Claire Saraux
email: claire.saraux@ifremer.fr

Elisabeth Van Beveren

Pablo Brosset
email: pablo.brosset@gmail.com

Quentin Queiros
email: quentin.queiros@ifremer.fr

Jean-Herve Bourdeix
email: jean.herve.bourdeix@ifremer.fr

Gilbert Dutto
email: gilbert.dutto@ifremer.fr

Eric Gasset
email: eric.gasset@ifremer.fr

Cyrielle Jac
email: cyrielle.jac@ifremer.fr

Sylvain Bonhommeau
email: sylvain.bonhommeau@ifremer.fr

Jean-Marc Fromentin
email: jean.marc.fromentin@ifremer.fr

Jean-Hervé Bourdeix

Small pelagic fish dynamics: A review of mechanisms in the Gulf of Lions

Keywords: population dynamics, top-down, disease, zooplankton, exploited species

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Knowledge on population dynamics is key to understanding ecosystem functioning and managing exploited populations. Small pelagic fish are widely known for their rapid and important population fluctuations, rendering their management especially difficult [START_REF] Bakun | Patterns in the Ocean: Ocean Processes and Marine Population Dynamics[END_REF]. Indeed, long time series of abundance proxies (i.e., fish scale deposition rates, [START_REF] Baumgartner | Reconstruction of the history of Pacific sardine and northern anchovy populations over the past two millennia from sediments of the Santa Barbara Basin, California[END_REF][START_REF] Mcclatchie | Collapse and recovery of forage fish populations prior to commercial exploitation[END_REF] demonstrate that large fluctuations are an inherent characteristic of these populations and occur very rapidly, i.e. within a few years. As a consequence, fish landings may drop from thousands or millions of tons to almost nothing in a matter of years (e.g. [START_REF] Lluch-Belda | World-wide fluctuations of sardine and anchovy stocks: the regime problem[END_REF]. Such rapid population changes are usually explained by the short lifespan of small pelagic fish and fluctuating, environment-dependent recruitment success [START_REF] Chambers | Early Life History and Recruitment in Fish Populations[END_REF][START_REF] Cushing | Plankton Production and Year-class Strength in Fish Populations: an Update of the Match/Mismatch Hypothesis[END_REF]. In addition, over the last decades, (over-)exploitation has also played a role in those fluctuations as fishing capacity and effort can hardly match such high level variations, generating collapses and longer periods of low abundance in small pelagic fish stocks [START_REF] Essington | Fishing amplifies forage fish population collapses[END_REF][START_REF] Toresen | Variation in abundance of Norwegian spring-spawning herring (Clupea harengus, Clupeidae) throughout the 20th century and the influence of climatic variations[END_REF].

Sardine and anchovy have a worldwide distribution and are especially known for their prevalence in the five boundary current systems (Benguela, Humboldt, California, Canary, Kuroshio), although their populations are also important in other areas, such as the Northeast Atlantic, south-west Australia and the Mediterranean and Black Seas [START_REF] Checkley | Climate Change and Small Pelagic Fish[END_REF]. While the main causes (including environmental and climatic drivers) for population variations as well as the apparent synchrony of the sardine/anchovy alternation have been largely studied in these five major ecosystems (see references within [START_REF] Checkley | Climate Change and Small Pelagic Fish[END_REF][START_REF] Schwartzlose | Worldwide large-scale fluctuations of sardine and anchovy populations[END_REF], the question arises on whether results might be transferable to smaller or non-upwelling systems (but see [START_REF] Katara | Environmental drivers of the anchovy/sardine complex in the Eastern Mediterranean[END_REF]. For those ecosystems, understanding the mechanisms involved in population fluctuations remains a key objective, especially with a view of improving management schemes, benefitting both ecosystem and economy. With a better insight in past and hence future fluctuations, management procedures can be better evaluated and selected, decreasing the probability of overexploitation [START_REF] Hilborn | Moving to Sustainability by Learning from Successful Fisheries[END_REF].

Besides an obvious interest for management, the topic is also interesting from an ecological point of view. To date, most studies have focused on co-variations between climatic or oceanographic parameters and population abundance, biomass or catch. However, those usually lack a comprehensive mechanistic. Using the Gulf of Lions as a pilot area, we attempt to deliver an integrated insight in the population dynamics by tackling the question from several complementary angles.

The Gulf of Lions is one of the most productive areas in the Mediterranean Sea, due to a wide (for the Mediterranean Sea) continental shelf and a combination of high discharge by the Rhone river and small-scale upwellings created by strong and transient winds [START_REF] Millot | The Gulf of Lions' hydrodynamics[END_REF][START_REF] Petrenko | Circulation in a stratified and wind-forced Gulf of Lions, NW Mediterranean Sea: in situ and modeling data[END_REF].

Recently, significant changes have been observed in the pelagic ecosystem of the Gulf of Lions (Fig 1).

Since 2008, the prevailing small pelagic species, i.e. sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) have exposed a lasting decrease in individual size and weight, making them far less profitable for fisheries in the absence of a market for fish of such a size. As a result, most pelagic trawlers that used to target sardine and anchovy have now turned towards other more valued species (such as European hake, Merluccius merluccius), or halted their activity altogether.

Consequently, landings of both species, but especially sardine, have dropped to insignificant levels during the last years [START_REF] Gfcm | Report of the working group on stock assessment of small pelagic species[END_REF]Fig 1). Conversely, the sprat (Sprattus sprattus) population which had been scantly present for the last two decades and is of no commercial interest in the region, showed a remarkable upsurge. Nonetheless, its mean size and body condition remained quite low or decreased (Fig 1).

Surprisingly, these changes in population dynamics neither appeared mediated by environmentallydriven changes in recruitment or overfishing (as often documented in small pelagic fish; see e.g. [START_REF] Barange | Habitat expansion and contraction in anchovy and sardine populations[END_REF][START_REF] Cushing | The biological response in the sea to climatic changes[END_REF][START_REF] Lindegren | Climate, fishing, and fluctuations of sardine and anchovy in the California Current[END_REF], as recruitment over this period remained high and the exploitation rate was rather low, even before 2008 (sardine exploitation rate < 15% since 2002; GFCM, 2016).

Facing this situation, we launched the EcoPelGol project to understand the underlying causes of recent small pelagic population changes in the Gulf of Lions. As no similar regime shift (in terms of oceanography or other biological compartments) had been observed at that period in the Gulf of Lions, the objective of the project was to provide an integrative assessment of causal mechanisms affecting population dynamics by jointly investigating top-down (predation by tuna or marine mammals) and bottom-up (environmental effects) controls as well as diseases and energy allocation trade-offs (see Fig. 2). Although all studies provide a piece of the puzzle, all need to be combined to provide an in-depth answer to the question. Here, we present a summary and cross-discussion of our findings. First, we present a set of studies investigating changes in size, growth, condition and demographic composition over the last two decades and then assessing whether the present situation really is unusual compared to the last 150 years in terms of fish landings. Then, observations and results from these studies allowed us to build alternative hypotheses regarding the control mechanisms of the populations. Each of these hypotheses will be presented and discussed here: (1) changes in size distribution are due to mortality or emigration; (2) observed decreases in small pelagic fish populations are due to top-down pressure exerted on small pelagics by the recently recovering bluefin tuna (Thunnus thynnus) population as well as dolphins (Tursiops truncatus and Stenella coeruleoalba); (3) changes in demographic composition and biological parameters of sardine and anchovy stocks are due to emigration or disease; (4) changes in population trends are due to bottom-up processes, such as environmental forcing and changes in planktonic preys. In the absence of long-term time series of plankton fields in the Gulf of Lions, we investigated potential changes in fish diet using a combination of stable isotope and stomach content data analyses. Finally, (5) we tested for the hypothesis that changes in fish size and mass should affect life-history trade-off between maintenance, growth and reproduction [START_REF] Stearns | Life-history tactics: a review of the ideas[END_REF]. To do this, we estimated the reproductive investment of sardine and anchovy along years through different proxies such as the reproductive period duration, the gonado-somatic index or the length at first maturity.

