between 11h00 and 24h00. This difference cannot be related to the different methods of analysis used i.e., GAM in this study and Sainsbury's method [START_REF] Sainsbury | Estimation of food consumption from field observation of fish feeding cycles[END_REF] in previous studies, since applying Sainsbury's method to our data does not change the results. So why did the main feeding period change? At least two hypotheses can be proposed to explain this difference: a shift in the ecosystem, or a problem of data collection. Anchovies in other upwelling systems are considered as daytime (e.g. northern anchovy E. mordax; [START_REF] Baxter | Summary of biological information on the northern anchovy Engraulis mordax Girard[END_REF][START_REF] Loukashkin | On the diet and feeding behaviour of the northern anchovy, Engraulis mordax (Girard)[END_REF][START_REF] Koslow | Feeding selectivity of schools of northern anchovy, Engraulis mordax, in the southern California[END_REF] or night-time (e.g. Benguela anchovy E. encrasicolus; James 1988) foragers. At first sight the main daytime trend we found appears surprising since zooplankton perform diel migrations and a large fraction of zooplankton is generally distributed within or below the oxycline during the day, out of reach of the anchoveta (e.g. Bertrand et al., 2008a;[START_REF] Escribano | Zooplankton associated with the oxygen minimum zone system in the northern upwelling region o f C h i l e d u r i n g M a r c h 2 0 0 0[END_REF]. However our results demonstrate that anchoveta can change their feeding period and forage at night if necessary. The HCS experienced 'cold' and 'warm' years during the period (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) of our study with a dominance of the upwelling-related cold coastal water along the Peruvian coast, except during the 1997-1998 El Niño (Swartzman et al., 2008). That anchoveta fed mainly during the day suggests that, even with the highly stratified conditions typical of enhanced upwelling periods, there were enough prey available during the day to sustain anchoveta. It seems therefore difficult to ascribe the difference between our results and previous studies to changes in climatic conditions. The period 1953-1982 that was previously studied included a wide range of climatic conditions: 'cold' until the beginning of the 1970s, and 'warm' from then until the mid 1980s. Hence we feel that it is more likely that the differences in anchoveta mean feeding periodicity between our and earlier studies was due to potential problems in the data collection (e.g. data sources varied from scientific cruises to fishing boats) for the 1953-1982 series or by the fact that the mean pattern hides important variability from one period to the other. Indeed a refined analysis of the data set used in previous studies revealed a pattern of several feeding periods per day, including some feeding during late night (Jarre- [Teichmann], 1992).

The plasticity in anchoveta foraging periodicity was even more obvious when considering the distance to the coast, latitude or SST, where mean patterns were not representative of the observations from a single survey. Inside its range of overall viable conditions anchoveta is therefore able to forage efficiently at any time, any place, or any temperature. In other words, inside its cold coastal water-related habitat (see Bertrand et al., 2004a;[START_REF] Gutiérrez | Anchovy (Engraulis ringens) and sardine (Sardinops sagax) spatial dynamics and aggregation patterns in the Humboldt Current ecosystem, Peru, from 1983-2003[END_REF]Bertrand et al., 2008b;[START_REF] Swartzman | The relationship of anchovy and sardine to water masses in the Peruvian Humboldt Current System from 1983-2005[END_REF], anchoveta were probably distributed where prey were abundant and available, rather independently of other parameters (Bertrand et al., 2008a).

A new vision of HCS functioning

If anchoveta is a predator that uses large zooplankters as the main source of dietary carbon, then our vision of HCS functioning must change. The HCS is by far the most productive eastern boundary current system in the world in terms of fish catches, but these high catches do not appear to be supported by exceptionally rates of primary production far higher than those observed in the other eastern boundary current systems [START_REF] Carr | Estimation of potential productivity in Eastern Boundary Currents using remote sensing[END_REF][START_REF] Carr | Production regimes in four Eastern Boundary Current systems[END_REF].

The high fish production in the HCS could be related to a more efficient use of primary production due to, for example, a short trophic chain, and this assumption has been prevalent in the last few decades since anchoveta was considered to feed directly on primary producers and hence rely on a complete very short and efficient food chain [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF][START_REF] Walsh | A carbon budget for overfishing off Peru[END_REF]. But if this is not the case, and anchoveta do not, in fact, benefit from a 2-step food chain, then the explanation for the high fish productivity in the HCS must be found elsewhere. [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF] stated that the ability of anchoveta to utilize food from several trophic levels and to choose energetically advantageous food enables it to achieve enormous biomass in a relatively short time. Yet all trophic models of the HCS developed to date, even recently, still consider that the anchoveta consumes at least as much carbon from phytoplankton as from zooplankton (e.g. [START_REF] Jahncke | Trends in carbon flux to seabirds in the Peruvian upwelling system: effects of wind and fisheries on population regulation[END_REF]Neira et al., 2004). Our results indicate that carbon flows in trophic models of the HCS must be re-evaluated, since the tremendous importance of anchoveta in the coastal pelagic ecosystem means that a major change in its diet should affect all the other components of that system. In particular, the fact that anchoveta forages at a higher trophic level than previously been considered implies that the primary and secondary production of the HCS may be higher than supposed (for primary production see [START_REF] Echevin | The seasonal cycle of surface chlorophyll in the Peru upwelling system: a modelling study[END_REF]. No absolute estimation of zooplankton production is available (see [START_REF] Ayón | Zooplankton volume trends off Peru between 1964 and 2001[END_REF]Ayón et al., , 2008b for trends in zooplankton biovolume) but we hypothesize that zooplankton production could be higher in the HCS than in other upwelling systems, and this higher secondary production could be related to a more efficient use of primary production by zooplankton and/or a strong connection between the coastal and the offshore pelagic ecosystems. This last argument may particularly apply to the euphausiids, which were the main source of dietary carbon for anchoveta and which inhabit both near-and offshore ecosystems.

The hypothesis of higher zooplankton biomass in the HCS compared to other systems highlights the urgent need for more studies on zooplankton diversity, abundance and dynamics in the HCS, particularly for large zooplankters like euphausiids [START_REF] Antezana | Adaptive behavior of Euphausia mucronata in relation to the oxygen minimum layer of the Humboldt Current[END_REF]. Studying these organisms is difficult, however, as they avoid standard samplers such as bongo nets, but new, multi-frequency acoustic measurements permit estimation of the biomass of the main zooplankton groups and facilitate the study of their distribution patterns in 2-D and 3-D (e.g. [START_REF] Lebourges-Dhaussy | Acoustic backscatter from zooplankton and fish explored through an optimised model framework[END_REF].

Our results also bring into question current thinking on anchoveta distribution. Because of the westward drift of water masses in the HCS there is usually a spatial separation between areas of peak phytoplankton abundance and areas of peak zooplankton abundance. Hence the relationship between anchoveta distribution and chlorophyll concentration, as observed from satellite, is not necessarily direct, and it is therefore important to take into account the characteristics of the available habitat, including zooplankton abundance, to understand anchoveta distribution patterns and also changes in abundance (cf. the habitat-based hypothesis in Bertrand et al., 2004a). It is not sufficient to use temperature or chlorophyll concentration as indicators of anchoveta distribution, since the plasticity of the genus Engraulis in general [START_REF] Bakun | Centro de Investigaciones biológicas del Nordeste, La Paz, Mexico and[END_REF] and E. ringens in particular allows it to feed successfully and distribute across a broad range of environmental variability. The dependence of anchoveta on zooplankton has previously been illustrated by [START_REF] Alheit | Regime shifts in the Humboldt Current ecosystem[END_REF] and Ayón et al. (2008b), who showed bottom-up control of anchoveta where its abundance in the HCS was linked to overall zooplankton abundance. At a smaller scale, Ayón et al. (2008b) also showed that this apparent bottom-up control was accompanied by a local depletion effect, as zooplankton biomass was lower in places were anchoveta was abundant.

Synthesis: plastic is fantastic!

Our results have demonstrated that the diet of anchoveta in Peru is based primarily on large zooplankters, but also that this species shows tremendous plasticity in its diet and feeding behaviour; anchoveta utilize food from several trophic levels, can choose energetically advantageous food types, and can fit its foraging period and duration to prey. Trophic plasticity is apparently an evolved adaptive strategy; other strategies include the ability to track and concentrate in refuge areas when conditions are adverse (Bertrand et al., 2004a), change its reproductive behaviour [START_REF] Buitrón | Aspectos reproductivos de la anchoveta peruana durante el período 1992-2000[END_REF], and distribute its population over a rather large temperature range (Bertrand et al., 2004a;[START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF]. In combination these characteristics may explain the 'anchoveta paradox': how a fish which (i) performs very small migrations and cannot escape adverse conditions, (ii) is mainly distributed in dense surface aggregations and is thus highly accessible to predators (fish, cephalopods, birds, mammals and fishers), and (iii) is very slow in its avoidance reactions to predators [START_REF] Gerlotto | Waves of agitation inside anchovy schools observed with multibeam sonar: a way to t r a n s m i t i n f o r m a t i o n i n r e s p o n s e t o predation[END_REF], can achieve such enormous biomass in a relatively short time.

Introduction

The Peruvian anchovy or anchoveta, Engraulis ringens Jenyns 1842, is the dominant pelagic nekton species in the coastal Northern Humboldt Current system (NHCS) off Peru. This fish has great ecological importance in channelling energy flow from primary and secondary production to higher trophic levels (Pauly et al., 1989b). During decades E. ringens was considered to depend mainly on phytoplankton (e.g., [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF][START_REF] Rojas De Mendiola | Stomach contents of anchoveta (Engraulis ringens)[END_REF][START_REF] Alamo | Variaciones alimentarias en Engraulis ringens y otros recursos pelágicos durante invierno-primavera de 1997[END_REF]. However, these studies were based on counts of E. ringens prey, a method considered to be inadequate for estimating dietary importance [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF][START_REF] Konchina | On methods of determining the trophic status of species in ichthyocenes[END_REF]. By applying an analytical method based on the estimation of prey carbon content, Espinoza & Bertrand (2008) showed that E. ringens f o r a g e m a i n l y o n macrozooplankton, in particular euphausiids and large copepods. Such trophic behaviour is a general pattern for Engraulis spp. [START_REF] Van Der Lingen | Chapter 7, Trophic dynamics[END_REF].

However some doubts subsisted about a potential temporal shift in Engraulis ringens diet towards more or less zooplankton. Indeed, E. ringens population was assumed to be dominated by individuals with higher affinity for zooplankton (shorter guts and less gillrakers) during the 1950s and 1960s than after the E. ringens collapse in 1972 [START_REF] Palomares | The Peruvian anchoveta and its upwelling ecosystem: three decades of change[END_REF][START_REF] Hutchings | Fish harvesting in a variable, productive environment -searching for rules or searching for exceptions?[END_REF]). An opposite change could have occurred in the last decades explaining the strong dominance of zooplankton in E. ringens diet observed by Espinoza & Bertrand (2008). To check for this we revisited available historical trophic diet data since 1954.

Then, to complete previous works, we describe the feeding behaviour of E. ringens according to fish size (ontogenetic), spatial (latitudinal and cross-shore), and diel patterns during 1996-2003, period with available fully spatialised data. We show that in this ecosystem characterised by a strong productivity but oxygen paucity, E. ringens diet seems mainly driven by the availability of large prey, in particular euphausiids.

Material and methods

Data

The study area covers the Peruvian coastal waters from the coastline to approximately 200 km offshore. In Peru, trophic data acquisition and sample analyses were strongly modified along time. In order to study both long term changes and spatial variability we had to use available data bases according to their characteristics. [START_REF] Rojas De Mendiola | Some observations on the feeding of the Peruvian anchoveta Engraulis ringens J. in two regions of the Peruvian coast[END_REF]. Individuals of E. ringens were randomly sampled and fixed in 10% formalin. Stomach contents items were identified and counted, taking as a unit for zooplankton at least the head and the whole cell for phytoplankton. We converted prey data count from [START_REF] Rojas De Mendiola | Some observations on the feeding of the Peruvian anchoveta Engraulis ringens J. in two regions of the Peruvian coast[END_REF] into carbon content by applied the methodology described in Espinoza & Bertrand (2008). We then pooled prey data expressed in carbon content into the following six taxonomic categories: (i) diatoms; (ii) other phytoplankters, (iii) other copepods, (iv) calanoid copepods, (v) euphausiids, and (vi) other zooplankters. Unfortunately, these data were not fully spatialised.

For the period 1974 -1988 we retrieved data from the so-called EUREKA operations; these employ about 20 fishing boats for a quasi-synoptic observation of the Peruvian coast and contributed to fish resource monitoring. The specific determination of trophic items was coarse and did not allow for a comprehensive diet analysis. However we could estimate the frequency of occurrence of euphausiids in stomach content. The same information was obtained from samples collected during scientific surveys or in factory plants in [START_REF] Crowder | Optimal foraging and feeding mode shifts in fishes[END_REF], 1988-1990, 1994, 1995and 2004-2009 (Annex 3.1) (Annex 3.1).

Data for the period 1996-2003 were collected on board research vessels of the Peruvian Sea Institute (IMARPE) during 22 acoustic surveys (Table 3.1). The samples and the methodology used for stomach content analysis and conversion to carbon were described in Espinoza & Bertrand (2008). A total of 13,430 fully georeferenced stomachs of Engraulis ringens were used.

The prey data was pooled into the same six taxonomic categories than for the period 1954-1969. We also pooled the data according to the following factors: size class, latitude, distance to the shelf break, and diel period (Table 3.2). Seasonal variability was not considered since no significant effects were depicted (Espinoza & Bertrand, 2008).

Analyses

From these different data sets and taking into account their characteristics we performed three kinds of analyses. First, we compared Engraulis ringens diet between 1954-1969 and 1996-2003. We did not enter in interannual variability because the first data set was based on only few samples of some years while the second was already described by Espinoza & Bertrand et al. (2008). Second, we constructed a time series of the frequency of occurrence of euphausiids between 1954 and 2009. Third, the fully spacialised data set from 1996 to 2003 allowed studying ontogenetic and spatial variability in E. ringens diet. Given that ANOVA assumptions for the data were not met (lack of normality), the non-parametric Kruskal-Wallis (KW) test [START_REF] Scherrer | Biostatistique[END_REF] was used to identify statistical differences in the carbon content-based contribution of different prey taxa according to the following factors: size-class, latitude, distance to the shelf break, and diel period. A Tukey-type test was used for a posteriori multiple comparisons [START_REF] Zar | Biostatistical Analysis[END_REF]. Finally, classification and regression trees (CART, [START_REF] Breiman | Classification and Regression Trees[END_REF] were used to assess the relative importance of size, latitude, distance to the shelf break, and diel period on E. ringens diet. This non-parametric method uses a partitioning algorithm to estimate a series of binary decision rules that divide the data into smaller homogeneous subgroups in an optimal way. The whole dataset is represented by a single node at the top of the tree. Then the tree is built by repeatedly splitting the data. Each split is defined by a simple rule based on a single explanatory variable. Splits are chosen to maximize the homogeneity of the resulting two nodes. Each terminal node (or leaf) of the final tree is characterized by a predicted prey distribution (carbon proportion for the 6 taxonomic groups), given the explanatory variables. For pruning the tree and validating the robustness of the CART results we used the recursive partitioning method (rpart; [START_REF] De'ath | Classification and regression trees: a powerful yet simple technique for ecological data analysis[END_REF]. Distance to the shelf break (km) Negative inshore, positive offshore: <-30, -30 to 0, 0 to 30, >30

Diel period (i) 07:00≤day<17:00; (ii) 17:00≤dusk<19:00 (iii) 19:00≤night<05:00 (iv) 05:00≤dawn<07:00

Results

Temporal variation

Only two periods (1954-1969 and 1996-2003) encompassed precise description of the Engraulis ringens diet. During 1954-1969, the diet of E. ringens was dominated by zooplankton; the total contribution of phytoplankton representing only 0.8% of the diet expressed in carbon (Fig. 3.1). Euphausiids contribution was higher than 92.3% so more than during 1996-2003 when the mean euphausiids contribution was 67.5%. Copepods fraction was also lower during 1954-1969 (6.3%) than during 1996-2003 (26.3%).

The frequency of occurrence of euphausiids varied along time (Fig. 3.2). Even if the time series is incomplete, a pattern emerges with less euphausiids during the mid 1970s-1980s (available data: 1974-1979, 1985, 1988, 1989) and maybe during the 1950s. In this last case the number of data (available data: 1954-1955) is insufficient to provide robust results. On the opposite euphausiids were more frequent during the late 1960s-early 1970 and the late 1990s -early 2000s. This pattern is rather similar to what was observed by Ayón et al. (2011) who studied the temporal changes in zooplankton dominant size group (between small, medium and large zooplankton, and euphausiids) in net samples. Indeed, euphausiids dominance in zooplankton from Ayón et al. (2011) matches the frequency of occurrence of euphausiids in E. ringens diet (Fig. 3.2), except during the mid 2000s when euphausiids dominance increases while the frequency of occurrence in stomach decreased.

Ontogenetic variation

Across all size classes the contribution of zooplankton to Engraulis ringens diet in terms of carbon content was greater or equal to 95 % (Fig. 3.3a). For all size-classes the most important prey groups were euphausiids and calanoid copepods, but their relative importance varied significantly with E. ringens size (KW test, p<0.001; Table 3.3, Annexes 3.2, 3.3, 3.4). The most noticeable pattern was the steady increase in euphausiids contribution with E. ringens size (Fig. 3.3a). Euphausiids represented 23.3% of the diet of the smallest studied E. ringens (4-6 cm) but reached 84.9% for the largest (18-20 cm). At the same time the proportional contribution of calanoid copepods markedly decreased from 68% in the smallest size class of E. ringens to 11.5% in the largest. The proportion of other zooplankters also diminished with increasing size, but not as regularly, from 8.8% in the smallest to 2% in the largest E. ringens. The contribution of other phytoplankters remained below 0.6%, and other copepods under 2.5% without clear trend at each size class. For diatoms, its contribution increased from 0.1% in the smallest sizeclass to 4.4% for the 10-12 cm size-class, then diminished to <1% in the larger size-classes.

Latitudinal variation

The contribution of zooplankton to Engraulis ringens d i e t w a s g r e a t e r o r e q u a l t o 9 5 % irrespective of latitude (Fig. 3.3b). Euphausiids and calanoid copepods, varied significantly with latitude (KW test, p<0.001, Table 3.3, Annexes 3.5, 3.6, 3.7) but no steady latitudinal trend was observed. Euphausiids were the main taxa with the exception of around latitudes 7ºS and 15ºS where the most important prey were calanoid copepods which represented 52.3 and 47.5%, respectively. The higher contributions of other zooplankters were observed close to Chicama (7ºS) and south of San Juan (16ºS) with 7.3 and 8.7%, respectively. The contribution of diatoms fluctuated between 0.04 % in the extreme north to a maximum of 4.4% at 06ºS with no apparent latitudinal trend (Fig. 3.3b).

Cross-shore variation

Euphausiids contribution varied significantly according to distance to the shelf break (Fig. 3.3c, KW test, p<0.001, Table 3.3, Annexes 3.8, 3.9, 3.10). It steadily increased from 45% in the more inshore samples to reach 80% at more than 30 km from the shelf break. Contribution by calanoid copepods followed an opposite trend with a highest contribution (46%) in the more inshore samples then a decrease to 15% at more than 30 km from the shelf break. The contribution of other zooplankters was highest in the more inshore areas (7.6%) and varied between 2.0 and 4.1 further offshore. Finally, the collective contribution of diatoms, other copepods, and other phytoplankters were marginal with <1.3% of carbon content in all cases.

Diel variation

Engraulis ringens diet (Fig. 3.3d) showed significant differences with the diel period (KW test, p<0.001, Table 3.3, Annexes 3.11, 3.12, 3.13). During the night, the diet was dominated by euphausiids (80.2%). This proportion decreased to 48.8% at dawn and 52.7% during the day, and reached a minimum of 39.8% at dusk. Calanoid copepods followed an opposite trend with 15.7% during the night, 35.3% at dawn, 40.9% during the day and 46.5% at dusk.

CART analysis

Results of the CART (Fig. 3.4) confirmed the above described but permitted to rank the effects this way: distance to the shelf break > diel period > latitude > Size > Year. When looking at the tree (Fig. 3.4) the first cut is due the distance to the shelf break with more copepods very close to the coast. Further to the coast, the second cut was related to fish size with more euphausiids in fish larger than 14 cm. For fish less than 14 cm, copepods were more important at specific latitude (4ºS, 6ºS, 7ºS, 8ºS, 10ºS, 11ºS, 12ºS and 15ºS), in particular during all diel periods but the night.

Discussion

Temporal variation

The quality of available data strongly varied with time. However the use of different data sources allows for a better vision of the temporal changes in Engraulis ringens diet. Results complete the one from Espinoza & Bertrand (2008) and show that the diet of E. ringens in Peru is based primarily on large zooplankters whatever the period (Fig. 3.1). Euphausiids contribution was even higher during the 1950-1960s (92.6%) than during the late 1990-early 2000s (67.5%) while copepods presented an opposite trend (6.3% and 26.3%). Euphausiids frequency of occurrence time series shows (Fig. 3.2) that euphausiids incidence decreased during the mid 1970-1980s. This trend is in agreement with the study of temporal changes in zooplankton sizegroup dominance by Ayón et al. (2011) who showed that euphausiids were strongly dominant during the late 1960 early 1970 (the 1950s were not included in their analysis) and since the 1990s.

