Paula Méndez-Fernandez
email: paula.mendez_fernandez@outlook.com

Lynda Webster

Tiphaine Chouvelon

Paco Bustamante

Marisa Ferreira

Angel F González

Alfredo López

Colin F Moffat

Graham J Pierce

Fiona L Read

Marie Russell

Maria B Santos

Jérôme Spitz

José V Vingada

Florence Caurant

An assessment of contaminant concentrations in toothed whale species of the NW Iberian Peninsula: Part II. Trace element concentrations

Keywords: trace elements, bioaccumulation, toothed whales, Northwest Iberian Peninsula

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

Introduction

A better understanding of marine mammal ecology and physiology, including changes related to anthropogenic activities such as concentrations of chemical contaminants, is needed.

Specifically there is a requirement to clarify the significance of contaminants in the wellbeing of marine mammal populations and the possibility of population level impacts from contaminants, with the objective of improved protection for these apex species. In Europe, recent legislation has been implemented with the aim of protecting and restoring ecological quality or integrity in estuarine and marine coastal and offshore systems (e.g. Water Framework Directive (WFD), Marine Strategy Framework Directive (MSFD), Habitats Directive) in which cetaceans are specifically recognised as an important component.

The Northwest Iberian Peninsula (NWIP) is situated at the northern limit of the NW African upwelling system [START_REF] Figueiras | Coastal upwelling, primary production and mussel growth in the Rias Baixas of Galicia[END_REF]. During the summer, the prevalent northerly winds favour the upwelling of nutrient-rich Eastern Atlantic Central Water (ENACW) that in turn sustains a high productivity and a considerable biodiversity. Nearly 300 species of fish, 78 species of cephalopods and at least 16 species of marine mammals have been reported in the area (Penas-Patiño and Piñeiro-Seage, 1989; Fernández de la [START_REF] De | Os cabaleiros do mar: baleas e golfiños das nosas augas[END_REF][START_REF] Guerra | Cephalopoda. Fauna Ibérica[END_REF].

During the last decades, industrial development and an increase in other human marinerelated activities in the area have intensified the pressures on the marine environment. To monitor the level of contamination, analyses of samples of sediments, seawater and commercial species such as shellfish are routinely carried out (e.g. [START_REF] Carro | Levels and distribution patterns of Polychlorinated biphenyl congeners in surface sediments from Galicia coast (Northwestern, Spain) according to granulometric characteristics[END_REF][START_REF] Prego | Twentieth century overview of heavy metals in the Galician Ria (NW Iberian Peninsula)[END_REF]. In addition, the monitoring of contaminants in the area increased after the Prestige tanker spill on 19 th November 2002 (e.g. Prego and Cobelo-Garcίa, 2004;[START_REF] Junoy | The macroinfauna of the Galician sandy beaches (NW Spain) affected by the Prestige oil-spill[END_REF]Pérez-López et al., 2006;[START_REF] Moreno | Seabird feathers as monitors of the levels and persistence of heavy metal pollution after the Prestige oil spill[END_REF][START_REF] Romero | Changes in the diet of coastal otters after the Prestige oil spill (Galicia, north-west Spain)[END_REF]. The presence of potentially toxic substances has occasionally been investigated in marine mammals from the NWIP since the 1980s (e.g. [START_REF] Borrell | Organochlorine compounds in common dolphin (Delphinus delphis) from the Atlantic and Mediterranean waters of Spain[END_REF][START_REF] Borrell | Organochlorine compounds and stable isotopes indicate bottlenose dolphin subpopulation structure around the Iberian Peninsula[END_REF][START_REF] Zegers | et alLevels of hexabromocyclododecane in harbor porpoises and common dolphins from western European seas, with evidence for stereoisomer-specific biotransformation by cytochrome P450[END_REF][START_REF] Pierce | Bioaccumulation of persistent organic pollutants in female common dolphins (Delphinus delphis) and harbour porpoises (Phocoena phocoena) from western European seas: Geographical trends, causal factors and effects on reproduction and mortality[END_REF][START_REF] Murphy | Assessing the Effect of Persistent Organic Pollutants on Reproductive Activity in Common Dolphins and Harbour Porpoises[END_REF].