Methods

This paper reviews the main findings of several studies. For clarity, we present here only an overview of the principal data and methods used in this project and refer to specific studies for more details.

Small pelagic data

Data on small pelagics come from two main sources: (i) data collected in July of each year since 1993 during scientific PELMED surveys, and (ii) fisheries data. PELMED surveys [START_REF] Bourdeix | PELMED -PELAGIQUES MEDITERRANÉE[END_REF] are acoustic and trawl surveys specifically designed to sample and assess small pelagic fish biomasses in the Gulf of Lions. Acoustic sampling is performed along 9 parallel and regularly spaced transects by means of echosounders (Simrad ER60). Discrimination between species is done both by echo trace classification and trawl allocation [START_REF] Simmons | Fisheries acoustics[END_REF]. Indeed, trawls are conducted each time new fish traces are observed on the echogram. The proportion of each species is then estimated by random sampling of the catch before sorting and weighing per species. Further, measurements such as weight, length, age and fat content are taken for the three main targeted species (anchovy, sardine and sprat) at each trawl. Depending on years, samples (entire fish or tissues) are also conserved (frozen at -20°C or in formol) and brought back to the lab for more complete analyses (e.g. histology, stomach contents, muscle lipid and isotopic analyses). Despite a very rich dataset with a high spatial resolution, these surveys only represent a snapshot of the summer situation and do not inform on potential seasonal patterns. To counteract this problem, a partnership with fishermen has been developed to acquire samples all year round. A few times a month, one crate of the catch is randomly sampled by fishermen on the boat and conserved on ice until they return to the harbour (note that fishing events last a maximum of 24h according to regulation). Individual measurements are then computed in the laboratory in the evening or the next day.

These data (~30 years) were complemented by data on sardine and anchovy fish landings spanning 1865-2013. This long-term time series was compiled by subregion of the Gulf of Lions by extensive data mining of the Mediterranean French fisheries records (see Van Beveren et al., 2016a for details), and provided us with a historical perspective in this ecosystem.

Environmental data

In a first approach, climatic and environmental variables were used as proxies of habitat quality and prey availability. We also used different global climatic indices including the North Atlantic Oscillation (NAO) or the Western Mediterranean Oscillation (WeMO), that integrate fluctuations of temperature, rainfall and wind mixing over large spatial and temporal scales and may be more relevant in explaining population fluctuations (see [START_REF] Stenseth | Review article. Studying climate effects on ecology through the use of climate indices: the North Atlantic Oscillation, El Niño Southern Oscillation and beyond[END_REF]. In addition, Sea Surface Temperature (SST) and Chlorophyll a concentration (Chla) satellite-derived data were extracted from MODIS aqua data (http://oceancolor.gsfc.nasa.gov). Rhone outflow, an indicator of nutrient availability, is measured at the Beaucaire station and data is distributed by the Compagnie Nationale du Rhône.

Biological parameters

Several biological parameters have been studied in this project.

Body condition: Body condition is commonly defined as the quantity of individual's energetic reserves [START_REF] Jakob | Estimating Fitness: A Comparison of Body Condition Indices[END_REF] and reflects the amount of energy extracted from the environment. As such, it may inform on prey availability or foraging efficiency [START_REF] Lloret | Condition and Health Indicators of Exploited Marine Fishes[END_REF]. Body condition is also often considered as a good indicator of individual well-being or performance, as it represents the amount of nutrients that can be mobilized for the different vital traits, thus mediating life-history trade-offs [START_REF] Schulte-Hostedde | Evaluating body condition in small mammals[END_REF]. We mostly used the LeCren body condition index [START_REF] Le Cren | The Length-Weight Relationship and Seasonal Cycle in Gonad Weight and Condition in the Perch (Perca fluviatilis)[END_REF] based on morphometrics measurements, as it allowed us to go back in time and investigate changes throughout the years. The Le Cren index is defined as the ratio between the weight of the fish and a theoretical weight for its length derived from measurements over the entire population: = where a and b are constants estimated from the length-weight relationships. This parameter has been shown to strongly correlate with bioenergetics measurements of fat contents using a fatmeter, as well as with direct biochemical measurements outside of the breeding period (Brosset et al., 2015a). During breeding, the morphometric index seems to represent a more integrative index of the fish condition (Brosset et al., 2015a).

Reproductive investment: Two parameters relating to reproduction were used. First, the length at first maturity L50 was defined as the length at which half of the population is mature using a sigmoid model. This was estimated each year or through periods of combined years. Second, the gonadosomatic index was used as a proxy of reproductive investment. It was estimated as the ratio between the weight of the gonad and the weight of the eviscerated fish.

Age: Age was assessed using otolith reading. However, as those were missing in some years, annual age compositions per species were computed using a Bayesian mixture model that uses age-length keys based on otolith readings as priors to slice size distributions (see [START_REF] Van Beveren | Rapid changes in growth, condition, size and age of small pelagic fish in the Mediterranean[END_REF] for more details).

Estimating predation on small pelagic fish

Predation by bluefin tuna: Top-down control by bluefin tuna was estimated by calculating the total amount of energy needed by bluefin tuna in the Gulf of Lions by combining (i) DEB (Dynamic Energy Budget) modeling to estimate the weight-specific energy requirement of an individual (adapted from [START_REF] Jusup | Simple measurements reveal the feeding history, the onset of reproduction, and energy conversion efficiencies in captive bluefin tuna[END_REF], (ii) population weight structure information from fisheries and (iii) population abundance as estimated from aerial surveys [START_REF] Bauer | Aerial surveys to monitor bluefin tuna abundance and track efficiency of management measures[END_REF]. This energy was then partitioned among the different consumed preys using stomach content analyses for 3 years of study and prey energy densities acquired from the literature. This allowed us to estimate the consumption of each prey species. As the PELMED survey delivers estimates of small pelagic fish stock abundances in the Gulf of Lions, we were then able to estimate the percentage of the population predated by bluefin tuna. To account for uncertainty associated with all parameters, calculations were bootstrapped (10 000 iterations) using the most appropriate error distributions for each parameter. Furthermore, as the predation pressure might have been more important on specific prey size classes, we also investigated bluefin tuna selectivity. To do so, we estimated the size distribution of consumed prey, based on undigested prey lengths and relationships between otolith and prey lengths for digested prey. Per species, prey size distribution was compared to the population size distribution, obtained from the PELMED survey. All methodological details can be found in [START_REF] Van Beveren | Predator-prey interactions in the face of effective management strategies: changes in Mediterranean small pelagics are not due to increased tuna predation[END_REF].

Predation by dolphins: A similar approach was used to estimate predation by the two main dolphin species inhabiting the Gulf of Lions (Tursiops truncatus and Stenella coeruleoalba). Plane and boat surveys were used to estimate dolphin abundance and population structure, while allometric energetic models were used to estimate individual energy requirement and stomach contents of stranded dolphins were used to determine prey composition in the diet. Uncertainty was estimated using a simulation approach (more details in Queiros et al., subm.).

Pathogen and disease analyses

To assess whether pathogens might be an additional source of mortality for sardine in the Gulf of Lions, monthly sampling was performed for one year in 2014-2015 (see details in Van Beveren et al., 2016b). Nine sampling events of 150 sardines each were used to perform a large-band search for bacteria, parasites (micro and macro) and viruses. Fresh samples were obtained from fishermen or the PELMED survey and were analysed within 24h. All fish were first measured, weighed and their fat content was assessed using a fatmeter (Brosset et al., 2015a), so that we could investigate the link between fish morphological characteristics and pathogen prevalence and intensity. Regarding viruses, we used both broad-spectrum cell cultures and specific PCR analyses targeting the pilchard herpes virus known to decimate sardine populations (see [START_REF] Whittington | Herpesvirus that caused epizootic mortality in 1995 and 1998 in pilchard, Sardinops sagax neopilchardus (Steindachner), in Australia is now endemic[END_REF] and the betanodavirus (the most common fish virus in the Mediterranean Sea). To check whether detected infectious agents could be harmful pathogens, we also ran tissue analyses through autopsy and histology.