Ontogenetic changes

Both, juvenile and adult Engraulis ringens w e r e l a r g e l y z o o p l a n k t i v o r o u s a n d f o r a g e d predominantly on euphausiids and calanoid copepods, while contribution of phytoplankton to dietary carbon was ≤4.4% irrespective of size-class. This is in contrast to that of [START_REF] Rojas De Mendiola | The food of the Peruvian anchovy[END_REF] and Rojas de Mendiola et al. (1969), who applied the numerical method and proposed that E. ringens become more phytoplanktivorous from juvenile to adult. However, prey-size range changed ontogenetically with a steady increase in euphausiids contribution from >25% in the smallest E. ringens (4-6 cm) to >80% in the largest size class (18-20 cm). For Engraulis spp., filter-feeding has a high metabolic cost relative to particulate feeding [START_REF] James | The relationship between respiration rate, swimming speed and feeding behaviour in the Cape anchovy Engraulis capensis Gilchrist[END_REF][START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF] and E. ringens select the largest available prey (Espinoza & Bertrand, 2008). Energetically oxygen does matter. Indeed, fish need sufficient amounts of both food and oxygen, but the latter might be more difficult to obtain than the former [START_REF] Pauly | Gasping fish and panting squids: oxygen, temperature and the growth of waterbreathing animals[END_REF]. This is particularly the case in the NHCS which encompasses an intense and shallow oxygen minimum zone and where, close to the coast, even surface waters are undersaturated in oxygen [START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF]. In this system oxygen appears as an important driving factor of small pelagic fish populations and the availability of large prey such as euphausiids provides an energetic advantage to E ringens compared with sardine Sardinop sagax Jenyns, 1842 [START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF]. Our study supports this contention because E. ringens appear to select the largest available prey they can capture. Indeed, the progressive change in diet composition with fish size can be attributed to the predator's ability to capture and handle the prey, which is reflected by the size-range of prey it can capture [START_REF] Crowder | Optimal foraging and feeding mode shifts in fishes[END_REF][START_REF] Christensen | Predator foraging capabilities and prey antipredator behaviours: preversus postcapture constraints on size-dependent predator-prey interactions[END_REF][START_REF] Lundvall | Size-dependent predation in piscivores: interactions between predator foraging and prey avoidance abilities[END_REF]. The upper size limit of prey is set by physical constraints of the predator, in particular mouth gape [START_REF] Hart | Pike as a selective predator. Effects of prey size, availability, cover and pike jaw dimensions[END_REF], swimming capacity [START_REF] Christensen | Predator foraging capabilities and prey antipredator behaviours: preversus postcapture constraints on size-dependent predator-prey interactions[END_REF][START_REF] Lundvall | Size-dependent predation in piscivores: interactions between predator foraging and prey avoidance abilities[END_REF], and its visual acuity and reaction distance to prey which also increases with increasing predator size [START_REF] Breck | Effect of fish size on the reactive distance of bluegill (Lepomis macrochirus) sunfish[END_REF]. During ontogenetic shifts in size, E. ringens have thus greater access to larger and more energetic prey such as euphausiids.

Such ontogenetic changes in diet were observed in the other species of anchovies: Engraulis mordax Girard 1854 in California [START_REF] Leong | A laboratory study of particulate and filter feeding of the northern anchovy (Engraulis mordax)[END_REF][START_REF] O'connell | The interrelation of biting and filtering in the feeding activity of the northern anchovy (Engraulis mordax)[END_REF], Engraulis encrasicolus L. 1758 in South Africa [START_REF] James | Effect of particle size and concentration on feeding behaviour, selectivity and rates of food ingestion by Cape anchovy Engraulis capensis[END_REF], the Northwest Mediterranean Sea [START_REF] Tudela | Trophic ecology of the European anchovy Engraulis encrasicolus in the Catalan Sea[END_REF][START_REF] Plounevez | Diet, feeding behaviour and trophic activity of the anchovy (Engraulis encrasicolus L.) in the Gulf of Lions (Mediterranean Sea)[END_REF] and the Bay of Biscay (Plounevez & Champalbert, 1999), and Engraulis anchoita Hubbs & Marini 1935 in Argentine [START_REF] Ciechomski | Investigations of food and feeding habits of larvae and juveniles of the Argentine anchovy Engraulis anchoita[END_REF][START_REF] Viñas | Gut analysis o f f i r s t -f e e d i n g a n c h o v y l a r v a e f r o m t h e Patagonian spawning areas in relation to food availability[END_REF].

This ability to adapting at different conditions was defined like dietary flexibility [START_REF] James | Effect of particle size and concentration on feeding behaviour, selectivity and rates of food ingestion by Cape anchovy Engraulis capensis[END_REF]. The above mentioned can be expressed in the foraging theory framework like a way to optimize the prey intake which provide high energy at low energetic -oxygen -cost and in turn can be digested and avoid the intra and interspecific competition [START_REF] Griffiths | Prey availability and the food of predators[END_REF][START_REF] Pyke | Optimal foraging theory: a critical review[END_REF][START_REF] Werner | The ontogenetic niche and species interactions in sizestructured populations[END_REF]. The calanoid copepods size ingested by Engraulis ringens fluctuated between ~0.65 and 4.0 mm while the mean size of the euphausiids was estimated in 12 mm (Espinoza & Bertrand, 2008). These differences in prey size imply that E. ringens display two feeding strategies: filtering on the smaller copepods and biting on the bigger prey. Such behaviour has been described in Engraulis mordax in California [START_REF] Leong | A laboratory study of particulate and filter feeding of the northern anchovy (Engraulis mordax)[END_REF][START_REF] O'connell | The interrelation of biting and filtering in the feeding activity of the northern anchovy (Engraulis mordax)[END_REF], Engraulis encrasicolus L. 1758 (formerly known as Engraulis capensis see [START_REF] Grant | Shallow population histories in deep evolutionary linkages of marine fishes: Insights from sardines and anchovies and lessons for conservation[END_REF] off South Africa ( J a m e s & F i n d l a y , 1 9 8 9 ) , t h e N o r t h w e s t Mediterranean Sea [START_REF] Tudela | Trophic ecology of the European anchovy Engraulis encrasicolus in the Catalan Sea[END_REF][START_REF] Plounevez | Diet, feeding behaviour and trophic activity of the anchovy (Engraulis encrasicolus L.) in the Gulf of Lions (Mediterranean Sea)[END_REF] and the Bay of Biscay (Plounevez & Champalbert, 1999). The threshold to shift from filtering to biting was estimated to 0.7 mm for Engraulis encrasicolus [START_REF] James | Effect of particle size and concentration on feeding behaviour, selectivity and rates of food ingestion by Cape anchovy Engraulis capensis[END_REF]) and 1.5 mm for Engraulis mordax (Chiappa-Carrara & [START_REF] Chiappa-Carrara | Estudio del régimen y hábitos alimentarios de la anchoveta Engraulis mordax Girard (Pises: Engraulidae), en Baja California, Mexico[END_REF].

Figure 3.4 Classification and regression tree o f E n g r a u l i s r i n g e n s d i e t a c c o r d i n g t o t h e spatiotemporal factors from years 1996-2003: distance of the shelf break (dsb), latitude (lat), size-class (size), diel period (period). Each terminal node of the tree is characterized by a dominant prey. Below a histogram shows the predicted prey distribution (carbon proportion) for

the taxonomic groups, given the explanatory variables: euphausiids (Euph), calanoid copepods (Cala), other copepods (Oth.Cop.), other zooplankton (Oth.zoo), diatoms (Diat) and other phytoplankton (Oth.phyt).

Spatial and diel changes

The zooplanktonic fraction dominated Engraulis ringens diet irrespective of latitude, refuting the contention of a latitudinal-dependent dietary shift of E. ringens being more zooplanktivorous in the south and phytoplanktivorous in the north of Peru previously proposed [START_REF] Rojas De Mendiola | Some observations on the feeding of the Peruvian anchoveta Engraulis ringens J. in two regions of the Peruvian coast[END_REF]Pauly et al., 1989b).

Interestingly, if euphausiids dominated E. ringens diet all along the coast, calanoid copepods dominated at 7ºS and 15ºS and were relatively abundant at 4ºS, 7ºS and 10-12ºS (Fig. 3.3b). At these latitudes the contribution of diatoms was also higher. In fact, by latitude, calanoid copepods were significantly correlated to diatoms (R2=0.50, p<0.001). These areas are known as active upwelling cells [START_REF] Morón | Características del ambiente marino frente a la costa peruana[END_REF], with 7ºS and 15ºS corresponding to strong upwelling cells of Chicama and San Juan, respectively. In such areas the upwelled cold coastal waters are advected far from the coast and disperse coastal productivity, including zooplankton from the shelf into the offshore domain [START_REF] Keister | Zooplankton distribution and cross-shelf transfer of carbon in an area of complex mesoscale circulation in the northern California Current[END_REF]. The abundance of diatoms and small non-vertically migrating zooplankton is usually higher within an upwelling filament than outside [START_REF] Keister | Zooplankton distribution and cross-shelf transfer of carbon in an area of complex mesoscale circulation in the northern California Current[END_REF].

Our results also question previous assumption according to which Engraulis ringens forage more on phytoplankton inshore and zooplankton more offshore [START_REF] Rojas De Mendiola | Observations on the food and feeding habits of the anchovy Engraulis ringens Jenyns made during cruise 6908-09[END_REF]Pauly et al., 1989b). Indeed, the zooplankton fraction dominated E. ringens d i e t irrespective of the distance to the shelf break. Nevertheless, calanoid copepods were three time more abundant in stomachs sampled in the more inshore station than those sampled at more than 30 km from the shelf break (Fig. 3.3c), while the larger zooplankton items i.e. the euphausiids strongly dominated the diet offshore. This pattern matches the cross-shore distribution of macrozooplankton off Peru with larger and more abundant zooplankton offshore than inshore [START_REF] Ballón | Is there enough zooplankton to feed forage fish populations off Peru? An acoustic (positive) answer[END_REF]. Also, in our sample, E. ringens size increased with the distance to the coast. In particular, no E. ringens < 10 cm long were observed at a distance from the coast > 160 km. Both larger fish and higher availability of large prey offshore explain the observed patterns. Similar results have been observed on Engraulis encrasicolus and Engraulis japonicus Temminck & Schlegel ( 1846), which feed on smaller prey inshore than offshore, following the trend in anchovy size distribution (Plounevez & Champalbert, 1999;[START_REF] Tanaka | Geographical variations in the trophic ecology of Japanese anchovy, Engraulis japonicus, inferred from carbon and nitrogen stable isotope ratios[END_REF].

Within its coldwater coastal habitat, Engraulis ringens is able to forage efficiently across a range of temporal, spatial and thermal scales (Espinoza & Bertrand, 2008). However a clear diel pattern appears from our E. ringens diet results, with euphausiids reaching 82% during night hours while copepods accounted for ~40% during other diel periods (Fig. 3.3d). Such patterns fit with the diel vertical migration of macrozooplankton [START_REF] Ballón | Is there enough zooplankton to feed forage fish populations off Peru? An acoustic (positive) answer[END_REF]. Indeed E. ringens are restricted to the surface oxygenated water (Bertrand et al., 2008a[START_REF] Bertrand | A c o u s t i c O b s e r v a t i o n o f L i v i n g O r g a n i s m s Reveals the Upper Limit of the Oxygen Minimum Zone[END_REF][START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF]. During the day it has little access to the bulk (~79%) of the macrozooplankton community which is distributed within the oxygen minimum zone [START_REF] Escribano | Zooplankton associated with the oxygen minimum zone system in the northern upwelling region o f C h i l e d u r i n g M a r c h 2 0 0 0[END_REF][START_REF] Ballón | Is there enough zooplankton to feed forage fish populations off Peru? An acoustic (positive) answer[END_REF]. The portion of macrozooplankton that stay in the upper layer during the day is mainly composed of small-sized subadult and juvenile euphausiids, and large copepods (Ayón et al., 2008a;[START_REF] Ballón | Acoustic study of macrozooplancton off Peru: biomass estimation, spatial patterns, impact of physical forcing and effect on forage fish distribution[END_REF][START_REF] Ballón | Is there enough zooplankton to feed forage fish populations off Peru? An acoustic (positive) answer[END_REF]. Similar patterns were observed in Engraulis encrasicolus in the southern Benguela Current system [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF].

In summary, Engraulis ringens take the bulk of its energy from zooplankton, in particular euphausiids and copepods. This is irrespective of the period, E. ringens size, distribution, and diel period. During the last decades the importance of euphausiids in E. ringens diet appears directly related to euphausiids abundance and not to physiological factors as assumed previously (e.g. [START_REF] Hutchings | Fish harvesting in a variable, productive environment -searching for rules or searching for exceptions?[END_REF]. This bottom-up effect is also observed at smaller scale since the euphausiids fraction increased with E. ringens size (concurrent with the capacity of E. ringens t o c a p t u r e a n d h a n d l e t h e l a r g e r p r e y ) a n d e u p h a u s i i d s a c c e s s i b i l i t y ( e . g . h i g h e r offshore and/or during night). Selecting the largest prey, the euphausiids, provide an energetic advantage for E. ringens in its ecosystem where oxygen depletion imposes strong metabolic constrain to pelagic fish [START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF]. Finally this study is another illustration of E. ringens plasticity (see Espinoza & Bertrand, 2008) which allows it to cope with the highly variable NHCS environment.

Abstract:

Sardines are one of the main small pelagic fish resources in eastern boundary upwelling systems (EBUS) where they play an important ecological role both as a predator of plankton and as prey of top predators. Sardine trophodynamics have been relatively well studied in three of the EBUS (the Benguela, California and Canary upwelling systems) but not in the Humboldt Current system. In this paper we describe the diet of sardine Sardinops sagax in the northern Humboldt Current system (NHCS) off Peru, using an analytical method which assesses relative dietary importance in terms of estimated prey carbon content. We assessed sardine diet by examining a total of 555 stomachs collected during six surveys conducted off Peru during the period 1996-1998, and compare our results with the diet of anchoveta Engraulis ringens off Peru and with the diets of sardines from the southern Benguela (also S. sagax) and the northern Canary (Sardina pilchardus) upwelling systems. The diet of sardine off Peru is based primarily on zooplankton, similar to that observed for anchoveta but with several important differences. Firstly, sardine feed on smaller zooplankton than do anchoveta, with sardine diet consisting of smaller copepods and fewer euphausiids than anchoveta diet. Secondly, whilst phytoplankton represents <2% of sardine dietary carbon, this fraction is dominated by dinoflagellates, whereas diatoms are the dominant phytoplankton consumed by anchoveta. Hence, trophic competition between sardine and anchovy in the northern Humboldt Current system is minimized by their partitioning of the zooplankton food resource based on prey size, as has been reported in other systems. Whereas sardine in the NHCS feed on smaller zooplankton than do anchovy in that system, sardine in the NHCS forage on larger prey and obtain a substantial portion of their dietary carbon from euphausiids compared to sardine from the northern Canary and southern Benguela Current systems.

Introduction

Sardines (the genera Sardinops and Sardina) and anchovy (the genus Engraulis) are the most important commercially-exploited small pelagic fish species worldwide, and are particularly abundant in the eastern boundary current systems (EBUS; including the Benguela, California, Canary and Humboldt Current systems). These fish feed on plankton and are the dominant forage of a variety of fish, marine mammal and seabird predators, and hence are also ecologically important.

A recent synthesis on the trophic dynamics of small pelagic fish from a variety of upwelling and other systems [START_REF] Van Der Lingen | Chapter 7, Trophic dynamics[END_REF] has shown that sardine and anchovy derive the bulk of their dietary carbon from zooplankton, a finding that contradicts many previous descriptions of exclusive or near-total phytophagy by these species. For example, Peruvian anchoveta (Engraulis ringens) was considered to feed directly on primary producers, which led to the belief that the large populations of anchoveta were supported by an unusually short and efficient food chain [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF][START_REF] Walsh | A carbon budget for overfishing off Peru[END_REF]. This assumption was recently refuted by Espinoza and Bertrand (2008), who estimated the carbon content of prey items, as opposed to using the numerical method of stomach content analysis which is considered to be subjective [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF], and showed that anchoveta forage mainly on zooplankton. Such findings improve our knowledge of the functioning of the northern Humboldt Current system (NHCS) off Peru, and illustrate the need for accurate and appropriate methods. Dietary information derived using carbon content is available for sardine and anchovy from the main EBUS (see van der Lingen et al., 2009) but not for sardine (Sardinops sagax) in the Humboldt Current system. To date, sardine in the Humboldt Current system was generally considered as feeding mainly on phytoplankton [START_REF] Rojas De Mendiola | Notas sobre la alimentación de la sardina y el machete en las aguas costaneras del Perú. I Seminario latinoamericano sobre el Océano Pacífico Oriental[END_REF][START_REF] Sanchez De Benites | Alteraciones en la dieta alimenticia de algunos peces comerciales por efecto del fenómeno ''El Niño[END_REF][START_REF] Alamo | Changes in the food and feeding of the sardine (Sardinops sagax sagax) during the years 1980-1984 off the Peruvian coast[END_REF]. This differs from recent studies on this and similar species' diet in other ecosystems, that demonstrated that sardine derive the majority of their dietary carbon from zooplankton (van der Lingen, 2002; [START_REF] Garrido | Diet and feeding intensity of sardine Sardina pilchardus: correlation with satellitederived chlorophyll data[END_REF][START_REF] Van Der Lingen | Chapter 7, Trophic dynamics[END_REF]. A single study on sardine diet in the NHCS [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF] mentioned that sardine consumed mainly small copepods and tunicates, but this study has seldom been considered.

In this paper we describe the diet of sardine in the NHCS based on a method which assesses relative dietary importance according to estimated prey carbon content [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF][START_REF] Van Der Lingen | Diet of sardine Sardinops sagax in the southern Benguela upwelling ecosystem[END_REF]. We compare our results on sardine diet and prey-size spectrum with those of the anchoveta in the NHCS. We also compare the diet of sardine from the NHCS with data on the diet of sardines in other systems, including Sardinops sagax from the southern Benguela (SBCS) off South Africa, Sardina pilchardus from the northern Canary system off Portugal (NCCS), and with published work describing clupeid diets in the California Current system (CalCS). We discuss how new insights into the functioning of these systems may be extracted from such a comparative approach. In particular, we re-examine the hypothesis [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF] that upwelling regions are so efficient in terms of fish production because of the direct link between phytoplankton and filter-feeding fishes.

Material and methods

Sardine diet in the northern Humboldt Current system (NHCS)

Sardines were collected during six acoustic surveys conducted by IMARPE research vessels between 1996 and 1998 with the aim of estimating pelagic fish abundance in the Peruvian EEZ (Table 4.1). This period encompasses the El Niño of 1997-98, one of the two strongest of the last century [START_REF] Wolter | Measuring the strength of the ENSO events: how does 1997/98 rank?[END_REF]. Fish were captured using a midwater trawl net ('Engel 124/1800', 12 mm codend mesh) and a sample of 5-20 sardines was randomly collected from each trawl. Individual sardine were measured (total length; cm) and weighed (wet body mass; g) and the cardiac and pyloric sections of their stomachs were extracted onboard and immediately fixed in 5% formalin for later laboratory analysis. In the laboratory stomachs were opened and their contents extracted and weighed (wet weight, WW) to the nearest 0.01 g. The stomach contents of individual fish were then pooled according to fish length (3 cm size-classes) for each sample, and pooled stomach contents were filtered through a 125 μm mesh. The filtered material was diluted to 100 ml using filtered seawater and a 0.1-ml sub-sample was examined under the compound microscope to identify and count phytoplankton prey. The material that remained on the mesh was diluted to 100 ml using filtered seawater and a 10-mL sub-sample was examined using a stereoscopic microscope to identify (to genus) and count zooplankton prey. All prey counts were then standardized to number per 100 ml, except for anchoveta eggs, for which the whole sample was examined. Different procedures were followed to estimate the carbon content of phytoplankton and zooplankton and hence determine their relative dietary importance, and have been described in detail in Espinoza and Bertrand (2008). In summary, we did not measure the size of all prey items but instead estimated prey size using information from the literature and from some direct measurements, taking into account the life cycle stage (e.g. adult Calanus were separated from copepodite stages). The lack of available references on size and volume for planktonic organisms off Peru led us to use a wide range of references published for other ecosystems. Phytoplankton size was converted to phytoplankton cell volume using references describing the organism's geometric shape and providing equations relating shape to volume, with the coefficients for each equation being obtained from the literature or from estimations made by IMARPE. To transform phytoplankton volume into carbon content we used equations given in [START_REF] Menden-Deuer | Carbon to volume relationships for dinoflagellates, diatoms, and other protist plankton[END_REF] and [START_REF] Verity | Relationships between lorica volume, carbon, nitrogen, and ATP content of tintinnids in Narragansett Bay[END_REF]. For zooplankton, prosome or total length of various zooplankters was taken from the literature or from some direct measurements made by IMARPE on zooplankton from stomach content samples. Zooplankton dry weight and carbon content were calculated using equations given by [START_REF] Alexandrov | Caloric content of the Black Sea invertebrates: 1 Zooplankton and meiobenthos[END_REF], [START_REF] Deibel | Feeding mechanism and house of the appendicularian Oikopleura vanhoeffeni[END_REF], [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF], van der Lingen (2002), [START_REF] Parsons | Biological Oceanographic Processes[END_REF]Sameoto (1971). All prey carbon content values were standardized to μg C, and the contribution to dietary carbon of a particular prey type was expressed as a percentage of the total estimated carbon content of all fish examined in each cruise. In order assess temporal (inter-survey) variability of sardine diet off Peru we pooled d i e t d a t a i n t o s e v e n b r o a d p r e y c a t e g o r i e s (dinoflagellates, diatoms, other phytoplankters, copepods, gastropods, euphausiids, and other zooplankters), and present the results for individual surveys and for all surveys pooled. All the equations and conversion factors are available in Tables 2-4 of Espinoza and Bertrand (2008).

Diet comparison

To compare the diet of sardine from the NHCS with that of anchoveta (Engraulis ringens) in the same system, and with those of sardines from the southern Benguela (SBCS) and northern Canary (NCCS) systems, we reanalysed data from published work that also examined dietary composition in terms of prey carbon content. For anchoveta we used results reported by Espinoza and Bertrand (2008), which comprised data on anchoveta stomach contents collected during individual surveys and aggregated for all surveys (as has been done for NHCS sardine in this paper). The diet of southern Benguela sardine (Sardinops sagax) was described by van der Lingen (2002), using samples collected from midwater trawls deployed during research surveys conducted in 1993 (number of trawls sampled = 10; stomach contents of 10 sardine per trawl were pooled for analysis) and 1994 (number of trawls sampled = 17; stomach contents of 10 sardine per trawl were pooled for analysis). The average contribution to sardine dietary carbon by prey type (%C Type ) and by prey size (%C Size ) was calculated for each survey, with the %C Type and %C Size v a l u e s f r o m e a c h s a m p l e r e c e i v i n g e qual weighting. For our comparison we calculated average %C Type and %C Size values from all 27 samples, again with equal sample weighting. [START_REF] Garrido | Diet and feeding intensity of sardine Sardina pilchardus: correlation with satellitederived chlorophyll data[END_REF] described the diet of 70 sardine (Sardina pilchardus) from the northern Canary Current system using samples collected fortnightly from purse-seine catches off the west and south coasts of Portugal, taken during 2002-2004, and those two datasets were also aggregated for our comparison. Further details regarding the sampling and analytical methods used for anchoveta, and sardines from the southern Benguela and northern Canary systems, are provided in Espinoza and Bertrand (2008), van der Lingen (2002), and [START_REF] Garrido | Diet and feeding intensity of sardine Sardina pilchardus: correlation with satellitederived chlorophyll data[END_REF], respectively.