In contrast to most organic chemicals, trace elements occurring in the marine environment are derived from both natural and anthropogenic (e.g. mining and industrial discharges) sources.

Trace elements can be divided into essential elements and non-essential elements. The essential elements include Cu, Zn, Se; these have a biological function. The non-essential elements include Hg, Cd, Pb). Many of these are potentially toxic, even at low concentrations [START_REF] Chappuis | Les Oligoéléments en médecine et biologie[END_REF]. Marine mammals, as long-lived apex predators, are potentially threatened by non-essential trace elements, since most are bioaccumulated and biomagnified through food webs [START_REF] Law | Metals in marine mammals[END_REF]Das et al., 2003a). Their distribution in tissues basically follows their chemical affinities. Hg, Cu, Zn and other elements exhibit the higher concentrations in liver, Cd accumulates in kidney and Pb in bones [START_REF] Honda | Distribution characteristics of heavy metals in the organs and tissues of striped dolphin Stenella coeruleoalaba[END_REF]André et al., 1990aAndré et al., , 1990b)).

The actual toxic effects of trace elements remain unclear since marine mammals have been exposed to these natural compounds for a very long time in evolutionary terms and as such have developed mechanisms to control and/or mitigate their toxic effects [START_REF] Law | Metals in marine mammals[END_REF]Gallien et al., 2001;Vos et al., 2003). As an example, the detoxification process of methylmercury by selenium through the formation of tiemanite in the liver [START_REF] Koeman | Mercury selenium correlation in marine mammals[END_REF][START_REF] Martoja | Identification of tiemannite as a probable product of demethylation of mercury by selenium in cetaceans. A complement to the scheme of the biological cycle of mercury[END_REF] leads to high concentrations of Hg, especially in toothed whales, without any obvious toxic effects (e.g. [START_REF] Koeman | Mercury selenium correlation in marine mammals[END_REF]Julshamn et al., 1987;[START_REF] Caurant | Mercury in Pilot Whales: possible limits to the detoxification process[END_REF][START_REF] Nyman | Current levels of DDT, PCB and trace elements in the Baltic ringed seals (Phoca hispida baltica) and grey seals (Halichoerus grypus)[END_REF]. Nevertheless, non-essential elements are more frequently monitored because of their threat and toxicity [START_REF] O'shea | Environmental contaminants and marine mammals[END_REF] and surrogate values from terrestrial mammals are used to evaluate their health effects on marine mammal populations.

The concentration of contaminants in marine mammal tissues varies primarily in relation to the prey they consume, but is also a function of their capacity to excrete these elements [START_REF] Aguilar | Biological factors affecting variability of persistent pollutant levels in cetaceans[END_REF]. Several biological factors have also been found to affect variations in trace element concentrations in marine mammals. These include, in particular, age (André et al., 1990a(André et al., , 1990b) ) but also body size and composition, nutritive condition, sex, health status, reproductive status/history duration of lactation and/or toxicodynamic and toxicokinetic processes (e.g. [START_REF] Muir | Organochlorine chemical and heavy metal contaminants in White-beaked dolphins (Lagenorhynchus albirostris) and pilot whales (Globicephala melaena) from the coast of Newfoundland, Canada[END_REF][START_REF] Caurant | Ecological and biological factors controlling the concentrations of trace elements (As, Cd, Cu, Hg, Se, Zn) in delphinids Globicephala melas from the North Atlantic Ocean[END_REF]Das et al., 2003a). Therefore, these factors have to be carefully taken into account when interpreting contaminant concentrations and their potential effects. This paper constitutes the part II of a two-part series; in this second part we extend the assessment of contamination status of the five most common cetacean species in the NWIP waters, initiated in part I (Méndez-Fernandez et al., submitted), comparing our results with data from previous studies on trace elements across the world and also including species from this geographic area that had not been previously analysed, such as the pilot whale. To this aim the liver and kidney of a total of 172 individuals were analysed for 14 trace elements: silver (Ag), arsenic (As), cadmium (Cd), cobalt (Co), chrome (Cr), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), selenium (Se), vanadium (V) and zinc (Zn). To evaluate their trace element contamination status in the area we also compare trace element patterns among the five species studied. Finally, the Spearman correlation coefficient test was calculated in order to determine any co-linearity between concentrations of different trace elements focusing on Cu, Se and Zn since they are known to participate in the Cd and Hg detoxification processes.