Planktonic data analyses

The study of bottom-up processes requires detailed information on prey (plankton) availability.

Unfortunately, surveys of phyto-and zooplankton are scarce in the Gulf of Lions and no historical datasets are available. Therefore, we used modeled plankton concentration estimates extracted from a coupled physical-biogeochemical model called SYMPHONIE -Eco3m-S [START_REF] Auger | Functioning of the planktonic ecosystem on the Gulf of Lions shelf (NW Mediterranean) during spring and its impact on the carbon deposition: a field data and 3-D modelling combined approach[END_REF][START_REF] Baklouti | Investigation of mechanistic formulations depicting phytoplankton dynamics for models of marine pelagic ecosystems and description of a new model[END_REF][START_REF] Marsaleix | Alternatives to the Robert-Asselin filter[END_REF]. This model includes 6 groups of plankton:

picophytoplankton, nanophytoplankton, microphytoplankton (mainly diatoms), nanozoopankton (small ciliates and flagellates), microzooplankton (large ciliates and flagellates) and mesozooplankton (mainly copepods).

We investigated whether fish diet had changed across time and tested whether this could be related to changes in small pelagic fish populations. We compared the diet of small pelagics before and after 2008 (i.e. when most of the changes occurred). We used a combination of complementary approaches, i.e. stomach content and stable isotope analyses [START_REF] Caut | Coupling stable isotopes with bioenergetics to estimate interspecific interactions[END_REF] on fish sampled from PELMED surveys. This ensured all samples were collected at a similar time of year and avoided any potential bias due to seasonal variation.

Results and Discussion

State of the population

As often in fisheries studies, the first alarming sign was observed from fish landings with a sudden crash in sardine landings around 2008, while anchovy landings had been decreasing at a slower rate over a longer time period. Looking at it from a historical perspective (Fig. 3) highlighted an even more alarming trend. Indeed, a first study describing the historical landings of sardine and anchovy in the Gulf of Lions showed that their present total landings are similar to those before the 1960s, when fishing was still non-industrial since sonars and large-scale trawlers had not yet appeared (Van Beveren et al., 2016a). Focusing on sardine, the recent situation (after 2008) is highly unusual, as recent landings are the lowest recorded over the past 150 years. Biomass and abundance estimates resulting from recent PELMED surveys did not reveal such drastic changes. Especially, fish abundance has been maintained if not increased lately contrary to the landings (Fig. 1). The contradiction between the crash in landings and an increase in abundance suggests a decrease in fishing effort.

One reason for this might be fish size. Indeed, the low price and quasi-absence of a market for small fish and the increase in fishing marginal costs (e.g. fuel) would lead to a halt in fishing effort. To test this hypothesis and obtain a better awareness of the current situation and population dynamics, we investigated changes in fish size, condition, age and length composition using PELMED survey data [START_REF] Van Beveren | Rapid changes in growth, condition, size and age of small pelagic fish in the Mediterranean[END_REF]. Sardine and anchovy size decreased around 2008 and fish remained small thereafter (Fig. 4). Annual anchovy size distributions are usually unimodal, so that the decrease in mean size corresponds to a shift of the entire mode towards small individuals due to a rather slow growth, as confirmed by von Bertalanffy growth curves [START_REF] Van Beveren | Rapid changes in growth, condition, size and age of small pelagic fish in the Mediterranean[END_REF]. Conversely, sardine distributions are bimodal and the decrease in size translated in the disappearance of the second mode containing large sardines. For sardine, which displayed the most significant size truncation, the shift towards smaller fish was explained by a combination of slower growth and the disappearance of older individuals (ages 2+, see [START_REF] Van Beveren | Rapid changes in growth, condition, size and age of small pelagic fish in the Mediterranean[END_REF].

Large old individuals might have disappeared because they emigrated towards different areas and/or they experienced a higher than usual mortality rate. If fish emigrated, they most likely did so towards Spanish waters, because of the main current direction and the presence of continental shelf in this area. However, a comparison between French and Spanish size distributions of the landings from 2002 to 2013 (Fig 5) reveals that size distributions are similar between both regions. In particular, when large fish disappeared from French landings, the same happened with Spanish landings. This not only suggests that large fish did not migrate to Spain, but also that the recent changes observed in the Gulf of Lions might take place at a larger scale in the North-Western Mediterranean Sea (although sardine exploitation has always been much more important in Spain; [START_REF] Palomera | Small pelagic fish in the NW Mediterranean Sea: An ecological review[END_REF]. As a consequence, it is more likely that large individuals experienced higher mortality rates.

Higher mortality is expected to occur if fish are in a lower condition [START_REF] Millar | Fasting endurance and the evolution of mammalian body size[END_REF].

Accordingly, since 2008 both sardine and anchovy populations have been characterized by a continuous trough of low body condition, compared to short, transient troughs in the past (Van Beveren et al., 2014, Fig 4). Thus, the current situation is the first to occur over such a long period.

Interestingly, the decrease was most pronounced for sardine, for which the largest decline in size and age was observed. Moreover older sardines were more affected by the decrease in condition than younger ones, supporting the hypothesis that natural mortality rates primarily increased for larger size classes (Brosset et al., 2015b).

As expected when fish populations decline (e.g. Ballon et al., 2008), sardines and anchovies matured earlier (length at maturity for sardine was about 12.1 cm prior to 2009, but about 9.6 cm after 2009 for sardines). Further, despite poor energy reserves, they maintained, or in some cases even increased, their reproductive investment (as expressed by the reproductive period duration as well as their gonadosomatic index, see Brosset et al., 2016b). These findings might explain why abundance and recruitment remained surprisingly high throughout the last decade (Fig. 1). However, such changes might also come at a cost and explain the disappearance of older sardines through a change in the life-history trade-off between reproduction and maintenance. This is especially true for sardines, which act as capital breeders reproducing in winter when feeding conditions are poorer. An especially low condition at the start of winter (Albo-Puigserver et al., 2017) and reproduction associated with an important reproductive effort may thus result in increased adult mortality. The change in the reproduction/maintenance trade-off might incur costs in terms of immunity for instance, so that fish might be more susceptible to disease [START_REF] Krams | Reproduction compromises adaptive immunity in a cyprinid fish[END_REF]. Such a change may also affect the amount of energy a fish can mobilize for muscle activity and as such decrease its swimming efficiency, so that weaker individuals might be more prone to predation [START_REF] Herting | The Role of Physical Fitness of Forage Fishes in Relation to their Vulnerability to Predation by Bowfin (Amia calva)[END_REF]. Ultimately, fish might reach a critical condition for their survival and die from starvation, if they are not able to find large quantity of prey especially at the end of winter.

Regardless of the exogenous pressures, one hypothesis is that the observed changes in life-history traits (slower growth, decreased size at first maturity) and associated trade-offs might result from population adaptation. For instance, fish facing an important selective exploitation on large individuals have been shown to react towards slower growth and earlier maturation [START_REF] Audzijonyte | How fast is fisheries-induced evolution? Quantitative analysis of modelling and empirical studies[END_REF][START_REF] Devine | Comparing rates of contemporary evolution in life-history traits for exploited fish stocks[END_REF][START_REF] Kuparinen | Fishing induced life-history changes degrade and destabilize harvested ecosystems[END_REF][START_REF] Mollet | Reconstructing the effects of fishing on lifehistory evolution in North Sea plaice Pleuronectes platessa[END_REF]). Yet, whether such changes derive from phenotypic plasticity or genetic changes remains mostly unknown. We recently aimed at experimentally disentangling those effects by capturing wild sardines in October 2015, which we placed in 4.5 cubic meter tanks for three months. Our results highlighted phenotypic plasticity rather than genetic adaptation. Despite the winter period over which the experiment was conducted and low water temperature at that time, sardines fed daily to 1.5% of their body mass (as often recommended in aquaculture) immediately exhibited compensatory growth (13.0 ± 0.1cm after 40 days vs. 11.2 ± 0.1cm at t0) and an important upsurge in their body condition (Le Cren index: 1.43 ± 0.02 at t40 vs. 0.93 ± 0.01 at t0). Such an increase would take 6 months up to one year to occur in the wild according to the von Bertalanffy curves estimated between 1993 and 2012 confirming that observed changes are a direct consequence of a dietary restriction in the wild.