In order to facilitate dietary comparisons between species and systems we first grouped diet data into the seven broad prey categories listed above, and present this information in terms of numerical dominance and relative contribution to dietary carbon. To examine the zooplankton component of sardine diet in more detail we then re-grouped the data (in terms of relative contribution to dietary carbon only) from each system/species into the following ten prey categories: (i) diatoms, (ii) dinoflagellates, (iii) other phytoplankters, (iv) calanoid copepods, (v) miscellaneous copepods (mainly cyclopoid, harpacticoid, and poecilostomatoid copepods), (vi) other copepods (including copepod nauplii), (vii) euphausiids, (viii) gastropods, (ix) fish eggs, and (x) other zooplankters (including crustacean eggs). We then compared the prey-size spectra between species and systems by examining the contribution to dietary carbon made by the following six size-classes: 0 < x < 0.5, 0.5 ≤ x 1.0 ≤ x 2.0, 2.0 ≤ x 4.0, 4.0 ≤ x 8.0, ≥8.0 mm. We used the Kruskal-Wallis rank test, a non-parametric alternative to ANOVA [START_REF] Scherrer | Biostatistique[END_REF], to assess whether diets between species and systems were significantly different. Because data from the SBCS are only available as percentage contribution to diet by size class and by taxonomic group, standard statistical methods (e.g. chi-square, contingency table, Kruskal-Wallis, and others) could not be used on those data. Consequently, Kruskal-Wallis tests were used to compare the diets of sardine and anchoveta from the NHCS, and of sardine from the NHCS and the NCCS, only.

Results

Sardine diet in the NHCS

A total of 555 sardine from the NHCS ranging from 10 to 30 cm total length were analysed (

Comparative trophic ecology by prey type

In all systems for which data are available the image of sardine diet is dramatically different depending on whether prey items are considered in terms of their numerical contribution or their relative contribution to dietary carbon (Fig. 4.1b). In terms of numerical dominance the phytoplankton fraction ranged between 47.8% (SBCS) and 99.2% (NHCS), whereas phytoplankton contributed between 1.5% (NHCS) and 17.0% (NCCS) of total dietary carbon.

Because the numerical method is not appropriate for adequately characterizing diet, subsequent dietary comparisons will be described in terms of dietary carbon only.

Marked differences are seen between the diet of sardine and anchoveta in the NHCS, and also between sardines from the NHCS, the NCCS, and the SBCS (Fig. 4.2). Results from the Kruskal-Wallis analysis performed between systems and species (excluding the SBCS sardine) on dietary carbon by prey categories showed highly significant (p< 0.01) differences for all prey categories except for ''Other zooplankters" (p> 0.05). Sardine from the NHCS had only two prey categories that accounted for more than 5% of dietary carbon, namely calanoid copepods (53.0% of dietary carbon) and euphausiids (38.7%). These two categories also dominated anchoveta diet, but in reverse order, with euphausiids (67.5%) being the most important category followed by calanoid copepods (26.3%). Sardine from the NCCS presented a very different prey spectrum and a more even spread of important prey categories, with six categories accounting for more than 8% of dietary carbon: other copepods (26.6%), fish eggs (23.8%), dinoflagellates (13.3%), miscellaneous copepods (12.3%), calanoid copepods (12.2%), and other zooplankters (8.4%). The diet of sardine in the SBCS is intermediate between sardine in the other two systems, with four prey categories accounting for more than 8% of dietary carbon: calanoid copepods (38.2%), miscellaneous copepods (18.2%), other zooplankters (17.5%), and fish eggs (14.6%).

Comparative trophic ecology by prey-size class

The differences observed in the contribution to dietary carbon by prey category between fish and systems are even more clearly observed when the prey-size spectra are examined (Fig. 4.3), and results from the Kruskal-Wallis analysis performed between systems and species (excluding the SBCS sardine) on dietary carbon by prey size showed highly significant (p< 0.01) differences for all size classes. In the NCCS the sardine prey-size spectrum presents a decreasing trend with smaller prey contributing the majority of total dietary carbon; prey smaller than 0.5 mm contribute 49.7% of dietary carbon and this proportion decreases to zero for prey larger than 4 mm. Sardine in the SBCS also derive the majority (>90% on average) of their dietary carbon from small (<2.0 mm) zooplankton prey, and their prey-size spectra is roughly dome shaped with a mode between 1 and 2 mm. No prey larger than 4 mm were observed in sardine from that system. Finally, the prey-size spectrum of sardine in the NHCS shows an increasing trend, with prey <0.5 mm making a negligible contribution to dietary carbon and prey larger than 8 mm contributing 39.6% of dietary carbon, this size corresponding mainly to euphausiids. A similar, and even more pronounced trend is observed for anchoveta in the NHCS, for which prey larger than 8 mm contribute up to 70.9% of dietary carbon. carbon fraction was dominated by copepods, in particular Eucalanus and Calanus, followed by euphausiids, and the importance of these taxa wa s previou sly reco gni se d by [START_REF] Alamo | Changes in the food and feeding of the sardine (Sardinops sagax sagax) during the years 1980-1984 off the Peruvian coast[END_REF] and [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF]. The phytoplankton carbon fraction was dominated by large dinoflagellates (mainly Protoperidinium, Ceratium, and Coscinodiscus), as previously observed [START_REF] Alamo | Changes in the food and feeding of the sardine (Sardinops sagax sagax) during the years 1980-1984 off the Peruvian coast[END_REF][START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF]. This very low proportion of phytoplankton could represent a limitation to sardine growth and development, since phytoplankton has more nitrogen (N) than zooplankton (i.e. the C:N ratio is higher for zooplankton than for phytoplankton). However, sardines (and anchovies) have significantly higher N absorption efficiencies from zooplankton than from phytoplankton (van der Lingen, 1998), and that sardine are better able to utilize zooplankton N (and also C and dry mass) than phytoplankton N suggests that this species derives more of its nutritional requirements from zooplankton (van der Lingen, 1998), which is supported by our results.

The contribution made by different prey categories to the diet of sardine in the NHCS strongly varied during the study period (Fig. 4.1a), illustrating the spatiotemporal opportunistic feeding behaviour of sardine which can forage on a variety of prey according to variations in plankton composition [START_REF] Van Der Lingen | Effect of particle size and concentration on the feeding behaviour of adult pilchard, Sardinops sagax[END_REF][START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF]. The relative contribution from phytoplankton was always low but reached a maximum in February-April 1997, prior to the El Niño, and then decreased to very low (<0.5%) levels during the El Niño of 1997-98. The proportion of sardine dietary carbon derived from copepods was highly variable and presented no clear trend (Fig. 4.1a), being highest in November-December 1996 and lowest a few months later in February-April 1997. Yet, when the contribution to dietary carbon by copepods decreased that from the euphausiids increased, and viceversa. The relative importance of copepods and euphausiids in sardine diet does not match those for anchoveta diet, described by Espinoza and Bertrand (2008) using data collected during the same surveys (Figs. 4.1a and Fig. 2.2). For instance, the highest copepod fraction observed in sardine diet in November-December 1996 corresponds to the lowest observed for anchoveta, although the very low number of sardine samples collected during this survey (Table 4.1) suggests that results should be interpreted with caution. However, this opposite trend was also observed for other surveys (in particular September-October1997andMarch-May1998) that had a larger sample size. These differences may reflect the fact that these species are ecological neighbours but not ecological analogues [START_REF] Gutiérrez | Anchovy (Engraulis ringens) and sardine (Sardinops sagax) spatial dynamics and aggregation patterns in the Humboldt Current ecosystem, Peru, from 1983-2003[END_REF], and seldom overlap spatially (Bertrand et al., 2004a;[START_REF] Gutiérrez | Anchovy (Engraulis ringens) and sardine (Sardinops sagax) spatial dynamics and aggregation patterns in the Humboldt Current ecosystem, Peru, from 1983-2003[END_REF][START_REF] Swartzman | The relationship of anchovy and sardine to water masses in the Peruvian Humboldt Current System from 1983-2005[END_REF].

Most of the previous studies on sardine diet in the NHCS were based on qualitative methods [START_REF] Rojas De Mendiola | Notas sobre la alimentación de la sardina y el machete en las aguas costaneras del Perú. I Seminario latinoamericano sobre el Océano Pacífico Oriental[END_REF][START_REF] Sanchez De Benites | Alteraciones en la dieta alimenticia de algunos peces comerciales por efecto del fenómeno ''El Niño[END_REF][START_REF] Alamo | Changes in the food and feeding of the sardine (Sardinops sagax sagax) during the years 1980-1984 off the Peruvian coast[END_REF], and indicated that sardines depend mainly on phytoplankton with zooplankton only occasionally being important. [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF] paper was unique in mentioning that zooplankton was by far the most important dietary component for sardine and anchoveta off Peru. However, his study was based on a relatively small sample size and was not fully recognised in Peru. The present study corroborates [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF] results, and clearly shows that in the NHCS sardine derives the bulk of its diet from zooplankton, as is the case for anchoveta (Espinoza and Bertrand, 2008). This result is in agreement with results from other studies (see review by [START_REF] Van Der Lingen | Chapter 7, Trophic dynamics[END_REF] on S. sagax diet in the SBCS (van der Lingen, 2002) and in the CalCS [START_REF] Parr | Is the presence of phytoplankton in the stomach contents of the California sardine caused by special pursuit or merely due to incidental ingestion?[END_REF][START_REF] Emmett | Pacific sardine (Sardinops sagax) abundance, distribution, and ecological relationships in the Pacific Northwest[END_REF][START_REF] Mcfarlane | Distribution and biology of Pacific sardines (Sardinops sagax) off British Columbia, Canada[END_REF], as well as with results for the Iberian sardine Sardina pilchardus off Portugal [START_REF] Garrido | Diet and feeding intensity of sardine Sardina pilchardus: correlation with satellitederived chlorophyll data[END_REF]. Given that our sampling had limited spatial and temporal coverage, we cannot conclude that this characterization of the diet of sardine off Peru we obtained is perfectly representative of its diet during different periods, e.g. when sardine was more abundant in the system (mid 1970s to the beginning of the 1990s).

Additionally, the fact that most of our data were collected during the El Niño of 1997-98 could also impact our dietary characterization, as El Niño events are known to alter the food web towards a higher dominance of pico-and nanophytoplankton (e.g. [START_REF] Iriarte | Phytoplankton size structure during and after the 1997/98 El Niño in a coastal upwelling area of the northern Humboldt Current system[END_REF].

The impact of El Niño on the food web and on fish diet has been observed in other cases (e.g. [START_REF] Brodeur | Effects of environmental variability on trophic interactions and food web structure in a pelagic upwelling ecosystem[END_REF][START_REF] Arntz | El Niño: experimento climático de la naturaleza[END_REF], although Espinoza and Bertrand (2008) did not observe differences in anchoveta diet during the El Niño of 1997-98 compared to other, non-El Niño periods. This may be due to the fact that during the El Niño of 1997-98 local upwelling cells in the NCCS were strong enough to avoid food limitation for secondary producers (Bertrand et al., 2004a). Furthermore, in an analysis of a 35-year time series of zooplankton volume, [START_REF] Ayón | Zooplankton volume trends off Peru between 1964 and 2001[END_REF] could not relate the variability in zooplankton biomass to El Niño events.

Diet comparison

The diets of sardine and anchoveta in the NHCS are broadly similar and in both species are dominated by copepods and euphausiids, although in different proportions (Fig. 4.2). A second difference is related to dinoflagellates, that are consumed noticeably only by sardine. The apparent similarity in the diet of sardine and anchoveta is puzzling, since these species have very different feeding capabilities and foraging behaviours, at least in the SBCS (van der Lingen et al., 2006). However, when sardine diet is compared between systems, important differences appear, even if zooplankton is always by far the most important dietary component (Figs. 4.1 and 4.2). Copepods are always the dominant prey item, but whereas calanoids are by far the most important copepod in the diet of sardine in the NHCS, they represent less than 40% in sardine from the SBCS and only 12.2% in those from the NCCS (Fig. 4.2). The contribution to sardine dietary carbon made by phytoplankton appears to be negligible (<2%) in the NHCS, and ranges between 7% and 17% in the other systems. Most of the samples from the NHCS were collected during the El Niño of 1997-98, which could have biased our results toward an underestimation of the phytoplankton fraction (note that the number of samples is also rather low in the case of the other systems), although even before El Niño the phytoplankton fraction accounted for less than 5% of sardine diet (i.e. less than in other systems). Finally, fish eggs are important prey items of sardine in the NCCS and SBCS, but apparently not in the NHCS. However, sardine in the NHCS have been observed to forage on fish eggs, with [START_REF] Santander | Egg mortality of the Peruvian anchovy (Engraulis ringens) caused by cannibalism and predation by sardines (Sardinops sagax)[END_REF] reporting an average of 16 anchoveta eggs in each sardine stomach for fish collected during August-September 1981, and the differences between that and our study likely reflect sampling limitations.

The differences in prey composition observed between sardine and anchovy and also between sardine from the three systems are related to marked differences in the modal sizes of ingested prey (Fig. 4.3). Within the NHCS, sardine and anchoveta forage principally on large items. This is particularly true for anchoveta, for which prey larger or equal to 8 mm contribute 70.9% of dietary carbon. Sardine in the NHCS derive the majority of their dietary carbon from items larger than 4 mm, and it is interesting to note that euphausiids observed in sardine stomachs were smaller (~8 mm) than those typically seen in anchoveta stomach contents (~12 mm). Also interesting is the difference in modal prey size of sardine diet between systems. If we take into account our results (Fig. 4.3) and published studies from the CalCS (e.g. [START_REF] Emmett | Pacific sardine (Sardinops sagax) abundance, distribution, and ecological relationships in the Pacific Northwest[END_REF][START_REF] Mcfarlane | Distribution and biology of Pacific sardines (Sardinops sagax) off British Columbia, Canada[END_REF][START_REF] Miller | Diets of and trophic relationships among dominant marine nekton within the northern California Current ecosystem[END_REF][START_REF] Van Der Lingen | Chapter 7, Trophic dynamics[END_REF], we can rank the size of sardine prey between systems in the following order: Canary < Benguela < California and Humboldt.

In the NCCS most of the sardine dietary carbon comes from small copepods, fish eggs and phytoplankton that are <2 mm and have a mode of <0.5 mm. The main prey of sardine in the SBCS (calanoid and cyclopoid copepods and fish eggs) are also <2 mm and have a mode between 1 and 2 mm. Finally it is only in the Humboldt and California systems that large prey, in particular euphausiids, are important in sardine diet. Therefore, it seems that in the NHCS sardine are possibly not primarily non-selective filter-feeders as in the SBCS (see [START_REF] Van Der Lingen | Diet of sardine Sardinops sagax in the southern Benguela upwelling ecosystem[END_REF][START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF] and probably in the NCCS [START_REF] Garrido | Laboratory investigations on the effect of prey size and concentration on the feeding behaviour of Sardina pilchardus[END_REF], possibly due to local adaptations to different environments. More detailed research on sardine and anchoveta feeding behaviour should be performed in the future.

The main difference in sardine prey composition between systems is related to the euphausiids, that are absent from sardine stomach contents in the eastern Atlantic (NCCS and SBCS) but are important in the eastern Pacific (NHCS, this study;and CalCS, Emmett et al., 2005;[START_REF] Mcfarlane | Distribution and biology of Pacific sardines (Sardinops sagax) off British Columbia, Canada[END_REF][START_REF] Miller | Diets of and trophic relationships among dominant marine nekton within the northern California Current ecosystem[END_REF]. This is a crucial finding because large prey such as euphausiids have a very high energy content per individual, which may favour or accelerate pelagic fish growth. In the Atlantic, euphausiids are less available to sardines, and fish eggs may supplant euphausiids in sardine diet, at least to a degree. We assume that the importance of euphausiids for sardine diet in the eastern Pacific is due to a higher availability of these organisms, attributable to a narrower shelf compared with Canary and Benguela systems.

In the NHCS for instance, the rich, cold coastal waters can extend far away from the shelf when the upwelling is strong. This allows the coastal community, including small pelagic fish, to extend their range of distribution far away from the shelf [START_REF] Swartzman | The relationship of anchovy and sardine to water masses in the Peruvian Humboldt Current System from 1983-2005[END_REF] and hence have access to the offshore vertically migrating communities, in particular the euphausiids. As stated by Espinoza and Bertrand (2008), a strong connection between the coastal and the offshore pelagic ecosystems may allow sardine and anchoveta to gain access to high energy prey such as euphausiids, which is likely one of the reasons for the high fish productivity of the NHCS (see Espinoza and Bertrand, 2008) compared to other systems (see [START_REF] Hutchings | Fish harvesting in a variable, productive environment -searching for rules or searching for exceptions?[END_REF][START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF].

Local and seasonal changes in food composition are high in all systems, reflecting the impact of environmental dynamics on the specific composition of prey patches ( K o n c h i n a , 1 9 9 1 ) . Gregarious fish such as sardine and anchovy are sensitive to processes (in particular patchiness) occurring at a continuum of spatial scales from the individual (dm) to populations (100s km) [START_REF] Fréon | Sustainable exploitation of small pelagic fish stocks challenged by environmental and ecosystem changes: a review[END_REF]Bertrand et al., 2008a). They exhibit a highly flexible feeding behaviour and can shift between filter or particulate feeding according to the available prey (van der Lingen, 2002; [START_REF] Garrido | Laboratory investigations on the effect of prey size and concentration on the feeding behaviour of Sardina pilchardus[END_REF], and this opportunistic foraging behaviour allows them to cope with variability in the conditions they meet.

Summary

Our characterization of the diet of sardine in the NHCS based on prey carbon contents has shown that the diet of sardine off Peru is based primarily on zooplankton, similar to that observed for anchoveta Engraulis ringens (Espinoza and Bertrand, 2008), but with several important differences. Firstly, sardine feed on smaller zooplankton (smaller copepods and fewer euphausiids) than does anchoveta. Secondly, whilst phytoplankton represents <2% of sardine dietary carbon, this fraction is dominated by dinoflagellates, whereas diatoms are the dominant phytoplankton consumed by anchoveta. Hence, trophic competition between sardine and anchovy in the NHCS is minimized by their partitioning of the zooplankton food resource based on prey size, as has been reported in other systems [START_REF] Van Der Lingen | Chapter 7, Trophic dynamics[END_REF]. Sardine in the NHCS (this study) and in the CalCS [START_REF] Emmett | Pacific sardine (Sardinops sagax) abundance, distribution, and ecological relationships in the Pacific Northwest[END_REF][START_REF] Mcfarlane | Distribution and biology of Pacific sardines (Sardinops sagax) off British Columbia, Canada[END_REF][START_REF] Miller | Diets of and trophic relationships among dominant marine nekton within the northern California Current ecosystem[END_REF] forage on larger prey and obtain a substantial portion of their dietary carbon from euphausiids, compared to those from the NCCS and SBCS. We relate this difference to the narrower shelf in the eastern Pacific compared with the eastern Atlantic that allows clupeids to distribute over the shelf break and in offshore areas where euphausiids are abundant.

Euphausiids have a high energetic content, which may favour fish growth relative to when feeding on other prey types. Additionally, the high availability of euphausiids in the eastern Pacific may limit sardine predation on fish eggs, which is very high in the NCCS and the SBCS compared with the NHCS. These last two points are probably key factors explaining the secret of the fish productivity in the NHCS. Finally our results provide an opportunity to re-examine the hypothesis of [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF] who attributed the high abundance of small pelagic fish in upwelling systems to their ability to feed directly on phytoplankton and hence benefit from a very short and efficient food chain. This theory based on phytophagy was questioned decades ago in other systems [START_REF] Cushing | Upper trophic levels in upwelling areas[END_REF][START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF] but was still considered to be the case off Peru. The 'Peruvian short food chain' hypothesis has now been discredited (this study; [START_REF] Konchina | Trophic status of the Peruvian anchovy and sardine[END_REF]Espinoza and Bertrand, 2008;[START_REF] Van Der Lingen | Chapter 7, Trophic dynamics[END_REF]. Off Peru, anchoveta and sardine not only forage mainly on zooplankton but select large, energetically-rich zooplankters. This indicates that the energy transfer from primary production to fish through zooplankton is particularly efficient, and that the zooplankton biomass is higher than supposed. Recent estimates of zooplankton biomass, based on acoustic methods, seem to confirm the presence of high biomass of macro-zooplankton off Peru (M.Ballón, IMARPE unpublished data), but more research in this field is crucial.

Introduction

The northern Humboldt Current system (NHCS) off Peru covers less than 0.1% of the World Ocean surface but sustains ~10% of the world fish catch [START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF]. The NHCS is subjected to bottom-up forcing at intraseasonal (Bertrand et al., 2008b) interannual [START_REF] Barber | Biological consequences of El Niño[END_REF], multidecadal [START_REF] Chavez | From anchovies to sardines and back; multidecadal change in the Pacific Ocean[END_REF][START_REF] Alheit | Regime shifts in the Humboldt Current ecosystem[END_REF]Ayón et al., 2008b) and centennial time scales [START_REF] Gutiérrez | Rapid reorganization in ocean biogeochemistry off Peru towards the end of the Little Ice Age[END_REF][START_REF] Sifeddine | Laminated sediments from the central Peruvian continental slope: a 500 year record of upwelling system productivity, terrestrial runoff and redox conditions[END_REF][START_REF] Valdés | 250 years of sardine and anchovy scale deposition record in Mejillones Bay, northern Chile[END_REF]Salvatteci, 2013). If the NHCS is the "heavyweight champion of the world" in terms of fish productivity, it generates considerably less primary production than the Northern Benguela Current System [START_REF] Bakun | The marine ecosystem off Peru: What are the secrets of its fishery productivity and what might its future hold?[END_REF].