Materials and methods

Sampling and study area

Sampling was carried out in the NWIP, from the northern limit of the Galician coast in Spain (43°3'N, 7°2'W) to Nazaré on the Portuguese coast (39°36'N, 9°3'W) during 2004 to 2008 (Fig. 1). Experienced members of the Spanish (Coordinadora para o Estudo dos Mamiferos Mariños, CEMMA) and Portuguese (Sociedade Portuguesa de Vida Salvagem, SPVS) stranding networks have been attending stranded and by-caught cetaceans for more than a decade and two decades respectively. Animals were identified to species, measured, sexed and, if the decomposition state of the carcass allowed, full necropsies were performed and samples were collected whenever possible. All procedures followed the standard protocol defined by the European Cetacean Society (ECS) including the decomposition state condition code [START_REF] Kuiken | Proceedings of the first European Cetacean Society workshop on 'Cetacean pathology: dissection techniques and tissue sampling[END_REF]. The animals recovered in a "fresh" state (a score of 1 to 3 from the ECS protocol, i.e. originally stranded alive, freshly dead or mildly decomposed) were selected. The common dolphin Delphinus delphis is the most frequently cetacean species in this area, which is believed to be a direct result of the large number of individuals being by-caught in NWIP fisheries [START_REF] López | Trends in strandings of cetaceans on the Galician coast, NW Spain, during the 1990s[END_REF][START_REF] López | Fishery by-catches of marine mammals in Galician waters: results from on-board observations and an interview survey of fishermen[END_REF], and therefore is the best represented species in the samples analysed (n = 114), but smaller numbers of four other species were also available: long-finned pilot whale Globicephala melas, n = 9; harbour porpoise Phocoena phocoena, n = 19; striped dolphin Stenella coeruleoalba, n = 21 and bottlenose dolphin Tursiops truncatus, n = 9.

Teeth were collected for age determination and liver and kidneys for trace element analyses.

After the necropsies, all samples of liver and kidneys for trace element analyses were removed and stored in polyethylene bags and stored frozen at -20°C until required for analysis.

Determination of age

At least five teeth were collected from each sampled individual, selecting the least worn/damaged and least curved teeth, to ensure sufficient material for replicate preparations.

Teeth were preserved frozen or in 70% alcohol. Age was determined by analyzing growth layer groups (GLGs) in the dentine of the teeth, following methods adapted from [START_REF] Lockyer | A report on patterns of deposition of dentine and cement in teeth of pilot whales, genus Globicephala[END_REF], [START_REF] Hohn | Protocol for obtaining age estimates from harbour porpoise teeth. Appendix 3, Report of the harbour porpoise age determination workshop[END_REF] and [START_REF] Rogan | Age determination in small cetaceans from the NE Atlantic[END_REF]. In brief, teeth were decalcified and then sectioned using a cryostat. The most central and complete sections (including the whole pulp cavity) were selected from each tooth, stained with Mayer's haematoxylin (modified by Grue) and 'blued' in a weak ammonia solution, then mounted on glass slides and allowed to dry. GLGs were counted under a binocular microscope. All readings were initially made blind (i.e. without access to individual biological data) and replicate counts were made by two independent readers.

Determination of trace elements

All the equipment used in the sample processing was cleaned, and subsequently decontaminated for 24 h in a solution composed of 35 mL HNO 3 (65%) and 50 mL HCl (36%) mixed into 1 L of Milli-Ro quality water. Frozen liver and kidney samples were freezedried and ground to powder using a Planetary Ball Mills Retsch PM 200. Two replicates for each sample were prepared for trace element analyses.