Factors affecting small pelagic populations

Although previously cited studies contributed to our general understanding of the core population dynamics, the underlying factors affecting population dynamics still needed to be determined. Given the exceptionality of the situation (high mortality of older fish rather than low recruitment), a large scope of potential mechanisms were scrutinised, including some which are commonly neglected in small pelagic fish research (e.g., predation and disease; Fig 2).

Top-down Control

First and foremost, fishing pressure was examined as overexploitation is a likely candidate when investigating decrease in fish size and condition. Due to differential catchability, fishing can cause or magnify a shift towards young, small and early-maturing individuals [START_REF] Audzijonyte | How fast is fisheries-induced evolution? Quantitative analysis of modelling and empirical studies[END_REF][START_REF] Devine | Comparing rates of contemporary evolution in life-history traits for exploited fish stocks[END_REF][START_REF] Kuparinen | Fishing induced life-history changes degrade and destabilize harvested ecosystems[END_REF][START_REF] Mollet | Reconstructing the effects of fishing on lifehistory evolution in North Sea plaice Pleuronectes platessa[END_REF]. Over the past 25 years, harvest rates (landings/population biomass) have generally been low (mean of 13% for anchovy and 10% for sardine) but varied significantly around these means (Fig. 6). A safe level of medium term exploitation for small pelagic fish is considered to be 0.4 [START_REF] Patterson | Fisheries for small pelagic species: an empirical approach to management targets[END_REF], a threshold that has only been crossed once in 1997 for sardine. This occurred right before sardine biomass peaked, indicating that there had been little effect on the population. Finally, since 2010 the sardine harvest rate has been reduced to 1% or less, but no signs of recovery in terms of growth, survival or condition have yet been recorded despite the high turnover rate of the species. These results suggest that fishery is not the main cause for high sardine and anchovy mortality. Further, stock assessments for sardines in this area based on a 2-stage biomass model, estimated an extremely low exploitation rate, confirming this vision (GFCM, 2016). As such, the General Fisheries Commission for the Mediterranean Sea judged the sardine stock ecologically unbalanced due to its lack of old individuals and problems of growth and body condition [START_REF] Gfcm | Report of the working group on stock assessment of small pelagic species[END_REF].

Similarly to the fishery hypothesis, another top-down regulating mechanism could be the effect of predators removing an important part of the adult population. However, despite sardine and anchovy being the two main prey species of bluefin tuna (accounting for more than 80% of the diet in terms of mass and abundance), top-down control by tuna has been estimated to be extremely limited, the percentages of each population consumed being lower than 2% [START_REF] Van Beveren | Predator-prey interactions in the face of effective management strategies: changes in Mediterranean small pelagics are not due to increased tuna predation[END_REF]. Further, bluefin tuna seemed to be opportunistic in terms of prey size, as no consistent sizeselective feeding was observed. Similarly, the fraction of the sardine and anchovy population ingested by dolphins was even lower (<0.1%, Queiros et al., submitted). The current situation of small pelagics in the Gulf of Lions is thus unlikely to be due to top-down processes.

Disease

A general veterinary study focusing at sardine was conducted, aiming to detect a wide range of potential pathogens, including parasites, viruses and bacteria (Van Beveren et al., 2016b). However, our analyses did not reveal any trace of macro-parasites or viruses (whether on culture or by specific PCRs) and detected only few bacteria, all of which with little prevalence (Van Beveren et al., 2016b).

Only micro-parasites were observed in 77% of the fish (Van Beveren et al., 2016b). Those microparasites observed in the liver were coccidies, a hepatic parasite, whose effect is mostly unknown. However, no clear damage to tissues and no link between pathogen presence and host size or condition were highlighted (Van Beveren et al., 2016b). This study therefore suggested that the probability of pathogens acting as a key mechanism affecting the sardine population is fairly low, although further research on the prevalence and harmfulness of coccidies as well as occurrence in different waters needs to be conducted for this hypothesis to be entirely refuted.

Bottom-up control

Rather, changes in growth, size and condition, and hence possibly increased mortality, might reflect bottom-up controls mediated by changes in plankton composition and/or concentrations. Indeed, we showed that the concentration in mesozooplankton (from biogeochemical models) was the only variable significantly affecting body condition of both sardines and anchovies, while river runoff, sea surface temperature (SST) or microphytoplankton (i.e. diatom concentration) affected condition in only one of the two species (Brosset et al., 2015b). Following up on this analysis and suggestions in a previous review [START_REF] Palomera | Small pelagic fish in the NW Mediterranean Sea: An ecological review[END_REF], we compared pre-and post-2008 diets for fish of similar size. Those exhibited important changes in carbon isotopic values as well as in consumed plankton composition and size for both sardines and anchovies (Brosset et al., 2016a). A principal component analysis on the diet composition of all sampled fish clearly showed segregation between the years prior to (2007 for sardines and 1994 for anchovies) and posterior to the changes (2011-2012) with a shift towards smaller prey species in both sardines and anchovies in recent years (Brosset et al., 2016a). Despite previous associations between small pelagic abundance and basic environmental parameters such as temperature [START_REF] Palomera | Small pelagic fish in the NW Mediterranean Sea: An ecological review[END_REF], a study on climatic (large-scale indices such as NAO, AMO and WeMO) and oceanographic parameters (SST and river runoff) did not show any significant change in 2008, suggesting that the mechanisms behind small pelagic changes might be more complex (Van Beveren et al., 2016a). Also, a study at a larger scale including most of the northern side of the Mediterranean Sea showed that fish size and body condition decreased in almost all areas but not concomitantly and suggests that the causes of these declines might be different and occurring at subregional scales [START_REF] Brosset | Spatio-temporal patterns and environmental controls of small pelagic fish body condition from contrasted Mediterranean areas[END_REF]. Nonetheless, smaller plankton species are known to be less energetic [START_REF] Zarubin | Intraspecific differences in lipid content of calanoid copepods across fine-scale depth ranges within the photic layer[END_REF], which might explain why small pelagics have experienced a trough of low body condition. Still, this result might be surprising as a regime shift towards smaller plankton size is thought to be favourable for sardines [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]. Indeed, sardines are known to feed on smaller plankton than anchovies probably because they are more efficient filter-foragers ( [START_REF] Van Der Lingen | Diet of sardine Sardinops sagax in the southern Benguela upwelling ecosystem[END_REF]. Why then would sardines be the ones most affected by a change in plankton communities towards smaller sizes? This observation might reflect an important difference in sardine feeding strategies between upwelling systems and other areas. Sardines from upwelling areas might have developed adaptations increasing their filtering efficiency because of high plankton densities. In the Mediterranean Sea, which is known to be rather productive in spring but mostly oligotrophic otherwise, sardines seem to have developed both feeding modes, i.e. filter and particulate feeding [START_REF] Nikolioudakis | Diet of sardine Sardina pilchardus: an 'endto-end' field study[END_REF]. Therefore, adaptations to filter-feeding might not have evolved so strongly in the Mediterranean Sea. Gill rakers for instance have a higher density in the upwelling area off the Iberian coast than in the North-Western Mediterranean Sea [START_REF] Costalago | Comparison of the feeding apparatus and diet of European sardines Sardina pilchardus of Atlantic and Mediterranean waters: ecological implications[END_REF]. Also, traditional differences in diet between sardines and anchovies ( [START_REF] Van Der Lingen | Diet of sardine Sardinops sagax in the southern Benguela upwelling ecosystem[END_REF] are not clearly observed in the Mediterranean Sea (e.g. LeBourg et al., 2015). Therefore, a change towards smaller prey might thus not have been beneficial for Mediterranean sardines, in contrast to what would have been expected in other systems.