Different hypotheses have been proposed to explain this paradox:

(i) the proximity to the equator, unique among the eastern boundary upwelling systems (EBUSs, allows strong upwelling with relatively weak winds so a weak turbulence [START_REF] Parrish | Comparative climatology of selected environmental processes in relation to eastern boundary current pelagic fish reproduction[END_REF], which may increase the occurrence of the optimal environmental windows for fish reproduction [START_REF] Cury | Optimal environmental window and pelagic fish recruitment success in upwelling areas[END_REF]; (ii) differences in the trophic structure within EBUSs (e.g., food chain length, Carr and Kearns 2003); (iii) the strong influence of ENSO inter-annual variability might 're-set' the ecosystem and could favour fast-growing populations like small pelagic fishes in the NHCS [START_REF] Bakun | The marine ecosystem off Peru: What are the secrets of its fishery productivity and what might its future hold?[END_REF]; (iv) in opposition with other EBUS, the main spawning periods match the season of maximal shelf retention of ichthyoplankton and food concentration in the NHCS for both anchovy and sardine [START_REF] Brochier | Investigating the 'northern Humboldt paradox' from model comparisons of small pelagic fish reproductive strategies in eastern boundary upwelling ecosystems[END_REF]). These hypotheses are not sufficient to understand the NHCS paradox and more emphasis should be put to better understand trophic relationships in this system. Espinoza andBertrand (2008, 2014) and Espinoza et al. (2009) showed that the Peruvian anchovy or anchoveta Engraulis ringens forage mainly on zooplankton refuting the prevalent direct short food chain hypothesis from diatoms to forage fish [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF]. These results suggest an ecological role for forage fish that challenges current understanding of its position in the foodweb, the functioning of the NHCS, and the current trophic models of the NHCS. In upwelling ecosystems, the role of plankton-feeding pelagic fish such as anchovy has often been enhanced, resulting for instance in anchoveta-centered model of the Peruvian ecosystem [START_REF] Pauly | The Peruvian anchoveta and its upwelling ecosystem: three decades of change[END_REF]Pauly et al., 1989a,b). However, other species can have important ecological role in this ecosystem. For example, a pelagic crustacean, the squat lobster Pleuroncodes monodon is particularly abundant in the NCHS since the mid-1990s (acoustic estimates ranging between 0.6 and 3.4 million tonnes), being potentially an important prey for seabirds, mammals and coastal fish [START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF]. As anchovy, it would then constitute an important prey for apex predators [START_REF] Arias-Schereiber | Informe sobre el estado de conocimientos y conservación de los mamíferos marinos en el Perú[END_REF][START_REF] Jahncke | Dieta del guanay Leucocarbo boougainvilii, del piquero peruano Sula variegata y otras aves de la costa peruana en abril y mayo de 1997[END_REF]. Since anchoveta and squat lobster partially overlap along the Peruvian coast [START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF], they could compete for food. However, squat lobsters have been seldom observed in anchoveta stomachs. The question of P. monodon potential role relative to anchovy and apex predators such as fur seals and birds is therefore still under debate, and central to understand the trophic functioning of the NHCS. Another important species in the ecosystem is the jumbo squid Dosidicus gigas that is exploited by an important fishery. This species has received attention in the NCSH through stomach content and stable isotope analysis to understand its life histories, habitat and resource uses [START_REF] Lorrain | Sequential isotopic signature along gladius highlights contrasted individual foraging strategies of Jumbo Squid (Dosidicus gigas)[END_REF]Argüelles et al., 2012;Alegre et al., 2014). However is necessary to better understand its actual role within the food web.

There is thus a need to extend to other species than anchoveta the revision of the trophic ecology in the NHCS to reach a comprehensive and revised vision of the trophic relationships in the NHCS. Still, all the previous results were based on stomach content analysis (SCA) only. This method has limitations since it is logistically difficult to apply for a wide suite of species; it underestimates the importance of soft bodied, rapidly digested or unidentifiable food items [START_REF] Tieszen | Fractionation and turnover of stable carbon isotopes in animal tissues: implications for δ 13 C analysis of diet[END_REF]; and only brings a snapshot of ingested prey items [START_REF] Olson | Food-web inferences of stable isotope spatial patterns in copepods and yellowfin tuna in the pelagic eastern Pacific Ocean[END_REF]. In contrast to SCA, the stable isotope analysis (SIA) of consumer tissues integrate dietary information over an extended period and has been proved to be a powerful tool to study the trophic ecology of marine organisms at the ecosystem scale [START_REF] Hobson | A stable isotope (δ 13 C,δ 15 N) model for the North Water food web: implications for evaluating trophodynamics and the flow of energy and contaminants[END_REF][START_REF] Michener | Stable isotope ratios as tracers in marine food webs: an update[END_REF][START_REF] Yasue | Trophodynamic similarities of three sympatric clupeoid species throughout their life histories in the Kii Channel as revealed by stable isotope approach[END_REF]). The δ 13 C values are generally used as a tracer of the habitat or the feeding zone of organisms [START_REF] Hobson | Tracing origins and migration of wildlife using stable isotopes: a review[END_REF][START_REF] France | Carbon-13 enrichment in benthic compared to planktonic algae: food web implications[END_REF][START_REF] Cherel | Geographical variation in carbon stable isotope signatures of marine predators: a tool to investigate their foraging areas in the Southern Ocean[END_REF] while δ 15 N values are mainly used as an indicator of the trophic position of organisms [START_REF] Hobson | Determination of trophic relationships within a high Arctic marine food web using δ 13 C and δ 15 N analysis[END_REF][START_REF] Lesage | Marine mammals and the community structure of the Estuary and Gulf of St Lawrence, Canada: evidence from stable isotope analysis[END_REF][START_REF] Loc'h | Benthic community and food web structure on the continental shelf of the Bay of Biscay (North Eastern Atlantic) revealed by stable isotopes analysis[END_REF][START_REF] Mèndez-Fernandez | Trophic ecology of five toothed whale species inhabiting the waters of the Northwest of the Iberian Peninsula using δ 13 C and δ 15 N isotopic signatures[END_REF]). However, stable isotopes do not allow reaching the taxonomic precision of the SCA, and it depends of the tissue turnover rates, i.e. how quickly changes the isotope signature in a body tissue following a dietary change [START_REF] Buchheister | Turnover and fractionation of carbon and nitrogen stable isotopes in tissues of a migratory coastal predator, summer flounder (Paralicthys dentatus)[END_REF]. Furthermore, the δ 15 N and δ 13 C at the base of the food web from which organisms draw their nitrogen and carbon can have a high spatial variability [START_REF] Macleod | Effects of light intensity, water velocity, and species composition on carbon and nitrogen stable isotope ratios in periphyton[END_REF][START_REF] Kitchell | Nutrient cycling at the landscape scale: the role of diel foraging migrations by geese at the Bosque del Apache National Wildlife Refuge, New Mexico[END_REF][START_REF] Vander Zanden | Primary consumer δ 13 C and δ 15 N and the trophic position of aquatic consumers[END_REF]. These geographical variations in primary producer stable isotope values, or nitrogen and carbon isotopic baselines, have been shown to propagate up the food web and are reflected in the isotope value of organisms up to top predators [START_REF] Popp | Insight into the Trophic Ecology of Yellowfin Tuna, Thunnus albacares, from Compound-Specific Nitrogen Isotope Analysis of Proteinaceous Amino Acids[END_REF][START_REF] Cherel | Geographical variation in carbon stable isotope signatures of marine predators: a tool to investigate their foraging areas in the Southern Ocean[END_REF][START_REF] Olson | Food-web inferences of stable isotope spatial patterns in copepods and yellowfin tuna in the pelagic eastern Pacific Ocean[END_REF][START_REF] Graham | Using isoscapes to trace movements and foraging behaviour of top predators in oceanic ecosystems[END_REF]. Accurate and reliable assessment of isotopic baselines is therefore critical to estimating a consumer's trophic position and concomitantly food chain length.

A prominent feature of the NHCS is the presence of a shallow (25-50 m) permanent oxygen minimum zone (OMZ) along the coast (e.g., [START_REF] Morales | Anchovy larval distribution in the coastal zone off northern Chile: the effect of low dissolved oxygen concentration and of a cold-warm sequence (1990-1995)[END_REF][START_REF] Strub | Coastal ocean circulation off Western South America[END_REF][START_REF] Fuenzalida | Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean[END_REF][START_REF] Bertrand | A c o u s t i c O b s e r v a t i o n o f L i v i n g O r g a n i s m s Reveals the Upper Limit of the Oxygen Minimum Zone[END_REF], which strongly impacts marine life (Bertrand et al. 2008a[START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF] and hence trophic interactions. In this OMZ, in particular south of ~7.5°S where the upper limit of the OMZ is shallow [START_REF] Bertrand | A c o u s t i c O b s e r v a t i o n o f L i v i n g O r g a n i s m s Reveals the Upper Limit of the Oxygen Minimum Zone[END_REF][START_REF] Mollier-Vogel | Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency[END_REF], both intense denitrification and anaerobic ammonium oxidation (anammox) occur [START_REF] Kuypers | Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation[END_REF][START_REF] Lam | Revising the nitrogen cycle in the Peruvian oxygen minimum zone[END_REF][START_REF] Lam | Microbial Nitrogen Cycling Processes in Oxygen Minimum Zones[END_REF], which should increase the nitrogen stable isotope values of the baseline [START_REF] Graham | Using isoscapes to trace movements and foraging behaviour of top predators in oceanic ecosystems[END_REF][START_REF] Mollier-Vogel | Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency[END_REF]). To assess these baseline variations, a primary consumer such as zooplankton can be sampled. Primary consumers can integrate isotopic variation at the base of the food web and may be less prone to short term fluctuations in stable isotope values than primary producers. As an example, [START_REF] Mcmahon | A review of ecogeochemistry approaches to estimating movements of marine animals[END_REF] compiled zooplankton δ 15 N values from the literature and produced a basin-wide isoscape for the Atlantic Ocean.

In this context, the main objective of this study is to determine the trophic pathways in the NHCS, following the route from zooplankton to t o p p r e d a t o r s i n a n e n v i r o n m e n t s t r o n g l y influenced by persistent upwelling and OMZ. For that purpose we analyzed the stable isotope values of 13 taxonomic categories collected off Peru from 2008 -2011, i.e., zooplankton, fish, squids and air-breathing top predators.

Material and methods

Sampling

Collection samples covered the entire Peruvian coast in the northern HCS (Table 5.1, Fig. 5.1; 03°30'-18°S). Zooplankton, fish and squid samples were collected from 2008 to 2011 during routine acoustic surveys performed by the Peruvian Institute of the Sea (IMARPE) (Table 5.1). Fish and squids were measured for standard and or mantle length (mm). In large individuals, a small piece (1 x 1 cm) of the anterior dorsal muscle was collected. In small fish, the head, gut, and caudal fin were removed, and the remaining whole body was collected. Euphausiids and copepods were analyzed as a whole. For the squat lobster, the telson was dissected and the exoskeleton removed to isolate and collect the internal muscle. All samples were stored frozen and processed later in the laboratory, except for jumbo squid samples that were stored in 70% ethanol. Seabird and fur seal blood were collected during field study in one island and a Peninsula (Table 5.1) via venipuncture, stored in 70% ethanol in the field and then stored at -20°C until analysis. Storage in 70% ethanol does not alter the bulk δ 15 N values of blood [START_REF] Hobson | Preservation of blood and tissue samples for stable-carbon and stable-nitrogen isotope analysis[END_REF][START_REF] Bugoni | Effects of preservation methods on stable isotope signatures in bird tissues[END_REF], while some studies reported a slight increase in bulk δ 13 C values of blood.

Samples preparation and analysis

All samples but squid samples were dried in an oven at 60°C for 48 h and ground to a fine powder. Squid samples were first evaporated and then freeze dried for 48 h. Lipid extraction was performed on muscle samples using 20 ml of cyclohexane on powder aliquots of about 1 g. Lipid free samples were dried at 60°C before processing. Lipids were not removed from seabird and fur seal blood samples, as it has been shown that the low lipid content of whole blood does not require lipid extraction prior to isotopic analysis [START_REF] Cherel | Isotopic discrimination between food and blood and feathers of captive penguins: implications for dietary studies in the wild[END_REF]. Carbonates were also removed for copepods and euphausiids by soaking them in diluted HCl for 5 mn. Samples (~300µg) were analyzed using an elemental analyzer (Flash EA1112, Thermo Scientific, Milan, Italy) coupled to an isotope ratio mass spectrometer (Delta V Advantage with a Conflo IV interface, Thermo Scientific, Bremen, Germany). Results are expressed according to international standards (Vienna Pee Dee Belemnite for δ 13 C and N 2 in air for δ 15 N) following the formula: δ 13 C or δ 15 N=[(R sample /R standard )-1]×10 3 (in‰), where R is 13 C/ 12 C or 15 N/ 14 N. Reference gas calibration was done using reference materials (USGS-24, IAEA-CH6, IAEA-600 for carbon; IAEA-N1, -N2, -N3, -600 for nitrogen). Analytical precision based on isotope values of the acetanilide (Thermo Scientific) used to estimate C and N content for each sample series was 0.1% both for carbon and nitrogen. Values are means ± SD.

Statistical analysis

For each species, δ 13 C and δ 15 N were first plotted versus body length, latitude and distance to the coast (in km). The δ 15 N isotope signature was strongly impacted by the species, the body length and the latitude. Also, the sampling design was very unbalanced in terms of body length and geographical position. To compare δ 15 N values and being able to estimate the trophic position (TP) of each species, a modelling approach taking into account the latitudinal and body length effects was necessary.

Linear mixed-effects (LME) models [START_REF] Pinheiro | Feeding behaviour and trophic environment of Engraulis encrasicolus (L.) in the Bay of Biscay[END_REF] are well suited for dealing with unbalanced sampling schemes and the different sources of variability affecting stable isotope data (e.g. [START_REF] Ménard | Isotopic evidence of distinct feeding ecologies and movement patterns in two migratory predators (yellowfin tuna and swordfish) of the western Indian Ocean[END_REF][START_REF] Lorrain | Nitrogen and carbon isotope values of individual amino acids: A tool to study foraging ecology of penguins in the Southern Ocean[END_REF][START_REF] Décima | Environmental perturbation effects on baseline ä 15 N values and zooplankton trophic flexibility in the southern California Current Ecosystem[END_REF]. The LME approach takes into account the data structure and the variables of interest. Here the data were grouped by species (subclass for copepods, order for euphausiids, and family for myctophids). The species effect was treated as random variations around a population mean, while the covariates of interest, i.e. body length and latitude, were assessed as two continuous covariates. LME allowed us to combine a random-effects analysis of variance model (variability amongst species) with linear regression models. Different models were performed with latitude and body length added sequentially as fixed effects or as random effects; these models were tested using Akaike criterion and likelihood ratio tests. The assumption of normality and independence for the random effect and the residuals were graphically assessed (not shown).

In the models we only considered δ 15 N values because it shows a stepwise enrichment between prey and consumer tissues (2-5‰) and as consequence is useful for examining trophic relationships [START_REF] Minagawa | Stepwise enrichment of 15N along food chains: further evidence and the relation between δ15N and animal age[END_REF]Wada, 1984, Peterson and[START_REF] Peterson | Stable isotopes in ecosystems[END_REF]. On the other hand, carbon isotopes ratios vary little (0-1‰) with tropic levels and are mainly used to identify feeding habitats, carbon pathways and sources of primary productivity [START_REF] Hobson | Tracing origins and migration of wildlife using stable isotopes: a review[END_REF]Welch, 1992, France and[START_REF] France | Ecosystem differences in the trophic enrichment of 13 C in aquatic food webs[END_REF].

A preliminary scrutinizing of the data showed a shift in δ 15 N at ~7.5ºS coherent with the impact of the OMZ on the baseline nitrogen stable isotopes (δ 15 N) [START_REF] Mollier-Vogel | Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency[END_REF]. Indeed, along the Peruvian coast, the OMZ get more intense and shallow south of ~7.5ºS [START_REF] Fuenzalida | Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean[END_REF][START_REF] Bertrand | A c o u s t i c O b s e r v a t i o n o f L i v i n g O r g a n i s m s Reveals the Upper Limit of the Oxygen Minimum Zone[END_REF][START_REF] Mollier-Vogel | Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency[END_REF]. We thus split our data base and constructed different models north and south of 7.5ºS. Then we predicted the δ 15 N at 11.7ºS for the southern zone and at 7.5ºS in the northern zone. These latitudes were selected to maximise the number of samples at one location. Population predicted δ 15 N values using the most parsimonious models were computed to compare the trophic positions of the different s p e c i e s i n t h e r a n g e o f t h e c o r r e s p o n d i n g observed body lengths for the fixed latitudes, or in the range of the observed latitudes for a fixed body length (Table 5.2).

Trophic position estimation

For comparison purposes with literature data, we estimated absolute TP values based on predicted δ 15 N values at 7.5°S and 11.7°S. The trophic position (TP) was estimated using the equation [START_REF] Post | Using stable isotopes to estimate trophic position: models, methods, and assumptions[END_REF]:

TP = γ + (δ 15 N consumer -δ 15 N baseline )/∆n
where δ 15 N consumer is the stable nitrogen signature of the consumer, δ 15 N baseline is the nitrogen signature of the baseline), γ is the trophic position of the organism used to establish baseline, and ∆n is the is the trophic enrichment factor (TEF), i.e., the enrichment in δ 15 N per trophic level [START_REF] Vander Zanden | Comparing the trophic position of littoral fish estimated using stable nitrogen isotopes (δ 15 N) and dietary data[END_REF][START_REF] Post | Using stable isotopes to estimate trophic position: models, methods, and assumptions[END_REF]. We used a mean TEF of 3.4‰ as the average enrichment between the muscle of marine fishes and invertebrates (e.g. [START_REF] Minagawa | Stepwise enrichment of 15N along food chains: further evidence and the relation between δ15N and animal age[END_REF][START_REF] Post | Using stable isotopes to estimate trophic position: models, methods, and assumptions[END_REF]. We based our food web estimates on the copepods (e.g. [START_REF] Miller | Prey dominance shapes trophic structure of the northern California Current pelagic food web: evidence from stable isotopes and diet analysis[END_REF]. The sampled copepods were mostly large species i.e. Eucalanus spp. and Centropages spp. These species are omnivorous feeders [START_REF] Boyd | Grazing patterns of copepods in the upwelling system off Peru[END_REF]. We thus assumed a TP of 2.5 for copepods as in [START_REF] Olson | Food-web inferences of stable isotope spatial patterns in copepods and yellowfin tuna in the pelagic eastern Pacific Ocean[END_REF]. The enrichment factor can vary with prey and predator tissue types. As suggested by [START_REF] Cherel | Isotopic niches and trophic levels of myctophid fishes and their predators in the Southern Ocean[END_REF], we used 1.7‰ as the enrichment factor between prey muscle and seabirds and marine mammal blood [START_REF] Hobson | Stable carbon and nitrogen isotopic fractionation between diet and tissues of captive seals: implications for dietary reconstructions involving marine mammals[END_REF][START_REF] Lesage | Diet-tissue fractionation of stable carbon and nitrogen isotopes in phocid seals[END_REF][START_REF] Cherel | Isotopic discrimination between food and blood and feathers of captive penguins: implications for dietary studies in the wild[END_REF], and we used the following equation to estimate TP for seabirds and fur seals: TP = γ + [(δ 15 N consumer -1.7 + ∆n) -δ 15 N baseline ]/∆n

Results

SIA results (without modelling)

In total we estimated δ 13 C and δ 15 N on 831 samples from 13 taxonomic categories of the Peruvian marine food web from 2008-2011 (Fig. 5.1, Table 5.1). 5.1, Fig. 5.2a). In the case of δ 13 C, the values varied between -14.7‰ for Peruvian booby and -20.3‰ for copepods (Table 5.1,Fig. 5.2a). The C:N ratios, ranged between 3.2 and 3.9, except for copepods which C:N mean value was 4.8 (Table 5.1).

Figure 5.3 : Body length -based variability of δ 15 N values (red=2008; green=2009; blue=2010; black=2011).

A steady decrease was observed in δ 13 C values of the sampled species when plotted according to the distance to the coast (Fig. 5.2b). Indeed δ 13 C values >-17‰ where only observed in the first 100 km from the coast. On the opposite, at a distance >200 km, δ 13 C values were lower than -18‰ (mesopelagic fishes). This decreasing trend in δ 13 C values with the distance from the coast seems species independent, i.e. appears as a general trend. Birds and squat lobster had the highest δ 13 C values (~-15‰) with very low variability (0.1 to 0.4‰) while copepods, euphausiids and mesopelagic fish had the lowest δ 13 C values (~-18.5‰) with high inter individual variability (0.9 to 1.3‰). Also note that fur seals showed rather low δ 13 C values (~-17‰) when taking into account the distance to the coast of the sampling site (0 km since they were sampled in a peninsula). The negative trend in δ 15 N values with the distance to the coast was less marked than the one observed in δ 13 C (Fig. 5.2c).

δ 15 N values showed a high variability for a given organisms body length (Fig. 5.3). Along-size trends in δ 15 N can be only studied on species for which the samples encompassed a large enough range of body lengths (i.e. anchoveta, jumbo squid, hake, Vincigerria, squat lobster and to a lesser extent, the jack mackerel). A clear increase in δ 15 N values with body length was observed on the jumbo squid and the hake. The other species presented no (Vincigerria, squat lobster, anchoveta) or a negative trend (jack mackerel). Such unexpected null or negative trends are due to interaction with latitude that was further corrected in the LME models.

Strong latitudinal variations were depicted in δ 15 N values (Fig. 5.4). They steadily decreased from 18ºS south to ~7.5ºS and then stabilised. Such trend is particularly clear on jumbo squid for which we have more samples. On the opposite, species only sampled north of 7.5º (hake, myctophids and euphausiids) do not show clear latitudinal trend. (red=2008; green=2009; blue=2010; black=2011).

LME Models

The LME models we used aimed at predicting δ 15 N values taking into account the variability in baseline δ 15 N values (with latitude) and the body size effect. As indicated in the Method section, two models were fitted on the data (north and south of 7.5ºS). Both models considered the species dependence but differed in their structure. The best model for the northern area had a fixed effect on both body size and latitude while the best model for the southern area had a fixed effect for body size and a random effect for latitude. Model results for fixed body size (Fig. 5.6) illustrate the different latitudinal effect on δ 15 N values. South of 7.5º δ15N values strongly decrease towards the north. The trends are species dependant (random latitudinal effect in the model) with higher slopes for anchoveta and jack mackerel than for jumbo squid or squat lobster. North of 7.5ºS the slopes are similar for all species (fixed latitudinal effect) and much less pronounced than further south. If we compare predicted δ 15 N values between species in northern and southern regions, strong differences are observed. At 7.5ºS the squat lobster had the highest δ 15 N value (12.0‰), before hake (11.7‰), mackerel (11.5‰), jumbo squid (11.1‰) and anchoveta (11.0‰). The lower δ 15 N values were observed on copepods (7.6‰), euphausiids (8.9‰) and the mesopelagic fishes (~9.3‰). At 11.7ºS a different pattern appeared. Top predators that were not sampled in the north presented the highest δ 15 N (fur seal: 15.9‰, jack mackerel: 15.1‰, guanay: 15.0‰, booby: 14.6‰). The following species in decreasing δ 15 N values were the jumbo squid (14.3‰), the squat lobster (13.6‰), the anchoveta (13.5‰), the mesopelagic fish Vinciguerria (12.6‰), the mackerel (12.3‰) and the copepods (9.8‰).

Trophic position

Mean TPs were estimated with predicted δ 15 N values for the fixed size at 11.7 and 7.5ºS (Table 5.2;Fig. 5.8). The trends are logically the same as the one previously described on predicted δ 15 N values. At 7.5ºS the predicted TPs varied between 2.9 for Euphausiids and 3.8 for squat lobster. At 11.7 the TPs ranged between 3.2 for mackerel and 4.8 for fur seal. Curiously, as already observed in δ 15 N values the rank between common species was different in predicted Since body length is an important factor structuring trophic links within communities and ecosystems, we looked at the potential relationships between the TP and the body length for the species for which a sufficient range of size was sampled in the northern and southern regions. The predicted TP increased with body size for all species (Fig. 5.7).