Ag, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Se, V and Zn analyses were performed with a Varian Vista-Pro for Inductively Coupled Plasma Atomic Emission Spectrophotometry (ICP-AES) and a Thermofisher Scientific XSeries 2 for Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Total Hg was determined using an Advanced Mercury Analyser (Altec AMA 254).

For ICP-AES and ICP-MS measurements, aliquots of dried samples, from 0.1 to 0.3 g, were digested with 6 mL 67-70% HNO 3 and 2 mL 34-37% HCl (both from Merck and of Suprapur® quality). Acid digestion of the samples was carried out overnight at room temperature and then in a Milestone microwave oven. The oven temperature was increased over a period of 30 min up to a final temperature of 120°C. The temperature was maintained at this maximal value for 15 minutes. After digestion, each sample was made up to a final volume of 50 mL with milli-Q water. For samples with a weight < 0.1 g, the mixture used was 3 mL 67-70% HNO 3 /1 mL 34-37% HCl and the samples were diluted to a final volume of 25 mL milli-Q water.

Mercury analysis was carried out with an Automatic Mercury Analyser spectrophotometer, ALTEC AMA 254 (AAS), which does not require an acid-digestion of the samples. Aliquots of 5 ± 0.5 mg dried sample were directly analysed after being inserted in the oven of the apparatus. After drying, the samples were heated under an oxygen atmosphere for 3 min, and the Hg liberated and subsequently amalgamated on an Au-net. The net was then heated to liberate the collected Hg, which was measured by AAS.

All trace element concentrations in tissues are reported in µg g -1 wet weight (wet wt.).

Quality control

All methods were validated by the replicate analysis of standards and samples, and through spiking experiments or analysis of certified reference materials (CRMs). For trace elements, two CRMs and three to six blanks, treated and analysed in the same way as the samples, were included in each analytical batch. The CRMs were National Research Council Canada [NRCC]) and TORT-2 (lobster hepatopancreas; NRCC). For all trace elements the measurement were in satisfactory agreement with the given certified values and showed mean recoveries of 94.5% for DOLT-4 and 97.5% for TORT-2. The limits of detection (LODs) (µg g -1 wet wt.) were 0.07 for Ag, Cd, Co, Cr and Pb; 0.67 for As; 0.2 for Ni; 0.35 for Se; 1.0 for V; 2.5 for Cu and Mn; 13 for Fe and Zn; 0.002 for Hg. Finally, these protocols were validated by an international intercalibration exercise [START_REF] Christopher | Description and results of the NIST/NOAA 2005 interlaboratory comparison exercise for trace elements in marine mammals[END_REF].

Data treatment

All data submitted to statistical tests were first checked for normality (Shapiro-Wilk test) and for homogeneity of variances (Bartlett test). When these conditions were satisfied, parametric tests were performed in the subsequent analyses; otherwise, data were transformed or nonparametric tests were used.

Variation in log-transformed trace element concentrations in toothed whales was investigated using analysis of covariance (ANCOVA). As age is an important factor influencing the accumulation of trace elements [START_REF] Aguilar | Biological factors affecting variability of persistent pollutant levels in cetaceans[END_REF], this parameter was used as the covariate in the ANCOVA to determine the influence of gender and species. When significant betweenspecies variation was detected, the ANCOVA was followed by Tukey multi-comparison tests to evaluate differences between each pair of species. In addition, discriminant analysis (DA) was used to examine differences in the trace element patterns among the species. As for ANCOVA analysis, only elements with concentrations above the limit of detection (see Table 1) were included in the DA.

Finally, the Spearman correlation coefficient test was calculated in order to determine any colinearity between concentrations of different trace elements. We focussed on Cu, Se and Zn since they are known to participate in the Cd and Hg detoxification processes. In addition, we evaluated the Hg detoxification process through the calculation of the Hg:Se molar ratio. This ratio was calculated as: Hg:Se = (Hg (µg g -1 wet wt.)/Se (µg g -1 wet wt.)) × (78.96 (g mol - 1 )/200.59 (g mol -1 )), where 200.59 g mol -1 and 78.96 g mol -1 are the atomic mass of Hg and Se, respectively.