Further research

While bottom-up control seems the most likely explanation to recent declines in small pelagic fish populations in the Gulf of Lions, the detailed mechanisms underlying the effect of plankton size and composition on fish life-history traits remain to be tested. In particular, the use of an experimental approach (wild fish maintained in tanks) where food size, quantity and quality may be manipulated for might help refine the understanding of individual and combined effects of food on fish foraging behavior, energy expenditure and gain, growth and reproduction. Further, the reconstruction of high-resolution zooplankton fields as well as a better understanding of the energy density of different plankton species or size-classes is urgently needed. If this indeed confirms a change in plankton, then oceanographic, atmospheric and hydrological parameters would need to be further analysed to better identify a potential regime shift of the ecosystem. In particular, we need to further examine the main factors affecting ocean productivity: nutrient input (through the Rhone river discharge) and water mixing (winds, local upwellings, and deep water convection). Additionally, density-dependence and competition might be important synergistic factors contributing to the dynamics of these populations, especially if resource partitioning is lower due to greater similarities in feeding behavior and prey preference. The total abundance of small pelagics in the Gulf of Lions has somewhat increased, in particular after the upsurge of sprats (Fig. 1). Further, these three species strongly co-occur in the Gulf of Lions [START_REF] Saraux | Spatial structure and distribution of small pelagic fish in the NorthWestern Mediterranean Sea[END_REF] and there has been an increase in the isotopic niche overlap of sardine and anchovy with sprat (Brosset et al., 2016a). A study of intraand inter-specific competition within and among these three species would be of high interest.

Finally, few ecotoxicological studies have been performed so far on small pelagic fish in the Gulf of Lions (but see COSTAS project; [START_REF] Tronczynski | Contaminants bioaccumulation and biomagnification in a short trophic system: phytoplankton, zooplankton, anchovy, sardine (COSTAS)[END_REF]). Yet, pollution or a decreased water quality could elicit a decrease in fish condition with direct or indirect mortality. Given that the Gulf of Lions contains considerable concentrations of pollutants [START_REF] Harmelin | Possible link between prey quality, condition and growth of juvenile hake (Merluccius merluccius) in the Gulf of Lions (NW Mediterranean)[END_REF] and is the most polluted Mediterranean area in terms of PCBs [START_REF] Bodiguel | Classical and novel organohalogen compounds (PCBs and PBDEs) in hake (M. merluccius, L.) from the Mediterranean and Atlantic coasts (France). Environmental Toxicology[END_REF][START_REF] Gomez-Gutierrez | Assessment of the Mediterranean sediments contamination by persistent organic pollutants[END_REF], this hypothesis would be worth a detailed examination. Pollutants might change fish physiology after ingestion [START_REF] Heath | Water Pollution and Fish Physiology[END_REF], whether or not after bioaccumulation or biomagnification of the contaminants along the trophic web. However, for the few pollutants (PCB, PBDE, Hg and 137Cs) looked at so far in anchovy and sardine of the Gulf of Lions (see [START_REF] Harmelin | Possible link between prey quality, condition and growth of juvenile hake (Merluccius merluccius) in the Gulf of Lions (NW Mediterranean)[END_REF], no exceptionally high levels were found when compared to other European regions [START_REF] Bocio | Concentrations of PCDD/PCDFs and PCBs in fish and seafood from the Catalan (Spain) market: Estimated human intake[END_REF][START_REF] Martí-Cid | Intake of chemical contaminants through fish and seafood consumption by children of Catalonia, Spain: Health risks[END_REF][START_REF] Naso | Persistent organic pollutants in edible marine species from the Gulf of Naples, Southern Italy[END_REF]. This indicates that it might be necessary to look at a mixture of pollutants, as the combined effect of numerous weakly present ones might become an influential cocktail. Further, pollutants might have an indirect effect on fish through the food web. In particular, planktonic species could be impacted [START_REF] Tiano | PCB concentrations in plankton size classes, a temporal study in Marseille Bay, Western Mediterranean Sea[END_REF], potentially resulting in a community and/or abundance change [START_REF] Arfi | Système planctonique et pollution urbaine: un aspect des populations zooplanctoniques[END_REF][START_REF] Uriarte | Differences in the abundance and distribution of copepods in two estuaries of the Basque coast (Bay of Biscay) in relation to pollution[END_REF]. However, a study on the aspects of PCBs (including multiple congeners) in the Gulf of Lions indicated that PCB biomagnification in plankton is relatively low, and concentrations are not correlated to the plankton biomass in the system (although such a relation might have been masked; [START_REF] Tiano | PCB concentrations in plankton size classes, a temporal study in Marseille Bay, Western Mediterranean Sea[END_REF]. While first results, mostly on PCB, were not particularly significant, ecotoxicology of fish and plankton is an important research venue to consider in order to further understand dynamics of small pelagics.

Conclusions

In conclusion, the important decrease in small pelagic landings in the Gulf of Lions resulted from a decrease in fish size and condition rather than from a decrease in abundance. Changes in size resulted from a decrease in growth (anchovy and sardine) and disappearance of old individuals (sardines). The EcoPelGol project enabled us to test for different control mechanisms that we discussed here. Namely, our results provide little evidence of top-down control by predators of those populations and suggested a very low probability of a control by disease and fisheries. Rather, our results suggest changes in diet of these species supporting a bottom-up control. Nonetheless, despite low energy reserves, the investment in reproduction does not appear to be impaired but might occur at a cost to adult maintenance. Future research should further explore the bottom-up control hypothesis to (1) confirm whether plankton communities have changed drastically in recent years and if so (2) examine which factors might explain such changes; and (3) investigate the ecophysiological mechanisms relating changes in diet to fish phenotypes.

Fig. 1 .

 1 Fig. 1. Biomass, abundance, landings and mean weight of anchovies (in green), sardines (in blue) and sprats (in black) in the Gulf of Lions.

Fig 2 :

 2 Fig 2: Schematics on the main drivers of small pelagics population dynamics. Significant drivers appear in bold, while less important ones appear in grey and in italics. Green items are perspectives for future work.

Fig 3 :

 3 Fig 3: Historical landings of sardines and anchovies adapted from Van Beveren et al., 2016a

Fig 5 :

 5 Fig 5: Comparison of the annual size distributions of sardine and anchovy landed in Spain (blue) and France (red). Vertical lines indicate the median size of each distribution.

Fig 6 :

 6 Fig 6: Harvest rates of sardine and anchovy as estimated by the total landings divided by the biomass of the stock assessed by acoustics. Dashed line indicates the reference point ofPatterson (1996)

  Fig 2 695

  Fig 3

Acknowledgements

We thank V.A. Viblanc, J. Alheit and two anonymous reviewers for their helpful suggestions to improve this manuscript. We thank the captain and the crew of the RV "L'Europe" as well as all the scientists on board for their assistance during the PELMED surveys. Our gratitude is extended to the fishermen who provided us with sardine samples all along the project as well as those who helped us capture wild sardines to keep them alive. Funding: PELMED surveys are cofinanced by Europe through the Data Collection Framework. Most of this work derives from the EcoPelGol programme (Study of the Pelagic ecosystem in the Gulf of Lions), funded by France Filière Pêche (FFP) and the "Région Languedoc-Roussillon". The rest of the work was funded through the FEAMP project MONALISA, which was co-funded by Europe and FFP.