Trophic position

Mean TPs were estimated with predicted δ 15 N values for the fixed size at 11.7 and 7.5ºS (Table 5.2; Fig. 5.8). The trends are logically the same as the one previously described on predicted δ 15 N values. At 7.5ºS the predicted TPs varied between 2.9 for Euphausiids and 3.8 for squat lobster. At 11.7 the TPs ranged between 3.2 for mackerel and 4.8 for fur seal. Curiously, as already observed in δ 15 N values the rank between common species was different in predicted TP values at 11.7 and 7.5ºS. The squat lobster had a higher TP (3.8) than a jumbo squid of 40 cm (3.5) at 7.5ºS but lower (3.6 vs. 3.8) at 11º7S.

Discussion

Using the stable isotope analyses, we focused on a simplified but representative (13 components) food chain in the NHCS. The δ 15 N baseline is strongly impacted by variability in the OMZ intensity along the Peruvian coast. We thus developed a modelling approach to account for such external variability. In spite of limitations due to the unbalanced nature of our data set some patterns emerge providing new insights on trophic relationships in the system. We confirm previous stomach content-based results on anchoveta TP and suggest the potential importance of an often neglected ecosystem component, the squat lobster.

Pattern of variability in δ 13 C

Our study revealed a general trend of increasing δ 15 N and δ 13 C values from copepods to large organisms (Fig. 5.2a). This classic pattern (e.g. [START_REF] Sherwood | Stable isotope analysis of some representative fish and invertebrates of the Newfoundland and Labrador continental shelf food web[END_REF][START_REF] Fanelli | Food web structure of the epibenthic and infaunal invertebrates on the Catalan slope (NW Mediterranean): Evidence from δ 13 C and δ 15 N analysis[END_REF][START_REF] Stowasser | Food web dynamics in the Scotia Sea in summer: A stable isotope study[END_REF]) illustrate a gain in heavy isotopes up the trophic food web.

We also observed an overall decreasing trend in δ 13 C values for multiple trophic levels with increasing distance to the coast (Fig. 5.2b). Such inshore offshore pattern is expected in many ecosystems and in upwelling systems (e.g. [START_REF] Sydeman | Trophic relationships among seabirds in Central California: combined stable isotope and conventional dietary approach[END_REF][START_REF] Miller | Carbon stable isotopes reveal relative contribution of shelf-slope production to the northern California Current pelagic community[END_REF]. The species δ 13 C values should decrease in the following order coastal > neritic > oceanic species, because δ 13 C values of particulate organic matter decreases from inshore to offshore waters with the decrease of productivity [START_REF] Hill | Biogeographic and nearshore-offshore trends in isotope ratios of intertidal mussels and their food sources around the coast of southern Africa[END_REF]. A decreasing cross-shore trend in productivity in the NHCS has also been estimated from Chl-a concentration [START_REF] Demarcq | Trends in primary production, sea surface temperature and wind in upwelling systems (1998-2007)[END_REF]. The species δ 13 C values can then be interpreted in terms of inshore or offshore foraging habitat.

Interestingly, the highest δ 13 C values were observed for birds and squat lobsters which would then indicate a much localized costal foraging area compared to mesopelagic fish and euphausiids that would forage more offshore. Tagging studies on birds indeed showed that these two species were feeding in the first tens of km of the colony very close to the coast [START_REF] Weimerskirch | Use of social information in seabirds: compass rafts indicate the heading of food patches[END_REF][START_REF] Weimerskirch | Foraging in Guanay cormorant and Peruvian booby, the major guano-producing seabirds in the Humboldt Current System[END_REF]. Anchoveta and squat lobster partly coincide in their δ 13 C values showing that anchoveta can feed both onshore and offshore, and has a larger foraging range than squat lobster that would be restricted to the coast. Fur seals, while sampled at the coast, had intermediate δ 13 C values, which can be explained by the fact that this species forage offshore at ~50-100 km from the coast (Y. Tremblay, unpublished data).

The δ 13 C signal could also be interpreted in terms of benthic vs. pelagic influence of food items (e.g. [START_REF] Hobson | A stable isotope (δ 13 C,δ 15 N) model for the North Water food web: implications for evaluating trophodynamics and the flow of energy and contaminants[END_REF]. However in this study we focused only on pelagic species except for the benthopelagic hake [START_REF] Lloris | Merluzas del mundo (Familia Merlucciidae). Catálogo comentado e ilustrado de las merluzas conocidas[END_REF]. As an example, the squat lobster Pleuroncodes monodon is benthic in Chile but pelagic off Peru where the intense and shallow OMZ impede it to access the sea floor [START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF]). Yet, in upwelling systems the primary production can create substantial deposition of pelagic production to sediments, which can be resuspended and reintegrated into both benthic and pelagic components. Such processes make difficult the precise estimation of δ 13 C-based benthic and pelagic productivities [START_REF] Iitembu | Comparison of ontogenetic trophic shift in two hake species, Merluccius capensis and Merluccius paradoxus, from the Northern Benguela Current ecosystem (Namibia) using stable isotope analysis[END_REF].

Pattern of variability in δ 15 N

Along with the increase with trophic level (Fig. 5.2a), the more noticeable pattern in δ 15 N is related to the latitude (Fig. 5.4). We indeed observed an increasing trend of δ 15 N values from north to south with a shift at ~7.5ºS. Such trend was already reported in previous studies [START_REF] Lorrain | Sequential isotopic signature along gladius highlights contrasted individual foraging strategies of Jumbo Squid (Dosidicus gigas)[END_REF]Argüelles et al., 2012) and can be related to changes in the δ 15 N values of the baseline modulated by the OMZ intensity along the Peruvian coast [START_REF] Bertrand | A c o u s t i c O b s e r v a t i o n o f L i v i n g O r g a n i s m s Reveals the Upper Limit of the Oxygen Minimum Zone[END_REF][START_REF] Mollier-Vogel | Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency[END_REF]. Indeed, in OMZ, strong denitrification and anammox are known to occur, and recent studied showed that anammox is the main N-loss process in the Peruvian OMZs [START_REF] Lam | Revising the nitrogen cycle in the Peruvian oxygen minimum zone[END_REF][START_REF] Kalvelage | Nitrogen cycling driven by organic matter export in the South Pacific oxygen minimum zone[END_REF]. Denitrification in anoxic water bodies has a large isotope effect of ~-25 ‰ (e.g., [START_REF] Brandes | Abiotic nitrogen reduction on the early Earth[END_REF][START_REF] Voss | Nitrogen isotope patterns in the oxygen-defficient waters of the Eastern Tropical North Pacific Ocean[END_REF]. To date, little is known about the isotope fractionation of anammox in the water column, but it is generally assumed that anammox has similar isotope fractionation [START_REF] Naqvi | Coastal versus open-ocean denitrification in the Arabian Sea[END_REF]. The most important implication is locally that denitrification and anammox can produce NO 3 and organic matter strongly enriched in 15 N (up to 20‰). The intensification of the oxygen deficient condition to the south of Peru south of ~8°S [START_REF] Bertrand | A c o u s t i c O b s e r v a t i o n o f L i v i n g O r g a n i s m s Reveals the Upper Limit of the Oxygen Minimum Zone[END_REF] would then contribute to latitudinal gradients in the δ 15 N baseline values, with highest δ 15 N values in the south. This North to South gradient of δ 15 N values has been observed in sediments along the Peruvian margin [START_REF] Mollier-Vogel | Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency[END_REF]. In the north (1°N to 8°S), sediment δ 15 N values are relatively low and uniform, ranging only between 4 to 5‰ while in the southern part, values generally increase southward but reveal a larger range between 4.5 and 13‰. This is exactly what we observe in our species δ 15 N values (Fig. 5.6).

Another factor impacting δ 15 N values was body size. Then, the following part of the discussion will be based on modelling results correcting for both latitudinal (baseline) and body size effects.

Predicted δ 15 N values and trophic positions

The modulation of the OMZ on the δ 15 N baseline isotopic values and the shift at ~7.5ºS led us to construct two separate LME models, south and north of 7.5ºS. The unbalanced sampling scheme and the actual species distribution (e.g. hake is mostly present in the north) explain that each model comprise different species. The lack of latitudinal and body size covertures for proposed a TP of 2 for the krill off Chile. In the California Current system, [START_REF] Sydeman | Trophic relationships among seabirds in Central California: combined stable isotope and conventional dietary approach[END_REF] estimated the TP of Euphausia pacifica to 2.5, while [START_REF] Miller | Prey dominance shapes trophic structure of the northern California Current pelagic food web: evidence from stable isotopes and diet analysis[END_REF] estimated the TP of Euphausia pacifica and Thysanoessa spinifera to 2.1 and 2.3, respectively. It thus seems that krill are at a higher TP in the NHCS that in the other comparables systems of the eastern Pacific. Yet krill can have a TP≥3. It is for instance the case of Euphausia frigida (3.0) and Euphausia tricantha (3.3) in Antarctica [START_REF] Stowasser | Food web dynamics in the Scotia Sea in summer: A stable isotope study[END_REF].

Above the euphausiids, mesopelagic fishes (Vinciguerria lucetia and myctophids) emerge as secondary consumers with a predicted TP between 3.0 and 3.2. These estimates are comparable to what have been observed in other EBUS: 3.4 in the California Current system [START_REF] Choy | Global Trophic Position Comparison of Two Dominant Mesopelagic Fish Families (Myctophidae, Stomiidae) Using Amino Acid Nitrogen Isotopic Analyses[END_REF] and 2.7 in the northern Benguela Current system [START_REF] Iitembu | Comparison of ontogenetic trophic shift in two hake species, Merluccius capensis and Merluccius paradoxus, from the Northern Benguela Current ecosystem (Namibia) using stable isotope analysis[END_REF].

Estimated TP for anchoveta (3.5-3.6) definitely refute the hypothesis of a short and efficient food chain with anchoveta foraging directly on phytoplankton [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF]. Rather they confirm the results from Espinoza and Bertrand (2008) who showed that zooplankton was by far the most important dietary component, with euphausiids contributing 67.5% of dietary carbon followed by copepods (26.3%). The TP estimates for various Engraulis spp. around the world (Table 5.3.) vary between 2 and 4, most ranging between 3.0 and 3.6. Our TP estimates for anchoveta are thus in the higher range of TP estimations but similar to [START_REF] Hückstädt | Stable isotope analysis reveals pelagic foraging by the Southern sea lion in central Chile[END_REF] who estimated the TP of E. ringens to 3.6. Such a rather high TP for E. ringens when compared with other Engraulis spp. is not surprising since E. ringens forage on larger prey than anchovies from other EBUS (Espinoza et al., 2009). Patterns for organisms with TP higher than the anchovy are less clear. Indeed the squat lobster showed the highest TP (3.8) in the northern region, i.e. above the hake (3.7) or the jumbo squid (3.5). In the southern region the estimated TP of squat lobster was still high (3.6) but lower than the jumbo squid (3.8). These differences are probably a result of our unbalanced data base. However in all cases, the estimated TP for squat lobster are high and in any case higher or equal to the one of anchovy. There is a lack of squat lobster diet study in the NHCS where it has a pelagic behaviour [START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF]. In Chile, Pleuroncodes monodon is benthic and forage on amphipods, zoeas, crustacean eggs, diatoms, foraminifers, bacteria, organic wastes

Width of isotopic niche

Some species such as jumbo squids, mackerel and anchoveta showed large variance in their isotopic signatures (Figure 5.2), which indicates long term inter-individual differences in their foraging ecology (different habitats and/or food items). In contrast, isotopic values of other species such as birds and, to a lesser extent, fur seals suggest a very localized and specialist diet, as already discussed for birds in the previous section. This can have strong implications in terms of resilience of species face to a changing environment, with species having a wider isotopic niche being more plastic and potentially more resistant to environmental changes.

Conclusion

In summary, despite some limitations inherent to the stable isotope method to identify precisely the diet of species, we provide a new vision of the relative trophic position of key ecosystem components in the NHCS (Fig. 5.8). We confirm previous stomach content-based results on anchoveta Engraulis ringens TP and highlight the potential remarkable importance of an often neglected ecosystem component, the squat lobster Pleuroncodes monodon. Indeed, our results support the hypothesis according to which this species could forage to some extent on fish eggs and larvae and can thus predate on the first life stages of exploited species. Further studies on this species as fatty acids or even compound specific isotope analysis of specific fatty acids need to be undertaken, in particular as we were not able to infer its role in the diet of airbreathing top predators.

The results presented in Chapters 2 to 5 provide significant elements to update our knowledge on the trophic relationships in the northern Humboldt Current system (NHCS). In this conclusion we discuss how these findings allows for an improved vision of the functioning of the NHCS and what are the key next steps. To do so we will use information from studies (van der Lingen et al., 2009;Ayón et al., 2011and Argüelles et al., 2012, Alegre et al., 2014) related with this dissertation.

When the same data provide different results: on the use of SCA

In Chapters 2 to 4 we focused on the main forage fish species of the NHCS, anchoveta (Engraulis ringens) and sardine (Sardinops sagax). Their trophic ecology was studied using a classic methodology i.e., stomach content analysis. Compared with previous works, the main difference of our approach was the way we quantified the diet. Indeed most previous works based on counts of fish prey concluded that these fish consume mainly phytoplankton (e.g. [START_REF] Rojas De Mendiola | The food of the Peruvian anchovy[END_REF][START_REF] Alamo | Stomach contents of anchoveta (Engraulis ringens), 1974-1982[END_REF]. Such method is known to be inadequate for estimating dietary importance since a phytoplankton cell accounts as much as a much larger and energetic prey [START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF][START_REF] Konchina | On methods of determining the trophic status of species in ichthyocenes[END_REF]. Methods based on prey weight (e.g. gravimetric) or on nutritional value (e.g. carbon content, caloric or energetic value) are more ecologically relevant [START_REF] Hyslop | Stomach contents analysis -a review of methods and their application[END_REF][START_REF] Koslow | Feeding selectivity of schools of northern anchovy, Engraulis mordax, in the southern California[END_REF][START_REF] James | Feeding ecology, diet and field-based studies on feeding selectivity of the Cape anchovy Engraulis capensis Gilchrist[END_REF]. Here we estimated the carbon content of the prey and provided a very different vision of anchoveta and sardine trophic ecology. Indeed zooplankton is by far the most important dietary component for these fish. Among zooplankters, euphausiids and large copepods were the main prey whatever the period. Euphausiids contribution was even higher during the 1950-1960s (92.6%) than during the late 1990-early 2000s (67.5%), while copepods presented an opposite trend (6.3% and 26.3%). Such zooplanktonic diet is similar to what was observed for Engraulis spp. and Sardinops sagax in other ecosystems [START_REF] Van Der Lingen | Chapter 7, Trophic dynamics[END_REF]. However, as indicated in Chapter 4, prey were larger in the NHCS compared with other eastern boundary upwelling systems (EBUS) and macrozooplankton constitute the bulk of forage fish diet even if sardine forage on smaller prey than anchoveta.

Revisiting the energetic transfer

These results constitute a major change in the understanding of trophic fluxes in the NHCS. Indeed they refute the previous hypothesis a c c o r d i n g t o w h i c h t h e g r e a t a b u n d a n c e o f anchoveta is based on its ability to graze on phytoplankton depicting a very short and efficient food chain [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF]. If forage fish consume mainly macrozooplankton then the trophic chain transfer should be efficient from phytoplankton to macrozooplankton and a high biomass of macrozooplankton should occurs. Sampling macrozooplankters such as euphausiids is complicated since these organisms are known to avoid nets [START_REF] Fleminger | Avoidance of towed nets by zooplankton[END_REF][START_REF] Brinton | Distributional Atlas of Euphusiacea (Crustacea) in the California Current region. Part I[END_REF][START_REF] Debby | Effect of net avoidance on estimates of diel vertical migration[END_REF][START_REF] Lawson | Euphausiid distribution along the western Antarctic Peninsula-part A: Development of robust multi-frequency acoustic techniques to identify euphausiid aggregations and quantify euphausiid size, abundance, and biomass[END_REF] because of both visual and mechanical disturbances [START_REF] Fleminger | Avoidance of towed nets by zooplankton[END_REF]. Robust biomass estimates are thus difficult to obtain from net sampling. To overcome such shortcoming, [START_REF] Ballón | Is there enough zooplankton to feed forage fish populations off Peru? An acoustic (positive) answer[END_REF] used acoustics to estimate macrozooplankton biomass. They estimated the macrozooplankton to about 105 g m -2 , i.e., two to five times more than previous estimates. This direct biomass estimation of macrozooplankton is in agreement with our new findings indicating that forage fish consume mainly macrozooplankton. This high biomass also supports the hypothesis we propose according to which the NHCS high fish production is due to very efficient energetic transfer toward macrozooplankton and the fish.

Trophic ecology and population dynamics

If zooplankters are the main prey for anchoveta and sardine, does the changes in zooplankton abundance and size spectra explain the variability of these fish populations? Extensive comparisons of the morphology of sardines and anchovies and of their feeding ecology suggested that differences in gill raker morphology allow sardines to efficiently filter feed on a larger size range of zooplankton (up to 1.23 mm and down to 10 μm) than anchovies, which do not appear to be able to effectively filter feed on zooplankton >0.7 mm (van der [START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF]. Thus, anchovies primarily feed by direct biting (particulate feeding) and show more size selective feeding, mostly taking zooplankton >1 mm, while sardines are apparently less size selective (van der [START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF], Espinoza et al., 2009).

Such differences led to the development of the trophic dynamics hypothesis to explain the variability between anchovy and sardine populations in other EBUS (e.g. [START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF]. This hypothesis (Fig. 6.1.) argues that changes in upwelling patterns and currents lead to changes in zooplankton size, species structure and spatial distribution. Having higher upwelling (more nutrients) may lead to larger phytoplankton (e.g. diatoms), which have lower surface-tovolume ratios than smaller phytoplankton (e.g. dinoflagellates), which predominate in warmer, lower nutrient conditions due to higher potential uptake rates under nutrient limitation [START_REF] Falkowski | Mix and match: how climate selects phytoplankton[END_REF][START_REF] Rykaczewski | I n f l u e n c e o f o c e a n w i n d s o n t h e p e l a g i c ecosystem in upwelling regions[END_REF]. In turn, larger zooplankton (i.e. large copepods and euphausiids, but not tunicates or gelatinous zooplankton) that may more effectively feed on large than on small phytoplankton [START_REF] Peterson | Life cycle strategies of copepods in coastal upwelling zones[END_REF][START_REF] Walker | Relationships between hydrography, phytoplankton production, biomass, cell size and species composition, and copepod production in the Southern Benguela upwelling system in April 1988[END_REF], thrive under higher upwelling conditions. Under warmer conditions, smaller phytoplankton may dominate and larger zooplankton may experience food limitation [START_REF] Painting | The dynamics of bacterioplankton, phytoplankton and zooplankton communities during the development of an upwelling plume in the southern Benguela[END_REF], resulting in dominance by smaller zooplankton. Ayón et al. ( 2011) examined whether changes in the relative size of zooplankton in the Peruvian HCS were associated with changes in sardine and anchovy biomass as suggested by the sizeselective-feeding hypothesis. To this end, zooplankton samples collected by the Peruvian Marine Research Institute (IMARPE) between 1963 and 2005 were used and the qualitatively dominance by different size classes (small, medium and large zooplankton, and euphausiids) in these samples was estimated. The temporal pattern for euphausiids dominance was highly cross-correlated (i.e. was in phase) with the time series for estimated biomass of anchovy, and small zooplankton dominance with that for estimated sardine biomass. These results support the focal hypothesis, which showed energetic advantages to sardine filter feeding on smaller zooplankton and to anchovy bite feeding on larger copepods and euphausiids. Although euphausiids predominate off the shelf break, anchovy biomass is generally highest on the shelf, suggesting a possible mismatch between anchovy feeding and euphausiids dominance. However, evidence concerning the offshore expansion of the anchovy range in cooler conditions, where both anchovy and euphausiids predominate, somewhat alleviates this apparent contradiction. A strong diel component to euphausiids and large zooplankton indicated diel migration for these zooplanktonic groups. That anchovy will preferentially eat euphausiids when they are more available (i.e. during the night and offshore) is supported by our results on anchovy diet dynamics (Chapter 3). Ayón et al. ( 2011) showed that sardine biomass was synchronous with the dominance of small zooplankton, which in turn corresponds to periods of generally warmer, lower upwelling conditions. These results support the hypothesis that sardine's capacity to efficiently filter-feed zooplankton over a large size range gives them a competitive advantage where small zooplankton is dominant (van der [START_REF] Van Der Lingen | Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?[END_REF]. Therefore, such results support the hypothesis that periods of low dominance of large zooplankton disfavors anchovy and gives an energetic advantage to predominately filter feeding sardines. Although sardine may have an advantage when small zooplankton dominate, Espinoza et al. (2009) showed that, at least from 1996 to 1998, sardine diets in the HCS contained surprisingly large amounts of euphausiids, comprising, on average, more than 1/3 of sardine prey carbon content. Therefore, even if filterfeeding by sardines is energetically more efficient than particulate feeding (van der Lingen, 1995), sardines in the NHCS appear to forage on large as well as small zooplankton. So why then do sardines not do better during cooler periods dominated by euphausiids? Indeed sardine could flourish whatever the zooplankton size spectra. This contradiction indicates that the conceptual hypothesis of how physical forcing may lead to environments that favour either sardine or anchovy from van der Lingen et al. ( 2006) (Fig. 6.1.) is not sufficient to explain sardine and anchovy dynamics in the NHCS. Other environmental factors could thus play a key role and allow us to provide a more comprehensive vision.

A fundamental environmental factor often neglected in marine ecosystem is the oxygen. Indeed fish need sufficient amounts of both food and oxygen, but the latter might be more difficult to obtain than the former [START_REF] Pauly | Gasping fish and panting squids: oxygen, temperature and the growth of waterbreathing animals[END_REF]. It is particularly the case in the NHCS, which encompasses one of the most intense and shallow OMZ [START_REF] Helly | Global distribution of naturally occurring marine hypoxia on continental margins[END_REF][START_REF] Chavez | The northern Humboldt Current system: brief history, present status and a view towards the future[END_REF][START_REF] Fuenzalida | Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean[END_REF][START_REF] Paulmier | Oxygen minimum zones (OMZs) in the modern ocean[END_REF]. [START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF] showed how the inclusion of this missing piece, the need to breath, allows the development of a comprehensive conceptual model of pelagic fish populations and change in an ocean ecosystem impacted by low oxygen. Oxygen demand also depends on the quality and quantity of food [START_REF] Pauly | Gasping fish and panting squids: oxygen, temperature and the growth of waterbreathing animals[END_REF]. The anchoveta gets most of its energy by visual selection and direct biting on macrozooplankton. Filter feeding is very expensive metabolically relative to biting for anchovies [START_REF] James | The relationship between respiration rate, swimming speed and feeding behaviour in the Cape anchovy Engraulis capensis Gilchrist[END_REF][START_REF] Van Der Lingen | Respiration rate of adult pilchard Sardinops sagax in relation to temperature, voluntary swimming speed and feeding behaviour[END_REF]. In sharp contrast, filter-feeding is energetically much cheaper for sardines (van der Lingen, 1995). These results lead [START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF] to suggest a conceptual model where the trophic size structure, the amount of available oxygen, and the feeding energetic cost are considered (Fig. 6.2). The last two periods of high macrozooplankton abundance (until the mid 1970s and since the late 1990s) are associated to low oxygen conditions. Sardine can shift toward a more energetically expensive direct biting on macrozooplancton but such foraging strategy became limiting if oxygen availability decrease as occurred in the late 1990s when sardine collapsed.

New insights provided by SIA

In the last two decades, the stable isotope analyses of nitrogen and carbon have been increasingly used to better understand marine food webs patterns and fluxes. In Peru so far very few studies were performed. [START_REF] Lorrain | Sequential isotopic signature along gladius highlights contrasted individual foraging strategies of Jumbo Squid (Dosidicus gigas)[END_REF]Arguelles et al. (2012) focused on the jumbo squid while this work is a first 'ecosystemic' contribution. Even if several shortcomings can blur our capability to fully describe the food webs (see Chapter 5) robust patterns emerged and we were able to propose a new vision of the trophic structures in the NHCS (Fig. 5.8).

Confirming SCA results on anchovy

Our results definitely refute the hypothesis of a short and efficient food chain with anchoveta foraging directly on phytoplankton [START_REF] Ryther | Photosynthesis and fish production in the sea[END_REF]. Rather they confirm the results from Espinoza and Bertrand (2008) who showed that zooplankton was by far the most important dietary component, with euphausiids contributing 67.5% of dietary carbon followed by copepods (26.3%), therefore having an estimated trophic position of ~3.5.

The squat lobster case

The squat lobster has become abundant along the Peruvian coast since the mid-1990s with acoustic estimates ranging between 0.6 and 3.4 million tons from 1998 to 2005 [START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF]. This species has been previously observe d in stomachs of hake ( Merluccius gayi peruanus), sharks [START_REF] Paredes | Estudio biológico pesquero del "camaroncito rojo" o "munida" Pleuroncodes monodon frente a Pucusana, Lima[END_REF], and jack mackerel (Trachurus murphyi) (Alegre et al., 2013). The squat lobster also constituted 75% of the diet of sea-lions and was important for sea birds in the zone of the Paracas Peninsula, 14°S [START_REF] Arias-Schereiber | Informe sobre el estado de conocimientos y conservación de los mamíferos marinos en el Perú[END_REF][START_REF] Jahncke | Dieta del guanay Leucocarbo boougainvilii, del piquero peruano Sula variegata y otras aves de la costa peruana en abril y mayo de 1997[END_REF]. Last years, abundance of the squat lobster has increased and many questions have risen about its role and for example potential predation of this species by anchoveta or competition between the two species. As shown in figure 6.4, large quantities of Pleuroncodes monodon can be found while fishing for anchovy. Our SIA results do not support the hypothesis that anchoveta would predate on squat lobsters as their trophic position (TP obtained from δ 15 N values) is nearly the same, with squat lobster even having a higher TP. Still, our results support the hypothesis that squat lobsters would forage to some extent on fish eggs and larvae, thus likely predating on the first life stages of anchoveta. Competition between these two species therefore seems to occur. However, the differing δ 13 C values of anchoveta and squat lobster suggest that these two species do not exactly share the same foraging habitat, with anchoveta having a larger and more offshore foraging range. This is in agreement with what [START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF] found using acoustic data from surveys performed between 1998 and 2006 along the Peruvian coast, with the squat lobster having a restricted and more coastal habitat compared to anchoveta. This would potentially reduce some direct competition and/or predation.

In any case, these results suggest the important role of the squat lobster the Humboldt Current system. More studies need to be done, in particular on its energetic content as some birds and seals have been found to forage on this species [START_REF] Arias-Schereiber | Informe sobre el estado de conocimientos y conservación de los mamíferos marinos en el Perú[END_REF][START_REF] Jahncke | Dieta del guanay Leucocarbo boougainvilii, del piquero peruano Sula variegata y otras aves de la costa peruana en abril y mayo de 1997[END_REF], which could impact their fitness. The junk-food hypothesis indeed attributes declines in productivity of marine birds and mammals to changes in the species of prey they consume and corresponding differences in nutritional quality of those prey [START_REF] Romano | Testing the junk-food hypothesis on marine birds: effects of prey type on growth and development[END_REF]. [START_REF] Lorrain | Sequential isotopic signature along gladius highlights contrasted individual foraging strategies of Jumbo Squid (Dosidicus gigas)[END_REF] and Argüelles et al. (2012) determined the habitat and resource use of Dosidicus gigas in the Northern Humboldt Current System from stable isotopes analyses. The two approaches were complementary. Using δ 13 C and δ 15 N values of serially sampled gladius (an archival tissue) [START_REF] Lorrain | Sequential isotopic signature along gladius highlights contrasted individual foraging strategies of Jumbo Squid (Dosidicus gigas)[END_REF] first examined high resolution variations in the trophic niche of five large (>60 cm mantle length) jumbo squids. They evidenced large inter-individual differences in jumbo squid foraging strategies with no systematic increase of trophic level with size, illustrating the high degree of plasticity of this species. Argüelles et al. (2012) then analyzed δ 13 C and δ 15 N values of mantle tissues of 234 jumbo squid individuals collected during 2008-2010 (note that these samples are considered within the samples used in Chapter 5). As observed globally for all species in Chapter 5, for δ 13 C values, distance to shelf break and latitude were the main explaining factors. The δ 13 C values followed a decreasing trend with increasing distance to the shelf break (Fig. 6.5.), which is consistent with higher δ 13 C values corresponding to highest productivity in coastal waters than in oceanic waters [START_REF] Pennington | Primary production in the eastern tropical Pacific: a review[END_REF][START_REF] Miller | Carbon stable isotopes reveal relative contribution of shelf-slope production to the northern California Current pelagic community[END_REF]. In turn, the δ 13 C values showed a clear bellshaped trend with lowest values north of Peru and highest values in central Peru. This variation (from north to central Peru), matched with the δ 13 C values of zooplankton and can be related to the upwelling off Peru and reflect latitudinal changes in productivity. Indeed, this area corresponds to the richest nearshore area ranges between 7 and 15°S (Guillén and Calienes, 1981;[START_REF] Echevin | The seasonal cycle of surface chlorophyll in the Peru upwelling system: a modelling study[END_REF]. The δ 13 C values also increased with jumbo squid mantle size, primarily reflecting increases in body mass [START_REF] Hesslein | Replacement of sulfur, carbon, and nitrogen in tissue of growing broad whitefish (Coregonus nasus) in response to change in diet traced by δ 34 S, δ 13 C, and δ 15 N[END_REF][START_REF] Herzka | Assessing connectivity of estuarine fishes based on stable isotope ratio analysis[END_REF], or carbon differences in δ 13 C that occur primarily at primary production, with small increases with increasing the trophic level (0.4‰, [START_REF] Post | Using stable isotopes to estimate trophic position: models, methods, and assumptions[END_REF]. These changes, suggest that jumbo squid juveniles and adults distribute over a large range of habitats, seasonally migrating to coastal waters to grow, mature, and feed, and then return to oceanic waters to spawn [START_REF] Nesis | The biology of the giant squid of Peru and Chile, Dosidicus gigas[END_REF][START_REF] Nesis | Dosidicus gigas[END_REF][START_REF] Nigmatullin | A review of the biology of the jumbo squid Dosidicus gigas (Cephalopoda: Ommastrephidae)[END_REF]. δ 15 N values were strongly affected by latitude (Fig. 6.5.) with increasing values from north to south were also found in zooplankton samples and were related to changes in isotope baseline values probably due to oxygen minimum zones that occur off Peru. This similar latitudinal trend in both zooplankton and D. gigas samples reveals that D. gigas is a relatively resident species at the scale of its isotopic turnover rate (i.e. a few weeks), even if this is not necessarily the case at the scale of its life. A small but significant size effect on δ 13 C values suggests that jumbo squid perform offshore-onshore ontogenic migration, with juveniles distributed offshore. For nitrogen, the high inter-individual variability observed with mantle length indicates that D. gigas can prey on a high variety of resources at any stage of their life cycle. Argüelles et al. (2012) thus provided further evidence that D. gigas have the capability to explore a wide range of habitats and resources at any stage of their life. Such findings were complemented by the present studies and by Alegre et al. (2014) who proposed a conceptual model of jumbo squid distribution and foraging patterns along the ontogenetic cycle taking into account the presence of the oxygen minimum zone (Fig. 6.6.). They show that small jumbo squid (400 mm) are mostly distributed far offshore where they largely forage on mesopelagic fish. As they grow, they move closer to the coast and increase their consumption of cephalopoda. However, off Peru, contrarily to other systems, D. gigas does not occupy very coastal waters where a huge biomass of anchovy is present. They hypothesized that jumbo squid cannot enter the coastal waters that present low surface oxygen saturation. Although jumbo squid can forage in hypoxic deep waters it needs surface normoxic waters afterwards. Oxygen concentration may thus limit the co-occurrence of both species and then preclude predator-prey interactions. Large squids move further offshore (without reaching the oceanic distribution of smaller jumbo squids), and increase their consumption of squids (including jumbo squid) and euphausiids. This comprehensive study on jumbo squid also illustrates the potential of SIA when a large number of individuals with a good spatial and temporal resolution can be achieved. The gladius study also demonstrates the potential of archival tissues for individual life history reconstructions. This is undoubtedly two important directions to follow for future SIA in Peru (see below).

The jumbo squid case

Perspectives for the future SIA in Peru

One main limit of this study is poor spatial and temporal resolution of sampling for most species such as squat lobster and copepods (in particular for the inshore offshore gradient). IMARPE cruises are mostly dedicated to fisheries evaluation and it was difficult to obtain time for other scientific sampling. By the way, the initial sampling protocol designed to decipher latitude, longitude and size effects, i.e., sampling the same species on the same cruise for small and large individuals at several latitudes together with onshore and offshore sampling could never be achieved. It was therefore difficult to test baseline effects properly and to assess inter annual variations and relate them to changing environmental conditions (e.g., cool vs. warm years, changes in the upper limit of the OMZ). As discussed previously in this thesis, both geographic patterns and trophic position influence the δ 15 N values of consumers, and resolving these two components for mobile pelagic organisms requires accurate and reliable assessment of isotopic baselines. In the future, an onshore offshore sampling of the base of the food web, i.e., copepods, at two specific locations (one south, one north) and at different times of the year would help confirming some hypotheses advanced in our study.

It would also be interesting to investigate potential changes in the foraging ecology of key species of the NSCH through time, to reveal potential shifts for example in strong El Niño vs. La Niña conditions or strong upwelling's years. F o r e x a m p l e , d o e s t h e f o r a g i n g h a b i t a t o f anchoveta extend offshore (lower δ 13 C values and higher SD) in cooler conditions (see earlier discussion in section 6.3)? Indeed, it would be a very efficient way to monitor long term changes in foraging ecology of important species. We therefore suggest for the future to start an historical/observatory sampling in one or two specific locations at the same period in Peru of copepods, euphausiids, the squat lobster, birds, anchoveta and jumbo squids to monitor potential shifts in the trophic structure of the system. This SIA sampling has to be performed in complement with the oceanographic survey and routine SCA sampling already performed by IMARPE. A time series on SIA should help to better establish the food chain length and variability in the NSCH and to provide a database essential to understanding bioaccumulation of contaminants, climate change impacts and fisheries responses in the NSCH.

Furthermore, past changes would also be interesting to measure through archival samples, such as zooplankton stored in formaldehyde, bird feathers from collections or anchovy scales from sediment cores. Past isotopic baseline variations could be assessed through zooplankton analyses. This step is indeed a prerequisite for further relevant interpretation of stable isotope values in higher trophic level organisms. Compound-specific isotope analysis of δ 15 N of some particular individual amino acids (δ 15 N CSIA-AA) is a rapidly growing tool in ecological studies to assess diet and trophic position (TP) in both modern and ancient foodwebs [START_REF] Mcclelland | Trophic relationships and the nitrogen isotopic composition of amino acids in plankton[END_REF][START_REF] Chikaraishi | Determination of aquatic food-web structure based on compound-specific nitrogen isotopic composition of amino acids[END_REF][START_REF] Germain | Nitrogen isotope fractionation in amino acids from harbor seals: implications for compound-specific trophic position calculations[END_REF]. A group of amino acids, the source amino acids (e.g., phenylalanine) do not fractionate through the food web and represent the isotopic baseline, whereas a second group of amino acids, trophic amino acids (e.g., glutamic acid), do fractionate consistently with each trophic level and can provide an estimate of the consumer's trophic position [START_REF] Mcclelland | Trophic relationships and the nitrogen isotopic composition of amino acids in plankton[END_REF][START_REF] Popp | Insight into the Trophic Ecology of Yellowfin Tuna, Thunnus albacares, from Compound-Specific Nitrogen Isotope Analysis of Proteinaceous Amino Acids[END_REF][START_REF] Chikaraishi | Determination of aquatic food-web structure based on compound-specific nitrogen isotopic composition of amino acids[END_REF][START_REF] Lorrain | Nitrogen and carbon isotope values of individual amino acids: A tool to study foraging ecology of penguins in the Southern Ocean[END_REF]. This method allows desiphering if variations in the bulk δ 15 N value of the consumer are driven by fluctuations at the base of the food web or changes in their trophic position. This method necessitates tissue only from the studied organism from which the isotopic composition of several amino acids can be simultaneously done, thus resolving complications associated with high resolution spatial and temporal baseline sampling. This method has successfully been applied in several types of organisms and recently in the jumbo squid D. gigas to decipher its foraging and expansion range (Ruiz-Cooley et al., 2013)

Concluding remarks

This study based on the combination of stomach content and stable isotopes analyses provided a new vision of the trophic structure of the NHCS. Several paradigms were revisited (e.g. anchoveta phytophagy) and some surprises came out (e.g. the relatively high TP of the squat lobster). These findings call for a strong revision of trophic models currently used in the NHCS. Also they indicate that the use of trophic levels from international data-base or trophic models outputs can lead to strong misunderstandings. For instance the trophic position of anchoveta ranges between 2.6 and 3.1 in FishBase, it was estimated to 2.3 from trophic models [START_REF] Bertrand | Impacts of Kelvin wave forcing in the Peru Humboldt Current system: scenarios of spatial reorganizations from physics to fishers[END_REF], while our results indicates much higher values (3.5-3.6). Still, this study reveals that

Figure 1

 1 Figure 1.1: Fish catch versus primary productivity for the four main eastern boundary coastal upwelling ecosystems for the years 1998-2005. Source: Chavez et al. (2008).

Figure 1 . 2 :

 12 Figure 1.2: Sea surface properties and oceanic circulation scheme. (a) Sea-Surface Salinity (SSS, color shading) and surface circulation. (b) Sea-Surface Temperature (SST, colour shading in °C) and subsurface circulation. The Ecuador-Peru Coastal Current (EPCC) and Chile-Peru Deep Coastal Current (CPDCC) are indicated by white arrows. SEC: South Equatorial Current; EPCC: Ecuador-Peru Coastal Current; POC: Peru Oceanic Current; PCC: Peru Coastal Current. EUC: Equatorial Undercurrent; pSSCC: primary (northern branch) Southern Subsurface Countercurrent; sSSCC: secondary (southern branch) Southern Subsurface Countercurrent; PCCC: Peru-Chile Countercurrent; PCUC: Peru-Chile Undercurrent; CPDCC: Chile-Peru Deep Coastal Current. Source: Chaigneau et al. (2013).

Figure 1 . 3 :

 13 Figure 1.3: Distribution of the main water masses in the HCS: Cold coastal water (CCW), Surface subtropical water (SSW), Surface equatorial water (SEW), Surface tropical water (STW). Source: Ayón et al. (2008a).

Figure 1 . 4 :

 14 Figure 1.4: Upper plot: OMZ thickness and upper boundary in the Eastern South Pacific. Thickness is colour-coded according to the colour bar on the right-hand side of the figure; units are in m. The upper boundary of the OMZ is shown in black contour lines with 50 m intervals; Source: Fuenzalida et al. (2009). Lower plot: Meridional variation of the upper OMZ averaged between the coast and 200 km offshore during a survey performed in 2005 (black solid line) and corresponding ± standard deviation (grey shaded area). Source: Bertrand et al. (2010).

Figure 1

 1 Figure 1.5: First ''ecosystem-based" diagram for the Northern Humboldt Current system. Source: Vogt (1948).

Figure 1

 1 Figure 1.8: Trophic flows average in the Northern Humboldt Current System, from the periods1964-1971 and 1973-1981. Flows of wet mass are in units of 10 3 kg.km -2 .yr -1 . Flows leave boxes on the upper half and enter them on the lower half. The width of the line indicates the order of magnitude of trophic flows. Biomass of detritus is a rough estimate. Source:[START_REF] Jarre | Two approaches for modelling fish stock interactions in the Peruvian upwelling ecosystem[END_REF] and[START_REF] Jarre-Teichmann | The potential role of mass-balance models for the management of upwelling ecosystems[END_REF].

Figure 1 .

 1 Figure 1.10: Conceptual model of (a) coastal and (b) open ocean pelagic ecosystems. Near coasts the nutrients are abundant, dense, colonial, centric diatom blooms form. The primary production can be transferred rapidly to small plentiful fish and the food chain is short and efficient. In the open ocean, under low nutrient input, photosynthetic picoplanktonic organisms dominate. The consumers of picoplankton are also very small and have growth rates similar to their prey. Scarce nutrients are recycled and retained in the upper layer. Modified from Chavez et al. (2011).

Finally a recent conceptual

  model, separating the coastal and open pelagic ecosystems (Fig. 1.10), indicated that nutrients were abundant and dense near coasts with the development of transient blooms of colonial centric diatoms. D i a t o m s a c c u m u l a t e b i o m a s s , w h i c h c a n b e transferred rapidly from to plentiful small fish

Figure 1 .

 1 Figure 1.11: Illustration of trophic enrichment in δ 13 C and δ 15 N from primary producers (plants and diatoms), to herbivores, to predators for A) terrestrial ecosystems and B) marine ecosystems. Panel A also shows differences in δ 13 C between food webs based on C 3 (black symbols) and C 4 plants (gray symbols). Values (mean ± SE) were adapted from the literature. Source: Ben-David and Flaherty (2012).

Figure 1 .

 1 Figure 1.12 Mean (±1 SE) δ 13 C versus mean (±1 SE) δ 15 N for various fish (solid symbols) and invertebrates (open symbols) of the Newfoundland and Labrador continental shelf food web.The abscissa represents a continuum of feeding types ranging from highly pelagic (more negative, or depleted in 13 C) to highly benthic (more positive, or enriched in 13 C). The ordinate should represent a continuum of trophic positions. Source:[START_REF] Sherwood | Stable isotope analysis of some representative fish and invertebrates of the Newfoundland and Labrador continental shelf food web[END_REF].

Figure 1 .

 1 Figure 1.13: Isotopic changes during nitrate turnover in the open ocean. Source: Gruber (2004).

Figure 1 .

 1 Figure 1.14 illustrates that even if the δ 15 N isotopic values of two consumers are identical (the two stars), they can have very different trophic position (TP) when adjusted to the corresponding isotopic baseline values.

Figure 1 .

 1 Figure 1.14: Conceptual representation of the calculation used to estimate trophic position of consumers (relative to assumed trophic position 2.3 for benthic or pelagic invertebrates) from the Newfoundland and Labrador continental shelf food web. The two stars have the same δ 15 N signature (i.e. 15.5‰), but very different trophic positions; the white star represents a nearshore, benthic consumer that is much closer to the baseline than the black star (an offshore, pelagic consumer). Calculation assumes classical trophic fractionation factors of 3.4‰ for δ 15 N and 0.8‰ for δ 13 C. Hence, consumers are offset to the right of their appropriate baseline δ 13 C signature, which is actually the δ 13 C signature of their prey (i.e. we assumed trophic fractionation of δ 13 C only between consumers and their prey). Source:[START_REF] Sherwood | Stable isotope analysis of some representative fish and invertebrates of the Newfoundland and Labrador continental shelf food web[END_REF].

Figure 2 .

 2 Figure 2.1: The locations of trawls (grey dots) from which anchovetas were collected for trophodynamic analysis. The black solid line indicates the 200 m isobaths

Figure

  Figure 2.2: Percentage contribution of phytoplankton (green), copepods (red), euphausiids (orange) and other zooplankters (grey) by number (total num) and to dietary carbon (total carb)for the total set of 21,203 anchoveta stomach contents. Also shown are the percentage contribution to dietary carbon by prey type for each of the 23 surveys from which anchoveta stomach contents were analyzed; surveys are identified along thex-axis, with the first two numbers corresponding to the year, the following two numbers the start month of the survey, and the last two numbers the end month of the survey.

Figure 2 . 3 :

 23 Figure 2.3: Scatter plot (grey dots) and cubic spline smoother fits (black solid lines) of GAM models (the black dashed lines show the 95% confidence limits) based on anchoveta stomach fullness according to time of day, latitude, distance from the coast (DC) and sea surface temperature (°C) for the complete set of 21,203 stomach contents (top panel) and for each of the 23 surveys from which stomach contents were analyzed (lower panels); surveys are identified along they-axis, with the first two numbers corresponding to the year, the following two numbers the start month of the survey, and the last two numbers the end month of the survey.The lefty-axis shows stomach fullness (% WW) and scaling along the righty-axes is relative and corresponds to the spline smoother that was fitted on the data such that ay-value of zero is the mean effect of the variables on the response.

Figure 2 .

 2 Figure 2.3 (continued)

Figure 3 . 1

 31 Figure 3.1 Per cent composition of prey groups (■ diatoms, ■ other phytoplankters, ■ calanoid copepods, ■ other copepods, ■ euphausiids and, ■ other zooplankters) to Engraulis ringens dietary carbon for the periods: (a) 1954-1969[START_REF] Rojas De Mendiola | Some observations on the feeding of the Peruvian anchoveta Engraulis ringens J. in two regions of the Peruvian coast[END_REF][START_REF] Christensen | Predator foraging capabilities and prey antipredator behaviours: preversus postcapture constraints on size-dependent predator-prey interactions[END_REF][START_REF] Levin | Oxygen minimum zone benthos: adaptation and community response to hypoxia[END_REF] (Espinoza & Bertrand, 2008).