Yacine Kchaou
email: yacinekchaou@yahoo.fr

Véronique Pelosin

Gilbert Hénaff

Nader Haddar

Khaled Elleuch

Failure mode analysis of SMAW welded UNS N08028 (Alloy28) superaustenitic stainless steel under crack growth tests

The Fatigue Crack Growth (FCG) behavior of welded UNS N08028 (Alloy28) superaustenitic stainless steel was investigated in this paper. Alloy28 superaustenitic stainless steel was welded using Shielded Metal Arc Welding process. The FCG behavior was studied under two load ratios (0.1 and 0.5) and for different specimen configuration: Base Metal (BM), Weld Metal (WM) and Welded Joint (WJ). The results indicated that FCG rates of WM specimen is lower than BM specimen at R = 0.1. Moreover, load ratio affects the FCG rate for BM and WM specimen. In addition, the observation of fatigue crack growth path of WM displayed that crack follows dendritic orientation giving a tortuous crack path, while it is less tortuous for BM specimens. Scanning Electron Microscope (SEM) observations of BM fractured surfaces showed that damage mechanism is intergranular for lower ΔK value, and transgranular for higher ΔK value. Moreover, SEM observations of WM fractured surface showed the presence of welding defects which can affect negatively fatigue crack growth resistance at R = 0.5.

Introduction

Superaustenitic stainless steels are widely used in the pressure vessel manufacturing. Their high mechanical and corrosion resistances make them the most used stainless steels for this application and they can be welded to assembly different structures. Welded zone is considered as a mechanical and microstructural heterogeneity which can affect fatigue resistance of the component during cyclic loading. Numerous investigations focused on fatigue resistance of welded austenitic stainless steel and especially their fatigue crack growth behavior [1-5]. They showed that crack growth rate is faster in Base Metal than Weld Metal.

Tsay et al. investigated the FCG behavior of AISI 304 stainless steel in air and H2 [START_REF] Tsay | The effect of microstructures on the fatigue crack growth in Ti-6Al-4V laser welds[END_REF]. They showed that the faster FCG rate of Base Metal compared to Weld Metal is caused by the presence of compressive residual stresses in the Weld Metal crack tip. Authors showed also that this result is due to a tortuous crack path and the high roughness of Weld Metal fractured surface. Li et al. investigated FCG behavior of a welded ferritic-martensitic cold rolled DP780 dual phase steel at room temperature [START_REF] Li | Microstructure and fatigue crack growth behavior in tungsten inert gas welded DP780 dual-phase steel[END_REF]. Authors found that Weld Metal has a FCG rate lower than Base Metal. They showed that this result is due to the hard martensite microstructure, which limits the size of the plastic zone at the crack tip. Deng et al. investigated FCG behavior of dissimilar 9Cr/CrMoV welded joint using different stress ratio and specimen configuration [START_REF] Deng | Microstructure correlation and fatigue crack growth behavior in dissimilar 9Cr/CrMoV welded joint[END_REF]. They found that stress ratio affects FCG of Base Metal, Weld Metal and Heat Affected Zone (HAZ). This is due to the presence of crack closure induced by plasticity. Authors found also that Weld Metal has a better FCG resistance compared to Base Metal and HAZ because of the presence of a tortuous crack path. Al-Haidary et al.

investigated Fatigue Crack Growth (FCG) behavior of welded 316L stainless steel at room temperature [START_REF] Al-Haidary | Fatigue Crack Propagation in Austenitic Stainless Steel Weldments[END_REF]. They pointed out the good resistance of Base Metal to crack propagation compared to Weld Metal. This is mainly due to the presence of tensile residual stress in welded zone.

In this context, this paper investigates FCG behavior of welded Alloy 28 superaustenitic stainless steel at room temperature. Microstructure, micro-hardness profile and tensile properties were analyzed on Base Metal and Weld Metal. The FCG rate on Base Metal, Weld Metal and Welded Joint specimens were determined for two different load ratios. Subsequently, morphological observations of fractured specimen and crack path were performed to better analyze the FCG behavior and failure mode.

Materials and methods

Hot rolled Alloy28 superaustenitic stainless steel sheets were used in this study. Alloy28 sheets were welded using Shielded Metal Arc Welding (SMAW) process in order to manufacture pressure vessels. Welding was made on Base Metal using commercially available filler metal (Sanicro 28). The stainless steel sheets were first prepared by chamfering and grinding the edges in order to make an X form. The industrial welding parameters and the heat input in each block of the 15 welding passes are given in Table 1.

Metallographic observations were carried out on several specimens prepared by grinding using 120, 240, 320, 600, 800, 1000, 2000 and 4000 grits of SiC papers, followed by the final polishing with 3µm and 1µm alumina powders. Then, specimens were etched using electrolytic etching in an aqueous solution of 10% wt. of oxalic acid at 5V during 20 seconds.

Microstructure was investigated using optical microscope and Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS) point analysis. Micro-indentation Vickers hardness tests were performed on Base Metal and weld Weld Metal at a load of 300 mN using a microhardness Fischer HM T3 XY-prog model.

Tensile tests were carried out at room temperature on cylindrical specimens machined in Base Metal and Weld Metal using an axial servo-electric machine (Instron 1362 100 kN) with a load train and compact digital electronics control. Base Metal specimens were taken along the rolling direction of the Alloy28 sheets. Tensile samples have a cylindrical form with a diameter of 4.3 mm and a length of 56 mm. Weld Metal samples were machined in the welding direction. WJ specimens were welded perpendicularly to welding direction and contain a 5 mm weld nugget in its center. For all tests, the applied strain rate was equal to 10 -3 s -1 and the strain was measured using an extensometer with a gauge length of 12.5 mm.

Tensile tests were performed following the standard ASTM: E8/E8M -16a Standard Test Methods for Tension Testing of Metallic Materials. FCG tests were conducted on a servohydraulic test machine at room temperature in a laboratory air environment (Fig. 1). FCG tests were performed according to ASTM E647 under constant load control with a sinusoidal loading waveform at a frequency of 20 Hz and load ratio ranging from 0.1 to 0.5. Crack length was measured continuously by the compliance method with a crack mouth clip gauge.

Optical measurements were periodically taken to calibrate the compliance method. A Compact Tension (CT) specimen with a 32 mm width, 8 mm thickness was used. Fig. 2 shows an illustration of the tested sample, including its dimensions. Three types of CT configuration were machined following different direction as shown in Fig. 3 a), b) and c) respectively: in the L-T direction in order to have a notch axis perpendicular to the rolling direction, named CT-BM, CT specimens notched in the weld metal named CT-WM, and CT specimens notched perpendicularly to Weld Metal named CT-WJ.

Results and discussions

Microstructure, Hardness profile and tensile properties

Chemical composition of Base Metal, Weld Metal and filler metal are showed in Table 2. It can be seen that Cr and Ni content are similar in Base Metal and Weld Metal which lead them compatible to welding. Optical micrographs showing the microstructure of Base Metal and Weld Metal are displayed in Fig. 4. The microstructure of Base Metal is fully austenitic and no delta ferrite or carbide precipitates were observed. The microstructure consists of equiaxed grains and twins and they can be observed in the austenitic matrix. The austenitic grains are randomly oriented with sizes range between 100 µm and 200 µm. The microstructure of Weld Metal presents dendritic grains oriented following welding passes direction. The microhardness profile across the weld bead is shown in Fig. 5. It can be noted that the hardness value of Weld Metal reaches 330 HV at the middle of the welding then decreases unto 220 HV in the Base Metal. The tensile properties of Base Metal and Weld Metal are showed in Fig. 6. Both of the tested materials present a ductile behavior but the mechanical properties are different. Indeed, it can be found that yield stress and tensile strength of Base Metal are lower than Weld Metal. Meanwhile, the elongation of Base Metal is also much higher than Weld Metal (A% = 15.2% and 7.5% respectively) which can be confirmed by the necking coefficient value (Z% = 62% and 41% respectively).