Figure 3 . 2

 32 Figure 3.2 Frequency of occurrence (grey bars -left y-axis) of euphausiids in Engraulis ringens stomachs, and euphausiids dominance from Ayón et al. (2011) in zooplankton net samples (additive model results; black line -right y-axis). No bar indicates absence of data.

Figure 3 . 3

 33 Figure 3.3 Percent contribution of prey groups ( diatoms, other phytoplankters, calanoid copepods, other copepods, euphausiids, other zooplankters) to dietary carbon of Engraulis ringens by size-class (a), degree of latitude (b), distance to the shelf break (in km, zero corresponds to the shelf break, values are negative inshore and positive offshore) (c) and diel period (d) from years 1996-2003.

Figure 4 . 2 :

 42 Figure 4.2: Average percentage contribution to dietary carbon (%C) by prey category for Engraulis ringens and Sardinops sagax in the Northern Humboldt Current system, S. sagax in the Southern Benguela system, and Sardina pilchardus in the Northern Canary Current system.

Figure 4 . 3 :

 43 Figure 4.3: Average percentage contribution to dietary carbon (%C) by prey size class (mm) forSardinops sagax in the Southern Benguela system, Sardina pilchardus in the Northern Canary system (Iberian coast), and S. sagax (sard) and Engraulis ringens (anch) in the Northern Humboldt system.

Figure 5 . 1 .

 51 Figure 5.1. Sampling station per year, red=2008, green=2009, blue=2010, black=2011; the blue line indicate the 200 m isobath.

Figure 5 . 2 :

 52 Figure 5.2: δ 13 C vs. δ 15 N biplot for zooplankton, nekton and top predators collected in the NHCS: (a) depicting isotopic niche space (b) δ 13 C according to the distance from the coast (km), (c) δ 15 N according to the distance from the coast (km). Values are x±SD (denoted inTable 1).

  δ 13 C and δ 15 N values showed a high variability between and within species (Fig.5.2a). The δ 15 N mean values varied between 18.3‰ (fur seal, Arctocephalus australis) and 7.9‰ (copepods) (Fig.5.2a). Higher δ 15 N values were found for the fur seal, the jack mackerel Trachurus murphyi a n d t h e s e a b i r d s ( G u a n a y c o r m o r a n t Phalacrocorax bougainvillii a n d Peruvian booby Sula variegata). Lowest δ 15 N values were encountered for copepods, Euphausiids and Myctophidae (Table

Figure 5 . 4 :

 54 Figure 5.4: Latitudinal variability of δ 15 N values. (red=2008; green=2009; blue=2010; black=2011).

Figure 5 . 5 :

 55 Figure 5.5: Cross shore (distance from the coast) variability of δ 15 N values. (red=2008; green=2009; blue=2010; black=2011).

  11.7 and 7.5ºS. The squat lobster had a higher TP (3.8) than a jumbo squid of 40 cm (3.5) at 7.5ºS but lower(3.6 vs. 3.8) at 11º7S.

Figure 5 . 6 :

 56 Figure 5.6: Predicted δ 15 N values from the two LME models: north and south of 7.5ºS. For each species the latitudinal range of prediction covers the one where individuals were sampled.

Figure 6

 6 Figure 6.1. Conceptual schematic of how physical forcing may lead to environments that favour either sardine or anchovy. Source: van der Lingen et al. (2006).

Figure 6 . 2 :

 62 Figure 6.2: (A) Anchovy and sardine biomass estimated using virtual population analysis (VPA). Source: Ñiquen et al. (2000), R. Guevara pers. com. (B) Dominance of euphausiids and small zooplankton from generalized additive model (GAM) results. Source: Ayón et al. (2011).

Figure 6 . 3 .

 63 Figure 6.3. Conceptual model of decadal changes in anchovy and sardine populations in the southeastern tropical Pacific. A: Schematic of the temporal evolution of large plankton and anchovy (blue solid line), and oxygen, small plankton and sardine (red solid line) between 1960 and 2010. B: Energetic costs of feeding on dominant plankton size-spectra for anchovy and sardine according to the scenarios from A. C: Schematic of the available habitat for anchovy (blue shaded area) and sardine (red shaded area). Source: Bertrand et al. (2011).

Fig. 6

 6 Fig. 6. 4. Anchovy and squat lobsters.

Figure 6

 6 Figure 6.5: GAMs of muscle δ 13 C and δ 15 N values as a function of latitude, mantle length (ML) and distance to shelf break (DSB). Source: Argüelles et al. (2012).

Figure 6 . 6 :

 66 Figure 6.6: Conceptual model and cross-shore profiles of oxygen and organisms distribution.The lower panel shows the mean (spline smooth) cross-shore profiles of dissolved oxygen saturation in % (grey 639 dashed line), depth of the 2 ml.l-1 isoline in m (black solid line) and the acoustic-estimated biomass of anchovy (blue solid line) and the jumbo squid acousticestimated biomass (red solid line). Oxygen and anchovy data come from[START_REF] Bertrand | Oxygen: A Fundamental Property Regulating Pelagic Ecosystem Structure in the Coastal Southeastern Tropical Pacific[END_REF]; jumbo squid data come from IMARPE, unpublished data. Note that the oxygen data cover the range 7ºS to 18ºS. The upper part shows the cross-shore distribution of jumbo squid along its ontogenetic cycle. The colours in the arrow represent the schematic range of distribution and proportional abundance of the three main prey groups i.e., the other cephalopoda, euphausiids and mesopelagic fish. Source:Alegre et al. (2014).

  En effet, nos résultats supportent l'hypothèse selon laquelle cette espèce s'alimenterait en partie sur les oeufs et larves d'anchois, menaçant ainsi les premiers stades de vie des espèces exploitées. Les valeurs différentes de δ 13 C suggèrent cependant que les anchois et les galathées pélagiques ne partageraient pas exactement le même habitat, ce qui réduirait potentiellement la compétition directe et/ou la prédation.

). Ainsi, la compétition trophique entre les sardines et les anchois est minimisée dans le NSCH par le partage de la ressource zooplancton selon sa taille, comme cela a déjà été montré dans d'autres écosystèmes. Ces résultats remettent en question la compréhension première de la position des petits poissons pélagiques (zooplanctonophage et non phytoplanctonophage) dans la chaine trophique ce qui implique de reconsidérer le fonctionnement et les modèles trophiques du NCHS.

Afin d'obtenir une compréhension plus globale de la position trophique relative des principaux composants du NHCS une approche basée sur des analyses d'isotopes stables (δ 13 C et δ 15 N) a été utilisée. Pour ce faire, la signature isotopique d'échantillons de 13 groupes taxonomiques (zooplancton, poissons, calmars et prédateurs supérieurs) prélevés entre 2008 et 2011 a été déterminée. Les valeurs de δ 15 N obtenues sont fortement impactées par l'espèce, la taille et la latitude. Le long de la cote péruvienne, la zone de minimum d'oxygène devient en effet plus intense et plus superficielle au sud de ~7.5ºS impactant fortement la valeur de δ 15 N de la ligne de base. Nous avons donc utilisé un modèle linéaire à effet mixte prenant en compte les effets latitude et taille afin de prédire la position trophique relative des composants clés de l'écosystème. Ces analyses isotopiques confirment les résultats issus des contenus stomacaux sur le régime alimentaire de l'anchois et mettent en évidence l'importance potentielle d'une composante souvent négligée de l'écosystème, la galathée pélagique Pleuroncodes monodon.

Mots clés: Système Nord du Courant de Humboldt, écologie trophique, zone de minimum d'oxygène, anchois Engraulis ringens, sardine Sardinops sagax, calmar géant Dosidicus gigas, Pleuroncodes monodon, isotopes stables, contenus stomacaux.

Table 1

 1

	Water mass	Abbreviation	Main characteristics
	Cold coastal water	CCW	Salinity: 34.80-35.05; superficial temperature: 15-17ºC in winter
			and 15-19ºC in summer. Influenced by upwelling, very productive
	Surface subtropical water	SSW	Salinity >35.10; superficial temperature: 17-25ºC in winter and 20-
			25ºC in summer. Oceanic characteristics, oligotrophic water mass
	Surface equatorial water	SEW	Salinity: 34.00-34.80; superficial temperature: 20-26ºC in winter
			and 21-26ºC in summer. Mesotrophic water mass
	Surface tropical water	STW	Salinity <34.00; superficial temperature: >23ºC in winter and >26ªC
			in summer.

.1 Main characteristics of the water masses presents in the Humboldt Current System (source:

Bertrand et al. 2004a

).

Table 2 .

 2

	Survey code	Start date	End date	Nº of trawls	Nº of stomach	Anchoveta length range (Min -Max)
	960204	10 Feb. 1996	01 Apr. 1996	49	633	9 -19
	960809	11 Aug. 1996	27 Sep. 1996	70	1021	5.5 -19.5
	961112	16 Nov. 1996	19 Dec. 1996	39	639	9.5 -19
	970204	13 Feb 1997	23 Apr. 1997	45	739	8 -19.5
	970910	04 Sep. 1997	05 Oct. 1997	51	1259	6.5 -19
	980305	27 Mar. 1998	01 My. 1998	35	687	9.5 -18
	980809	23 Aug. 1998	17 Sep. 1998	36	624	4 -16
	981112	30 Nov. 1998	21 Dec. 1998	36	798	4 -16.5
	990203	14 Feb. 1999	28 Mar. 1999	75	1266	10 -17
	990809	28 Aug. 1999	17 Sep. 1999	19	336	10 -18
	991112	12 Nov. 1999	14 Dec. 1999	70	1412	8 -18
	000102	20 Jan. 2000	26 Feb. 2000	124	2277	6.5 -18.5
	000607	10 Jun. 2000	06 Jul. 2000	50	928	7.5 -18.5
	000809	28 Aug. 2000	23 Sep. 2000	38	574	7 -18.5
	001011	11 Oct. 2000	13 Nov. 2000	45	846	6.5 -18
	010204	03 Mar. 2001	10 Apr. 2001	78	1630	8.5 -18.5
	010708	05 Jul. 2001	06 Aug. 2001	83	1596	6.5 -19
	020203	21 Feb. 2002	18 Mar. 2002	83	1650	7 -18
	0208	10 Aug. 2002	31 Aug. 2002	30	281	9.5 -17
	021011	01 Oct. 2002	13 Nov. 2002	36	416	10 -17
	030203	26 Feb. 2003	31 Mar. 2003	45	524	6.5 -17.5
	030809	16 Aug. 2003	15 Sep. 2003	26	243	7.5 -17.5
	031012	24 Oct. 2003	10 Dec. 2003	36	824	4.5 -17.5

1: Survey code, start and end date, the number of trawls, and the number, length range (in cm) of anchoveta collected for stomach content analysis during this study.

Table 2 .

 2 2: Type and size of zooplankton recorded from anchoveta stomach contents; length was estimated by IMARPE except where indicated.

	Name	Type	Length (mm)
	Acartia	Calanoida	0.9375 a
	Aetideus	Calanoida	1.5
	Calanus	Calanoida	2 b
	Calocalanus	Calanoida	1 c
	Candacia	Calanoida	1.1666 a
	Centropages	Calanoida	1.5 a
	Clausocalanus	Calanoida	0.65 b
	Clytemnestra	H a r p a c t i c o i d a	0 . 6 6 b
	Copilia	Poecilostomatoida	1.75 a
	Corycaeus	Cyclopoida	1 a
	Corycella	Poecilostomatoida	0.65
	Euaetideus	Calanoida	0.9 b
	Eucalanus	Calanoida	4
	Euchaeta	Calanoida	1.08 b
	Euchirella	Calanoida	2.5 b
	Euterpina	Harpacticoida	0.5 c
	Haloptilus	Calanoida	1.2 a
	Lubbockia	Poecilostomatoida	0.8
	Lucicutia	Calanoida	1.03 a
	Macrosetella	Harpacticoida	0.7
	Mecynocera	Calanoida	0.899 a
	Microsetella	Harpacticoida	0.7 c
	Nonocalanus	Calanoida	1.5
	Oithona	Cyclopoida	0.5 a
	Oncaea	Poecilostomatoida	0.667 a
	Paracalanus	Calanoida	0.8 b
	Phaena	Calanoida	0.8
	Pleuromamma	Calanoida	1.2 b
	Pontellina	Calanoida	1.5
	Rhincalanus	Calanoida	4
	Saphirina	Poecilostomatoida	2.3 b
	Scolecithrix	Calanoida	1.7 b
	Scolecithtricella	Calanoida	1.8 b
	Temora	Calanoida	1.2 b
	Harpacticoida		0.6 c
	Copepoditos		0.5 a
	Restos de copépodos		0.25
	Euphausiacea		12 b
	Amphipoda		8 b
	Ostracoda		0.7
	Zoea		2
	Megalop		3
	Cirriped larvae		0.1
	Emerita sp. Larvae		5
	Decapod undetermined larvae		5
	Pagurus sp.		5
	Pleuroncodes monodon		7
	Decapoda Reptantia n/i		3
	Galatheidae		7
	Bivalvia		1.25
	Gastropoda		1.25
	Anchoveta eggs		vol: 0.27 mm 3d
	Apendicularia		10
	Echinoderm larvae		1
	Chaetognata		8 b
	Squid remains		10
	Polychaeta		10
	Anchoa sp. Eggs		vol: 0.27 mm 3
	Engraulidae eggs		vol: 0.27 mm 3
	Fish eggs		vol: 0.27 mm 3
	Fish larvae		15
	Fish undetermined		15
	Engraulidae		15
	Vinciguerria sp.		15
	Myctophidae		15

a http://earth.leeds.ac.uk/cyclops/data/ncfs-zooplank.xls b

[START_REF] Santander | Catalogo de zooplancton en el mar peruano[END_REF]

c

[START_REF] Tudela | Trophic ecology of the European anchovy Engraulis encrasicolus in the Catalan Sea[END_REF]

d

[START_REF] Llanos-Rivera | Latitudinal and seasonal egg-size variation of the anchoveta (Engraulis ringens) off the Chilean coast[END_REF]

Table 2 . 3 :

 23 Relationships employed to calculate dry weight and carbon content of the different categories of phytoplankton and zooplankton. Carbon contents not in μg were transformed in μg for comparison; PL: prosome length (μm); TL: total length (mm); DW: dry weight (μg); WW: wet weight (μg).

	Phytoplankton	Volume (µm 3 ) to Carbon (pg)
	Diatoms	C=0.288 Vol 0.811a
	Dinoflagellates and other phytop.	C=0.760 Vol 0.819a
	Tintinnids, Foraminifera, Radiolaria	C=445.5 + 0.053 Vol b
	Zooplankton	Length to dry weight (µg) regression	dry weight (µg) to carbon (µg)
	Calanoid copepods	Ln(DW) = 2.74ln(PL) -16.41 c	C = 0 . 4 2 4 D W c
	Cyclopoid copepods	Ln(DW) = 1.96ln(PL) -11.64c	C=0.424 DW c
	Harpacticoid copepods	Ln(DW) = 1.96ln(PL) -11.64 c	C = 0 . 4 2 4 D W c
	Poecilostomatoid copepods	Ln(DW) = 1.96ln(PL) -11.64 c	C = 0 . 4 2 4 D W c
	Euphausiacea	DW=0.0012 TL 3.16ci	C = 0 . 4 2 4 D W cj
	Amphipoda, Echinoderm larvae	DW=0.005 TL 2.311ci	C = 0 . 3 7 0 D W dj
	Ostracods, zoea, megalops, crustacea larvae, hermit crab and Galatheidae	DW=3.946 TL 2.436c	C = 0 . 4 2 4 D W c
	Bivalvia, Gasteropod	DW=47.386 TL 3.663d	C = 0 . 4 2 4 D W c
	Fish eggs	DW=0.093 Vol+0.0012 ck	C = 0 . 4 5 7 D W c
	Apendicularia	DW=11.3TL 1.77e	C = 0 . 3 8 7 D W e
	Chaetognaths	DW=0.00097 TL 2.2365fi	C = 0 . 2 9 0 D W f
	Fish, squid and fish larvae	DW=0.0001 TL 3.582di	C = 0 . 3 8 D W g
	Polychaeta	WW=0.01 TL 2.136h	C=0.518 DW hj
		DW=0.157 WW hi

a Menden-Deuer and Lessard (2000) b Verity and Langdon (1984) c van der Lingen (2002) d James (1987) e Deibel (1986) f Sameoto (1971) g Parsons et al. (1984) h Alexandrov (2001) i DW in mg j C in mg k volume= 0.27 mm 3 , from Llanos-Rivera and Castro (2004) as anchoveta eggs dominated fish egg fraction

  2.2). Phytoplankton reached maximum levels (40.7% of total carbon content) in anchoveta diet in August-September 1996, and minimum levels (0.07%) in June-July 2000. The contribution of euphausiids was high (≥80% of total carbon content) during November-December 1996, February-April and September-October 1997, August-September 1999, January-February and August-September 2000, and July-August 2001, and the lowest value (5.8%) was recorded in November-December 1998. The maximum contribution to dietary carbon (86.7%) from copepods occurred during November-December 1998, at the end of the 1997-1998 El Niño event, and the lowest value (2.0%) was observed during November-December 1996.

  who reported that Calanus, Centropages and Corycaeus were frequently-observed in anchoveta stomach contents. During the 1997-1998 El Niño, various studies confirmed the numerical i m p o r t a n c e o f c o p e p o d s a n d e u p h a u s i i d s ; whereas amphipods, anchoveta eggs, Vinciguerria sp., and Myctophidae were of minor importance

Table 3 .

 3 1. Year, start and end date, number of trawls, total number of stomachs with prey contents, and length range (cm) ofEngraulis ringens during 1996Engraulis ringens during -2003. .

	Year	Start date	End date	N of trawls	Nº of stomachs (with prey)	Fish total length (Min -Max)
	1996	10 Feb.	01 Apr.	49	157 (147)	11.0 -18.5
		16 Nov.	19 Dec.	39	338 (329)	9.5 -19.0
	1997	13 Feb.	23 Apr.	45	186 (184)	10.5 -19.5
		04 Sept.	05 Oct.	51	117 (106)	11.5 -18.5
	1998	27 Mar.	01 May.	35	148 (124)	10.0 -17.0
		23 Aug.	17 Sept.	36	348 (308)	4.0 -15.0
		30 Nov.	21 Dec.	36	274 (232)	4.0 -16.0
	1999	14 Feb.	28 Mar.	75	1197 (1059)	10.0 -17.0
		28 Aug.	17 Sept.	19	279 (178)	10.0 -18.0
		12 Nov.	14 Dec.	70	1243 (999)	8.0 -18.0
	2000	20 Jan.	26 Feb.	124	1965 (1798)	6.5 -18.5
		10 Jun.	06 Jul.	50	928 (859)	7.5 -18.5
		28 Aug.	23 Sept.	38	512 (486)	7.0 -18.5
		11 Oct.	13 Nov.	45	822 (717)	6.5 -18.0
	2001	03 Mar.	10 Apr.	78	1584 (1446)	8.5 -18.5
		05 Jul.	06 Aug.	83	1274 (1208)	6.5 -19.0
	2002	21 Feb.	18 Mar.	83	1362 (1094)	7.0 -18.0
		10 Aug.	31 Aug.	30	269 (267)	9.5 -17.0
		01 Oct.	13 Nov.	36	415 (365)	10.0 -17.0
	2003	26 Feb.	31 Mar.	45	524 (513)	6.5 -17.5
		16 Aug.	15 Sept.	26	243 (237)	7.5 -17.5
		24 Oct.	10 Dec.	36	798 (774)	4.5 -17.5

Table 3 .

 3 2. Factors used to pool the data.

	Factors	Classes
	Engraulis ringens size class (cm)	By 2 cm size-class: [4-6>, [6-8>, [8-10>, [10-12>, [12-14>, [14-16>, [16-18>, [18-20>
	Latitude (ºS)	By degree of latitude from 03º30'S to 18ºS

Table 3

 3

	.3 Results of non-parametric ANOVA (Kruskal-Wallis test); n=1938 (the 13,430 stomachs pooled
	by size class).
		Size-class	Latitude	Distance to the shelf break	Diel period
	Items	d.f.=7	d.f.=15	d.f.=3	d.f.=3
	Diatoms	H=60.5, p<0.001 H=47.7, p<0.001	H=73.0, p<0.001	H=16.7, p=0.001
	Other phytoplankters H=53.1, p<0.001 H=51.3, p<0.001	H=22.6, p<0.001	H=14.2, p=0.003
	Calanoid copepods	H=24.2, p=0.001 H=44.4, p<0.001	H=10.1, p=0.018	H=80.5, p<0.001
	Other copepods	H=35.8, p<0.001 H=49.5, p<0.001	H=11.0, p=0.012	H=64.2, p<0.001
	Euphausiids	H=94.7, p<0.001 H=84.1, p<0.001	H=115.6, p<0.001	H=72.5, p<0.001
	Other zooplankters	H=31.0, p<0.001 H=111.8, p<0.001 H=36.8, p<0.001	H=20.1, p<0.001

Table 4 .

 4 1: Survey characteristics.

				Latitudinal	Transect extension		N° of sardine	Sardine Length range
	Survey code	Start date	End date			N° of trawls
				limits	(nmi)		stomachs	(min. -max.)
	1996/11-12	16 November 1996	19 December 1996	5°S-18°S	100	1	16	25-30
	1997/02-04	13 February 1997	23 April 1997	5°S-18°S	80-90	5	67	21-30
	1997/09-10	04 September 1997	05 October 1997	3º30'S-18ºS	80	6	79	10-30
	1998/03-05	27 March 1998	01 May 1998	3º30'S-18ºS	50	18	287	11-29
	1998/05-06	26 May 1998	24 June 1998	4ºS-18ºS	70	7	59	16-21
	1998/08-09	23 August 1998	17 September 1998	5°S-18°S	80	4	47	18-22

  Table4.1). Ninety-one prey taxa were identified, including 24 diatom genus, 10 dinoflagellates, by euphausiids (38.7%) and then other zooplankton (5.3%). The copepod carbon fraction was dominated by Eucalanus (58.4% of the total copepod contribution to dietary carbon), Calanus (21.5%), Rhincalanus (5.4%) and Centropages (4.5%). The phytoplankton carbon fraction was dominated by the dinoflagellates Protoperidinium (31.4% of the total phytoplankton contribution to dietary carbon) and Ceratium (16.2%), and the diatom Coscinodiscus (27.3%).

	When the surveys are considered individually strong variability in dietary composition is seen
	(Fig. 4.1a). The maximum contribution to dietary carbon by copepods (90.8%) occurred during
	November-December 1996, whilst the minimum (33.3%) was observed during February-April
	1997. The maximum contribution by euphausiids was observed in February-April 1997 (61.2%)
	and August-September 1998 (47.5%), whilst the minimum (5.8%) occurred in November-
	December 1996. The maximum contribution by other zooplankters occurred in March-May
	1998 (16.2% of dietary carbon) and June 1998 (26.9%), with gastropods also having their
	maximum contributions during these surveys. The contribution to dietary carbon made by
	phytoplankton was very low (<0.3%) during the El Niño 1997-1998, whereas the highest value
	(4.9%) was observed in February-April 1997 just before the El Niño event.

Table 5 .

 5 1. Data description.

	Taxonomic category	Code	Year range	Storage	Sample	n	Body length	Latitudinal	Longitudinal	δ 13 C	δ 15 N	C:N
							range (mm)	range (ºS)	range (ºW)	mean ± SD	mean ± SD	mean ± SD
	Copepod	Copepod	2008-2010	Frozen	Full organism	33		3.85 -14.65	76.07 -83.57	-20.3 ± 1.3	7.6 ± 1.3	4.8 ± 0.6
	Euphausiids	Euph	2008-2011	Frozen	Full organism	17	19 -24	3.85 -14.65	76.07 -81.87	-18.1 ± 0.9	8.8 ± 0.6	3.9 ± 0.2
	Pleuroncodes monodon	SLobst	2008-2011	Frozen	Muscle tissue	43	8 -22.9	4.42 -12.43	77.35 -81.55	-15.2 ± 0.4	13.3 ±1.2	3.9 ± 0.2
	Dosidicus gigas	JSquid	2008-2011	Alcohol	Muscle tissue	337	20 -988	3.48 -18.32	71.12 -84.25	-17.1 ± 0.9	13.4 ± 3.4	3.2 ± 0.1
	Myctophidae	Myct	2008-2010	Frozen	Muscle tissue	35	35 -100	5.25 -7.33	81.40 -83.57	-19.0 ± 1.0	9.0 ± 1.2	3.4 ± 0.3
	Vinciguerria lucetia	Vincig	2008-2011	Frozen	Muscle tissue	23	32 -65	5.25 -15.05	80.25 -83.57	-18.5 ± 0.6	10.7 ± 2.8	3.4 ± 0.1
	Engraulis ringens	Anch	2008	Frozen	Muscle tissue	165	31 -165	4.42 -16.02	74.47 -82.25	-16.3 ± 1.2	12.1 ± 2.3	3.3 ± 0.1
	Scomber japonicus	Mackerel	2008-2011	Frozen	Muscle tissue	8	32 -230	5.25 -15.85	76.15 -82.25	-17.6 ± 1.7	11.8 ± 3.3	3.2 ± 0.1
	Trachurus murphyi	JM	2008-2011	Frozen	Muscle tissue	14	118 -310	4.42 -15.85	76.15 -81.55	-17.2 ± 0.6	17.5 ± 2.9	3.2 ± 0.0
	Merluccius gayi peruanus	Hake	2008	Frozen	Muscle tissue	84	100 -480	3.70 -7.47	80.28 -81.38	-16.3 ± 0.6	11.4 ± 0.5	3.2 ± 0.1
	Sula variegata	Booby	2009-2010	Alcohol	Blood	37	n.d.	11.75	77.47	-14.7 ± 0.2	14.7 ± 0.2	3.3 ± 0.0
	Phalacrocorax bougainvillii	Guanay	2009-2010	Alcohol	Blood	37	n.d.	11.75	77.47	-15.0 ± 0.2	15.1 ± 0.2	3.3 ± 0.1
	Arctocephalus australis	Fur seal	2010	Alcohol	Blood	16	1200 -1490	15.37	75.18	-16.9 ± 0.3	18.3 ± 0.4	3.5 ± 0.0

Table 1 )

 1 .

Table 5 .

 5

	Species	Fixed body	Predicted δ 15 N at	Predicted	Predicted δ 15 N at	Predicted
		size (mm)	7.5°S	TP at 7.5°S	11.7°S	TP at 11.7°S
	Copepod	1	7.6	2.5*	9.8	2.5*
	Euphausiid	2	8.9	2.9
	Pleuroncodes monodon	20	12.0	3.8	13.6	3.6
	Myctophidae	50	9.3	3.0
	Vinciguerria lucetia	50	9.4	3.0	12.6	3.2
	Engraulis ringens	140	11.0	3.5	13.5	3.6
	Scomber japonicus	200	11.5	3.6	12.3	3.2
	Trachurus murphyi	250			15.1	4.0
	Merluccius gayi peruanus	280	11.7	3.7
	Dosidicus gigas	400	11.1	3.5	14.3	3.8
	Sula variegata	400			14.6	4.4
	Phalacrocorax bougainvillii	400			15	4.5
	Arctocephalus australis	1300			15.9	4.8

2. Predicted δ 15 N (‰) values and trophic position (TP) at 7.5°S and 11.7°S. * Assumed

  Annex 3.6 Results of Tukey-type multiple comparisons post hoc analyses of calanoids contribution in Engraulis ringens diet by 1º of latitude. In bold, significant differences (p<0.05).

								ANNEXES
	Annexe 1
		3 º 3 0 '	4	5	6	7	8	9	1 0	1 1	1 2	1 3	1 4	1 5	1 6
	3º30'
	4	0.999
	5	0.991	1.000
	6	1.000	0.999	0.930
	7	0.997	1.000	1.000	0.628
	8	1.000	0.999	0.993	0.999	0.891
	9	0.981	1.000	0.999	0.266	0.999	0.521
	10	0.997	1.000	1.000	0.624	1.000	0.919	0.999
	11	0.839	0.999	0.998	0.036	0.933	0.111	0.999	0.992
	12	1.000	0.999	0.440	0.999	0.053	0.954	0.014	0.100	0.002
	13	1.000	0.999	0.454	0.999	0.053	0.980	0.013	0.105	0.001	1.000
	14	0.999	1.000	0.999	0.857	1.000	0.987	0.999	1.000	0.928	0.134	0.188
	15	0.994	1.000	1.000	0.822	1.000	0.983	0.999	1.000	0.987	0.243	0.279	1.000
	16	1.000	0.999	0.997	0.999	0.984	1.000	0.793	0.976	0.300	0.997	0.999	0.998	0.995
	17	1.000	0.999	0.999	0.999	0.999	1.000	0.989	0.999	0.769	0.999	0.999	0.999	0.999	1.000
	18	0.999	1.000	0.999	0.994	0.999	0.999	0.999	0.999	0.890	0.551	0.649	1.000	0.999	0.999	0.999
		3 º 3 0 '	4	5	6	7	8	9	1 0	1 1	1 2	1 3	1 4	1 5	1 6
	3º30'
	4	0.956
	5	0.604	0.999
	6	0.876	1.000	0.970
	7	0.916	1.000	0.856	1.000
	8	0.779	1.000	0.996	1.000	0.999
	9	0.365	0.979	0.999	0.110	0.038	0.268
	10	0.867	1.000	0.972	1.000	1.000	1.000	0.150
	11	0.502	0.999	1.000	0.850	0.716	0.982	0.999	0.925
	12	0.937	1.000	0.836	1.000	1.000	0.999	0.050	0.999	0.647
	13	0.803	1.000	0.999	0.999	0.999	1.000	0.520	1.000	0.996	0.999
	14	0.444	0.996	0.999	0.397	0.208	0.680	1.000	0.479	0.999	0.232	0.821
	15	0.624	1.000	0.999	0.999	0.999	1.000	0.995	0.999	1.000	0.998	0.999	0.999
	16	0.525	0.999	1.000	0.861	0.820	0.988	0.999	0.939	1.000	0.784	0.994	1.000	0.999
	17	0.214	0.932	0.996	0.175	0.124	0.402	0.999	0.262	0.963	0.135	0.478	0.999	0.947	0.997
	18	0.453	0.967	0.999	0.362	0.246	0.640	1.000	0.452	0.999	0.263	0.788	1.000	0.997	0.999	0.999

Figure 2.3 (continued)

Published in the Journal of Fish Biology Espinoza, P. & Bertrand, A. (2014) Ontogenetic and spatiotemporal variability in anchoveta (Engraulis ringens) diet off Peru. Journal of Fish Biology, 84: 422 435.

Acknowledgements

............................

Acknowledgments

The authors gratefully thank the 'laboratorio de ecología trófica' staff from Instituto del Mar del Perú (IMARPE) for having facilitated the use of the data. This work is a contribution of the Research Unit 'Upwelling Ecosystems' UR 097 and of the Interdepartmental Thematic Action ''Humboldt Current System" from IRD. We warmly thank Patricia Ayón, François Gerlotto, Mariano Gutiérrez, Astrid Jarre, Salvador Peraltilla, Gordon Swartzman, Jorge Tam and Marc Taylor; particular thanks to Blanca Rojas de Mendiola for her very valuable help when interpreting the results and to Francis Juanes for interesting comments and English edition. Finally we thanks Andrew Bakun and Gary Sharp for their interesting comments and Carl van der Lingen for his continuous and determinant help during all the process of this work.

Acknowledgements

The authors gratefully thank the 'laboratorio de ecología trófica' staff from the Instituto del Mar del Perú (IMARPE) for having facilitated the use of the data. T. Miller is warmly thanked for English edition and useful comments. A. Alegre and F. Ménard are thanked for their help in data processing. This work is a contribution to the cooperative agreement between the Instituto del Mar del Peru (IMARPE), the Institut de Recherche pour le Developpement (IRD), and of the LMI DISCOH. PE was supported by an individual BEST grant from the IRD managed by Campus France.

Acknowledgements

The authors gratefully thank the 'laboratorio de ecología trófica' staff from Instituto del Mar del Perú (IMARPE) for having facilitated the use of the data. We warmly thank Patricia Ayón and Jorge Tam for very helpful discussions. We also warmly thank the three Referees their helpful insights and comments. This work is a contribution of the Research Unit 'Upwelling Ecosystems' UR 097 from IRD.

Acknowledgements

This work is a contribution to the cooperative agreement between the Instituto del Mar del Peru (IMARPE), the Institut de Recherche pour le Developpement (IRD), and of the LMI DISCOH. This publication was made possible through support provided by the ANR TOPINEME. We would like to thank J.M. Munaron for its invaluable help in stable isotope sample preparation, together with G. Guillou and P. Richard from the LIENSs laboratory for stable isotope analysis. PE was financially supported provided by the BEST grant from IRD and managed by Campus France.

CHAPTER 2 Revisiting Peruvian anchovy (Engraulis ringens) trophodynamics provides a new vision of the Humboldt Current system

Published in Progress in Oceanography Espinoza, P. & Bertrand, A. (2008) Revisiting Peruvian anchovy (Engraulis ringens) trophodynamics provides a new vision of the Humboldt Current system. Progress in Oceanography,79,[215][216][217][218][219][220][221][222][223][224][225][226][227]. diatoms, dinoflagellates, other phytoplankters, copepods, euphausiids, gastropods and other zooplankters, by number (Total Nb.) and by contribution to dietary carbon (Total Carb.) for (a) the pooled sardine stomach content data (two left histograms), and for each of the 6 surveys from which sardine stomach contents were analyzed off Peru (surveys are identified along the x-axis, with the first two numbers corresponding to the year, the following two numbers the start month of the survey, and the last two numbers the end month of the survey), and (b) by number (%N) and by contribution to dietary carbon (%C) for Sardinops sagax in the Southern Benguela current system (SBCS), Sardina pilchardus in the Northern Canary current system (NCCS), and S. sagax in the Northern Humboldt Current system (NHCS).

Numerical analysis of the diet when data from all the surveys were combined indicated that phytoplankton strongly dominated sardine diet and comprised 99.2% on average of all ingested prey, with copepods representing only 0.7%, euphausiids <0.1%, and other zooplankters <0.1% of the total number of prey (Fig. 4.1a). However, this view of sardine diet changes when prey carbon content is considered (Fig. 3.1a); zooplankton was by far the most important component and contributed 98.5% of dietary carbon, whilst phytoplankton contributed the remaining 1.5%. Among zooplankters, copepods dominated and contributed 54.5% of dietary carbon, followed

CHAPTER 5

Trophic pathways in the northern Humboldt Current system: new insight from stable isotopes analysis Abstract:

Here we aim at better understand the trophic relationships in the northern Humboldt Current system (NHCS) following the route from zooplankton to top predators in an environment strongly influenced by persistent upwelling and oxygen minimum zone (OMZ). For that purpose we analyzed the δ 13 C and δ 15 N stable isotope values of thirteen taxonomic categories collected off Peru from 2008 -2011, i.e., zooplankton, fish, squids and air-breathing top predators. The δ 15 N isotope signature was strongly impacted by the species, the body length and the latitude.

Along the Peruvian coast, the OMZ get more intense and shallow south of ~7.5ºS impacting the baseline nitrogen stable isotopes. Also, the sampling design was unbalanced in terms of body length and geographical position. To compare δ 15 N values and being able to estimate the trophic position (TP) of each species, we employed a linear mixed-effects modelling approach taking into account the latitudinal and body length effects. We provide a new vision of the relative trophic position of key ecosystem components. Also we confirm previous stomach content-based results on anchoveta Engraulis ringens TP and highlight the potential remarkable importance of an often neglected ecosystem component, the squat lobster Pleuroncodes monodon. Indeed, our results support the hypothesis according to which this species forage to some extent on fish eggs and larvae and can thus predate on the first life stages of exploited species. However, the δ 13 C values of anchoveta and squat lobster suggest that these two species do not exactly share the same habitat, with anchoveta having a larger and more offshore foraging range. This would potentially reduce some direct competition and/or predation.

Key words: Peru, trophic ecology, oxygen minimum zone, anchoveta Engraulis ringens, zooplankton, top predators, trophic position, Squat lobster Pleuroncodes monodon, jumbo squid Dosidicus gigas and fish scales [START_REF] Gallardo | Relaciones ecológicas de las comunidades bentónicas y bentodemersales de la plataforma continental de Chile central[END_REF]. In the California Current, pelagic P. planipes feeds on protists, zooplankton (mainly copepods, euphausiids and chaetognaths) and large diatoms; the diatom fraction being higher when diatoms are abundant [START_REF] Longhurst | The role of pelagic crabs in the grazing of phytoplankton off Baja California[END_REF]. According to [START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF] the squat lobster may also forage on eggs or larvae including those of anchoveta. Since the anchoveta and the squat lobster share a similar ecological niche, [START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF] stated that "a potential negative competition and predation exists between these species". The high TP we obtained for the squat lobster seems to support the hypothesis according to which this species forage to some extent on fish eggs and larvae and can thus predate on the first life stages of exploited species. The δ 13 C values of these two species however suggest that anchoveta and squat lobster do not exactly share the same habitat, with anchoveta having a larger and more offshore foraging range. This is indeed what [START_REF] Gutiérrez | Ecological niches and areas of overlap of the squat lobster 'munida' (Pleuroncodes monodon) and anchoveta (Engraulis ringens) off Peru[END_REF] observed through acoustic survey performed between 1998 and 2006 along the Peruvian coast, with the squat lobster restricted to a more coastal region when compared to anchoveta. If these hypotheses are confirmed, this would potentially reduce some direct competition for resources. Specific diet studies should be performed on the squat lobster in the NHCS to conclude. Espinoza and Bertrand (2008) following the method from [START_REF] Christensen | ECOPATH II-a software for balancing steady-state ecosystem models and calculating network characteristics[END_REF] The estimated TP for jumbo squid ranged between 3.5 (north) and 3.8 (south) for a jumbo squid of 40 cm. Such results are in the accordance with [START_REF] Hückstädt | Stable isotope analysis reveals pelagic foraging by the Southern sea lion in central Chile[END_REF] who estimated jumbo squid TP to 3.8 in Chile. However such estimates seem low considering diet studies. Indeed Alegre et al. (2014) showed that Dosidicus gigas mainly forage on cephalopods and fish. Lower TP organisms such as euphausiids and other crustaceans and mollusks accounted to less than 18%. We could thus have expected a TP ≥ 4. The same occurs for the hake (estimated TP: 3.7) that mainly forage on fish and squids [START_REF] Tam | Changes in the diet of hake associated with El Niño 1997-1998 in the northern Humboldt Current ecosystem[END_REF], crustacean accounting to only ~15%. However our estimates are similar with those for Merlucius gayi gayi in Chile: 3.6 [START_REF] Hückstädt | Stable isotope analysis reveals pelagic foraging by the Southern sea lion in central Chile[END_REF], Merlucius productus in California: 3.3 [START_REF] Miller | Prey dominance shapes trophic structure of the northern California Current pelagic food web: evidence from stable isotopes and diet analysis[END_REF], and Merlucius capensis in South Africa: 3.0 -3.6 [START_REF] Iitembu | Comparison of ontogenetic trophic shift in two hake species, Merluccius capensis and Merluccius paradoxus, from the Northern Benguela Current ecosystem (Namibia) using stable isotope analysis[END_REF].

The mackerel Scomber japonicus showed a lower trophic level (3.2-3.6) than the jack mackerel Trachurus murphyi (4.0) in accordance with the diet differences between these species (Alegre

CHAPTER 6

General conclusions and perspectives organism diet is highly variable in time and space. In the NHCS, marine life needs to have a high (trophic) plasticity to cope with its highly variable environment. Such variability and the role of the oxygen minimum zone on the structure and functioning of the ecosystem -as well as on the baseline nitrogen isotope values -plead for a regular trophic monitoring of the main components of the ecosystem combining stomach content and stable isotopes analyses and other new technologies such as CSIA on amino acids.

Annex 1: Shape, size and volume of phytoplankton cells recorded from anchoveta stomach contents; cell volume was extracted from the literature (see references) or calculated from published geometric shapes and published or estimated cell sizes.

Annexe 2

Annex 2: Contribution by number and carbon content of anchoveta dietary items (data from all surveys combined).

Dietary items

Total number Total Carbon (µg) DIATOMS Actinoptychus 2 2 4 9 0 0 0 1 7 3 3 Amphiprora 1 0 7 0 0 0 1 9 Amphora 1 8 6 0 0 0 6 4 Asterionellopsis 2 6 5 0 4 3 0 0 0 2 8 7 4 2 Asteromphalus 7 1 9 0 0 0 1 5 6 Bacteriastrum 7 9 1 0 0 0 2 5 9 7 Cerataulina 2 6 0 0 0 3 2 Chaetoceros 2 0 6 8 6 6 0 0 0 4 9 0 6 0 Cocconeis 4 0 0 0 0 . 2 6 Corethron 2 1 7 0 0 0 3 0 7 Coscinodiscus 6 0 9 9 4 0 0 0 1 2 8 4 6 3 9 Cylindrotheca 4 0 0 0 0 . 0 7 Detonula 3 2 5 5 1 0 0 0 6 0 9 1 0 Ditylum 8 9 0 9 0 0 0 0 1 9 3 7 3 0 Eucampia 1 6 3 1 3 0 0 0 8 1 0 7 Fragilariopsis 1 4 8 7 0 0 0 3 0 Grammatophora 3 0 0 0 1 Guinardia 1 8 3 1 0 0 0 8 0 4 5 Gyrosigma 1 0 8 2 2 0 0 0 2 4 4 8 1 Hemiaulus 1 5 0 0 0 1 4 Lauderia 7 0 0 0 3 6 Leptocylindrus 2 0 0 0 0 . 2 3 Licmophora 1 6 0 0 0 9 Lioloma 1 1 7 5 7 0 0 0 1 4 9 6 3 Lithodesmium 1 9 6 8 6 0 0 0 7 2 7 3 Navicula 5 0 2 5 0 0 0 9 5 9 Odontella 1 5 1 3 0 0 0 9 5 0 Planktoniella 4 5 5 7 0 0 0 1 1 8 4 7 Pleurosigma 2 2 6 6 0 0 0 3 2 9 9 Proboscia 9 0 0 0 6 Pseudonitzschia 3 3 4 9 5 0 0 0 1 8 3 4 Pseudosolenia 2 7 0 0 0 3 7 2 Rhizosolenia 4 4 9 4 0 0 0 1 5 6 9 9 Skeletonema 9 7 2 3 9 5 0 0 0 4 0 3 6 1 Stephanopyxis 5 7 2 0 0 0 2 0 0 4 Thalassionema 3 9 0 3 2 0 0 0 3 6 5 6 Thalassiosira 5 1 3 4 1 1 0 0 0 3 4 8 3 7 1 Tropidoneis 2 0 0 0 1 DINOFLAGELLATES Ceratium 8 0 4 6 0 0 0 4 2 7 4 6 Dinophysis 6 0 9 0 0 0 3 3 9 9 Diplopelta 1 7 0 0 0 9 5 Diplopsalis 2 2 0 0 0 1 2 3 Dissodium 1 2 4 8 0 0 0 6 9 6 6 Goniodoma 1 2 5 6 0 0 0 7 0 1 1 Gonyaulax 4 4 0 0 0 2 4 6 Gymnodinium 2 0 0 0 1 9 Oxophysis 5 3 0 0 0 2 9 6 Podolampas 6 0 0 0 3 3 Pronoctiluca 1 0 0 0 6 Prorocentrum 1 3 4 0 0 0 0 2 6 1 1 Protoperidinium 1 1 4 6 1 0 0 0 1 6 0 6 5 1 Pyrocystis 7 0 4 0 0 0 3 9 3 0 Pyrophacus 1 6 0 0 0 8 9 Scrippsiella 1 0 0 2 0 0 0 5 5 9 3 SILICOFLAGELLATES Dictyocha 8 4 6 9 0 0 0 2 9 2 6 2 Octactis 5 6 6 0 0 0 1 9 5 6 PHYTOFLAGELLATES Tetraselmis 6 0 0 0 2 MICROFLAGELLATES Olisthodiscus luteus 1 2 1 4 0 0 0 9 8 0 4 TINTINNIDS Amphorellopsis 1 0 0 0 0 7 9 Codonella 3 4 0 0 0 0 2 6 9 9 Codonellopsis 8 3 0 0 0 6 5 9 Dictyocysta 3 3 7 0 0 0 2 6 7 5 Eutintinnus 2 1 6 0 0 0 1 7 1 4 Favella 9 7 0 0 0 7 7 0 Helicostomella 4 0 0 2 0 1 7 6 2 2 7 Aetideus 1 6 0 8 0 2 5 6 6 6 7 Calanus 1 1 4 3 0 0 4 0 1 2 9 3 2 Calocalanus 2 1 9 0 1 1 5 0 9 Candacia 4 7 1 2 0 3 7 7 7 7 5 Centropages 1 2 9 4 4 0 2 0 6 6 1 0 3 Clausocalanus 1 1 2 3 4 0 1 8 1 3 4 [ [