Fatigue Crack Growth tests on Alloy28 Base Metal (CT-BM)

FCG tests results

In order to study the effect of load ratio on FCG rate, Fatigue Crack Growth tests were carried out on CT-BM specimens with a load ratio equal to 0.1 and 0.5. The evolutions of crack length versus the number of cycles as well as FCG rate (da/dN) versus stress intensity factor ΔK were depicted in Fig. 7 and8 respectively. Crack growth rate was determined using the Eq. 1:

= (1)
ai and Ni are the crack length and number of cycles at the measurement time i respectively.

The stress intensity factor amplitude ΔK for CT specimens could be calculated according to Eq. 2.

[9] :

∆ = ∆ √ × , , , , , (2)
where B is the specimen thickness, W is the specimen width, ΔP is the applied loading amplitude, α is the fatigue crack coefficient defined by and a is the crack length.

Generally, the fatigue crack growth process can be divided into three regions: slow expanding region (I), stable expanding region (II) and rapid expanding region (III). In this study, the tests were mainly performed in the region (II), which is the most valuable to estimate the fatigue life. The conventional Paris model [10-12], given by Eq. ( 3), was applied to describe the FCG behavior of the investigated steels:

= ! " # (3)
da/dN is the Fatigue Crack Growth rate, ΔK is the stress intensity factor range, C and m are constants that depend on the tested materials.

The Fatigue Crack Growth rate data for CT-BM specimens are summarized in Fig. 8 in terms of the relationship between the FCG rates (da/dN) and stress intensity factor (ΔK). Paris law parameters extracted from da/dN curves are reported in Table 3. It can be seen that FCG rate is higher for R = 0.5 test. For example, for ΔK = 15 MPa√m, da/dN = 1.3x10 -8 m/cycle at R = 0.5 and da/dN = 3x10 -9 m/cycle at R = 0.1. In order to understand the origin of the difference between FCG rate at R = 0.1 and 0.5 and to clarify the fatigue fracture process, examinations of the fatigue fracture surfaces of Base Metal specimens are essential.

Morphological analysis of CT-BM specimens

The fatigue crack growth path and fractography observations could be used to reveal the microstructure detail where crack grows through, to identify the characteristics of the FCG as well and the fine-scale fatigue fracture features. Therefore, optical microscopy of crack path and SEM observations of fractured CT-BM specimens tested at R = 0.1 and R = 0.5 were carried out.

Fig. 9 shows the crack path observation of CT-BM specimen tested at R = 0.5. It can be seen that crack path presents few deviations with some crack branching. Moreover, it seems that at lower ΔK value (10 and 11 MPa√m), crack path is dependent on microstructure and propagates in grain or twin boundaries. For higher ΔK value, crack propagates and crosses grains and twins giving a transgranular propagation mode. In order to explain the relationship between FCG behavior and microstructure, the reversible plastic zone (RPZ) size was determined for lower and higher ΔK value in the stage II of crack propagation. The RPZ size (rp) under plane stress condition was defined as presented in Eq. 4

[14]:

$ % = & ' ( & ∆) * +. ( (4)
Where ΔK is the stress intensity factor range, and 0.2 is the yield strength. Rp values corresponding to different ΔK values are summarized in Table 4. highlighted the presence of crack closure by crack tip plasticity of a 304L austenitic stainless steel at low load ratio (R = 0.1). In order to underline the effect of plasticity on crack closure, Pommier [START_REF] Pommier | Plane strain crack closure and cyclic hardening[END_REF] showed that there is a relation between crack closure and cyclic behavior of a 316 austenitic stainless steel. This latter reveals an elevated Bauschinger effect which shows a high kinematic hardening. Pommier showed also that crack closure is directly linked to kinematic hardening in crack tip [START_REF] Pommier | Plane strain crack closure and cyclic hardening[END_REF].

It can be considered that crack closure of CT-BM specimen is due to the roughness of crack surfaces as well as high plasticity in crack tip zone. As a conclusion, the difference between FCG rates of CT-BM specimens can be attributed to the roughness-induced and plasticityinduced crack closure phenomenon during the unloading process at R = 0.1.

Fatigue Crack Growth tests on Alloy28 Weld Metal (CT-WM)

FCG tests results

Fatigue Crack Growth tests were also performed on CT-WM specimen for the two load ratios: 0.1 and 0.5. All results were revealed from binocular measurement, which limits the number of point in the da/dN-ΔK curves. Fig. 13 summarizes the FCG behavior of the CT-WM specimens. It can be seen that FCG rate is significantly faster at R = 0.5 in the studied ΔK range. For example, at ΔK = 20 MPa√m, FCG rate is equal to 3.2x10 -8 m/cycle at R = 0.5 and to 10 -8 m/cycle at R = 0.1. Table 5 shows the characteristic of da/dN curves in the Paris domain. In order to understand the origin of this discard, fatigue crack propagation path and fracture surface observations of CT-WM specimens were performed.

Morphological analysis of CT-WM specimens

In this section, only specimen tested at R = 0.5 is investigated.

Comparison of Crack Growth Rate between CT-BM and CT-WM specimens

The Fatigue Crack Growth rate data for Base Metal and Weld Metal at R = 0.1 and 0.5 are summarized in Fig. 17. It can be observed that at R = 0.1, FCG rate of CT-BM and CT-WM is lower than tests at R = 0.5. This result is probably due to crack closure phenomenon which appears for tests performed at R = 0.1. As shown in Fig. 17, FCG behavior of CT-WM specimen is close to that of CT-BM specimen tested at R = 0.5. In contrast, da/dN-ΔK curve of CT-WM at R = 0.1 is lower to that of CT-BM specimen and especially at high ΔK values.

This result can be explained by the high tortuosity of fatigue crack path of CT-WM specimen which enhances FCG resistance. Therefore, the combined effect of crack deflection, dendritic orientation and crack closure at R = 0.1 enhanced the crack growth resistance in CT-WM specimen more than in coarse-grained stainless steel such as CT-BM specimen. It was also deduced from previous work that welded zones present an important value of tensile or compression residual stresses [5]. In the case of this study, residual stresses were not been quantified. However, it can be supposed that residual stresses can be combined with other phenomena cited above and induce the contact between crack lips at R = 0.1, and then reduce FCG rate in the welded structure compared to R = 0.5.

Despite the high tortuosity of CT-WM crack path, FCG rate doesn't seems to be affected by this morphology and it is close to FCG rate of CT-BM at R = 0.5. These phenomena can be balanced producing a slight variation of FCG rate between Base Metal and Weld Metal zones in CT-WJ specimen.

Conclusions

The Fatigue Crack Growth behavior of SMAW welded Alloy28 superaustenitic stainless steel is investigated in this study. The main conclusions are drawn as follows:

(1) Fatigue Crack Growth rate is affected by load ratio for Base Metal and Weld Metal specimens. This result is due to the presence of roughness-induced and plasticity-induced crack closure at lower load ratio.

(2) Base Metal specimens present a crack propagation mode influenced by microstructure for lower ΔK values, while for higher ΔK, crack propagation mode is transgranular with the presence of fatigue striations.

(3) Damage mechanisms of Weld Metal specimens are highly dependent to microstructure at different ΔK value.

(4) Based on microstructural observation of fatigue crack growth path of Weld Metal specimen, it is found that crack follows dendritic orientation giving then a tortuous crack path which enhances Fatigue Crack Growth resistance at R = 0.1.

(

Figs. 10

 10 Figs. 10 show SEM observations of fractured CT-BM specimen tested at R = 0.5 for different ΔK values. At ΔK = 11 MPa√m (da/dN = 3.2x10 -9 m/cycle), local propagation is observed on a grain or twin, which confirm a transgranular propagation mode. At ΔK = 15 and 20 MPa√m, fractured surface presents several cleavage facets, relief appearance and secondary cracks.

  At relatively low stress intensity factor range (ΔK = 11 and 15 MPa√m), the reversible plastic zone (RPZ) size of Base Metal specimens is smaller than austenitic grain size, so the fatigue crack propagates by a mechanism that follows individual deformation mode of the localized area inside the grain and adjacent to the grain or twin boundaries[START_REF] Goto | Formation of a high-cycle fatigue fracture surface and a crack growth mechanism of ultrafine-grained copper with different stages of microstructural evolution[END_REF]. At higher ΔK value (ΔK = 20 MPa√m), the RPZ size of Base Metal specimens is larger than the grain size, which enables the crack to interact with several grains during cyclic loading and to grow along grain boundaries, presenting a transgranular fracture mode[START_REF] Ho | Fatigue properties of ultrafine grained low carbon steel produced by equal channel angular pressing[END_REF]. Additionally, RPZ values in BM show that plastic zone size increases and exceeds grain size value when ΔK reaches 20 MPa√m, inducing then plasticity-induced crack closure at lower load ratio. In fact, at R = 0.5 crack closure are assumed to be limited, which is not the case at R = 0.1[START_REF] Xiong | Fatigue crack growth behavior in fusion zone of 16MnR steel weldment[END_REF]. Otherwise, Alloy28 Base Metal shows a high Bauschinger effect[START_REF] Kchaou | Microstructural, compositional and mechanical investigation of Shielded Metal Arc Welding (SMAW) welded superaustenitic UNS N08028 (Alloy28) stainless steel[END_REF]. This can produce the contact between crack lips and causes crack closure. In addition, slip bands appearing on crack lips during cyclic loading can produce crack surface contact and contribute directly to crack closure phenomenon.Referring to previous work, Trudel[START_REF] Trudel | Fatigue crack growth in the heat affected zone of a hydraulic turbine runner weld[END_REF] showed that crack path deviations of a martensitic stainless steel reduce FCG at R = 0.1. In this work, it has been found that crack closure is induced by plasticity, roughness, oxide, viscous fluid and phase transformation[START_REF] Newman | Mechanics of fatigue crack closure[END_REF].Vor [21]

Fig. 16 .

 16 Fig.[START_REF] Ho | Fatigue properties of ultrafine grained low carbon steel produced by equal channel angular pressing[END_REF]. At ΔK = 11 and 15 MPa√m, fractured surface seems to have a roughly aspect with the presence of several secondary cracks. On the other hand, it can be observed the presence of brittle particles in the whole of fractured surface of the Alloy28 stainless steel as found in a previous work[START_REF] Kchaou | Microstructural, compositional and mechanical investigation of Shielded Metal Arc Welding (SMAW) welded superaustenitic UNS N08028 (Alloy28) stainless steel[END_REF]. These particles are considered as welding defect which can produce a

)

  It can be supposed that different phenomena can be compensated in CT-WM specimen, such as plasticity, welding defects, residual stresses, tortuosity and roughness, and lead FCG behavior similar to CT-BM specimens at R = 0.5. Concerning CT-WJ specimen, it presents an almost stable Fatigue Crack Growth rate between Base Metal and Weld Metal zones which can be attributed to combination of crack deflection, roughness-induced crack closure, plasticity-induced crack closure, welding defects and residual stresses.

Figure 1 .Figure 2 . 2 Figure 4 .Figure 5 .Figure 7 .Figure 8 .Figure 9 . 5 Figure 10 .Figure 18 .Figure 19 .Figure 20 . 14 Figure 21 .

 122457895101819201421 Figure 1. Fatigue crack growth test machine

  Lieurade et al. indicated that generally fatigue resistance of most weld metals depends not only on mechanical In addition, as the crack closure phenomenon is absent in CT-WM specimen at R = 0.5, welding defects can accelerate crack growth rate and lead to decrease the FCG resistance and make it close to FCG behavior of CT-BM.Fatigue Crack Growth tests were performed on CT-WJ specimen notched perpendicularly to welded joint as shown in Fig.3 c). The test was carried out at constant ΔK = 20 MPa√m and at R = 0.1. Crack propagates initially in the Base Metal in the rolling direction, and then reaches Weld Metal zone. This test enables to determine FCG rate evolution when crack reaches Weld Metal. The cycle counter was initialized to zero when crack begins to propagate in Weld Metal. Crack length evolution versus number of cycles curve as well as FCG rate versus crack length in Base Metal and Weld Metal zone can then be determined (Fig.18and Fig.19 respectively). Crack length evolution presents a linear law with a slope equal to 4x10-6 mm/cycle and 3x10 -6 mm/cycle for Base Metal and Weld Metal zone respectively. Although the microstructure of Weld Metal is mainly constituted of hard dendritic grains, which limits the size of plastic zone at crack tip as showed in table 5, FCG rate is almost close to the one of Base Metal zone, with a slight decrease. This can be explained by the presence of different phenomenon such as welding defects and residual stresses which can be compensated and balanced that leads to have a similar FCG rate in Weld Metal and Base Metal zone. In order to understand this behavior, SEM observation of fractured CT-WJ specimen is necessary.Optical micrograph of crack path of Weld Metal specimen is showed in Fig.20. It can be seen that crack path is more tortuous when crack reaches Weld Metal zone. It can be noted also that dendritic orientation is perpendicular to crack propagation axis, giving then a cellular shape contrarily to CT-WM specimens. The fatigue crack path deflection of CT-WM is more pronounced than CT-WJ specimen. The difference of crack path morphology and of microstructure orientation can induce a slight increase of FCG rate in Weld Metal zone compared to CT-WM specimen (3x10 -8 m/cycle and 10 -8 m/cycle respectively). The optical micrograph of fractured surface of CT-WJ specimen is displayed in Fig.21. It can be seen that there is a difference between surface morphology of Base Metal zone and Weld Metal zone with a rougher aspect in the Base Metal zone. SEM observations of the different zone of CT-WJ specimen were carried out and showed in Fig.22. It can be observed that Base Metal presents a high roughness surface compared to Weld Metal. The stability of FCG rate between the two studied zones can be explained by the presence of multiple phenomena, as roughnessinduced crack closure in Base Metal which decelerates FCG rate, and also the presence of brittle particles combined to residual stress in Weld Metal which can accelerate FCG rate.

	3.5. Fatigue Crack Growth tests on Alloy28 welded joints
	3.5.1 Crack Growth rate curves
	3.5.2. Morphological analysis of CTWJ specimens

properties, but also on internal defects in the structure

[26]

. Welding defects, as porosites between dendrites, microcracks and brittle particles formed after solidification, are considered as stress concentration zones which can increase FCG rate.

Table 1 .

 1 Welding parameters for shielded metal arc welding process

	Number of	Filler metal	Current (A) Voltage (V)	Welding	Heat input
	passes	diameter			speed	(KJ/cm)
		(mm)			(cm/min)
	1	2.5	60		24.6	3.8
	2-4				29.31	7.45
	5-8 9-12	4	140	26	31.4 33.16	6.96 6.58
	13-15				32	6.81

Table 2 .

 2 Chemical composition of Base Metal and Weld Metal

	Element	Fe	Ni	Cr	Mo	Si
	(wt.%)
	BM	36.40	31.95	28.00	3.41	0.22
	WM	35.85	33.36	27.00	3.14	0.64
	Filler metal	Bal.	30.9	27.1	3.8	0.9

Table 3 .

 3 Paris law parameters of Base Metal specimens

	R	C	m
	0.1	6x10 -15	4.87
	0.5	2.75x10 -14	4.84

Table 4 .

 4 Value of RPZ and da/dN of Base Metal and Weld Metal specimens

	ΔK (Mpa√m)	rp (µm)
		BM	WM
	11	43	19
	15	80	35.5
	20	141	62.8

Table 5 .

 5 Paris law parameters of Weld Metal specimens

	R	C	m
	0.1	9x10 -16	5.28
	0.5	4x10 -14	4.63