The levels of significance for statistical analyses were always set at α = 0.05 and analysis were performed using R version 2.11.1 (R Development Core Team, 2010).

Results and discussion

Concentrations of trace elements in the light of biological and ecological factors

Trace element concentrations in the tissues of marine mammals are well-documented on a global basis, with a focus on the non-essential elements Cd and Hg, and the essential ones, Cu, Zn and Se for their role in the detoxification of Cd and Hg. However, few studies have investigated the wider range of trace elements present in the liver and kidney of marine mammals as studied here (e.g. [START_REF] Bustamante | Trace elements in two odontocete species (Kogia breviceps and Globicephala macrorhynchus) stranded in New Caledonia (South Pacific)[END_REF][START_REF] Bryan | Establishing baseline levels of trace elements in blood and skin of bottlenose dolphins in Sarasota Bay, Florida: Implications for non-invasive monitoring[END_REF][START_REF] Griesel | Trace element concentrations in blood of harbor seals (Phoca vitulina) from the Wadden Sea[END_REF][START_REF] Stavros | Concentrations of trace elements in blood and skin of Florida manatees (Trichechus manatus latirostris)[END_REF]. The reason is probably that the trace elements analyzed are either not bioaccumulated in large amounts in mammalian tissues [START_REF] Thompson | Heavy metals in marine vertebrates[END_REF]; or liver and kidney are not the organs of storage for some of them. In humans, for example, baseline data on As, Ag, Co, Cr, Mn, Ni or V is effectively more often available in blood than in tissues, and recent studies in marine mammals that have investigated as many or even more trace elements than in this study, dealt with blood or skin as non invasive monitoring samples [START_REF] Bryan | Establishing baseline levels of trace elements in blood and skin of bottlenose dolphins in Sarasota Bay, Florida: Implications for non-invasive monitoring[END_REF][START_REF] Griesel | Trace element concentrations in blood of harbor seals (Phoca vitulina) from the Wadden Sea[END_REF][START_REF] Stavros | Concentrations of trace elements in blood and skin of Florida manatees (Trichechus manatus latirostris)[END_REF].

The trace elements analysed exhibit different status. Cd, Pb, Hg and Ag are toxic elements for which no biological function has been demonstrated so far. In contrast, Fe, Cu, Co, Zn, Se and Cr are essential elements whose deficiency induces pathology in human and more generally in mammals [START_REF] Chappuis | Les Oligoéléments en médecine et biologie[END_REF][START_REF] Underwood | Trace elements in human and animal nutrition[END_REF]. The status of As depend on the physicochemical forms to which mammals are exposed. In humans, the toxicological risk of As is due to inorganic forms to which they can be exposed in occupational situations. In the marine environment, tissues of marine invertebrates and fish contain high concentrations of As (1 to 100 μg g -1 dry wt.) in the form of organoarsenic compounds, particularly arsenobetaine [START_REF] Neff | Ecotoxicology of arsenic in the marine environment[END_REF]. Our results revealed that concentrations of the essential elements Co, Pb and V were below the detection limit in all species for both kidney and liver, while the concentrations of Ag and Mn were below the detection limit in kidney for all species (Table 1). Concerning As, only pilot whale and bottlenose dolphin exhibited detectable concentrations of this element and probably as a consequence of a higher exposure through their diet. [START_REF] Kubota | Arsenic accumulation in the liver tissue of marine mammals[END_REF] suggest that cetacean feeding on cephalopods and crustaceans leads to higher retention of As concentration than a piscivorous diet, which is the case of pilot whales. The remaining elements being above the detection limit could be categorized into two distinct groups, within which the patterns were generally similar across all species. Four of the elements (Cu, Fe, Hg and Se) exhibited higher concentrations in liver than in kidney (see Hg shown as an example in Fig. 2a). In contrast, Cd, Cr and Ni showed lower concentrations in the liver compared to kidney (see example of Cd, Fig. 2b).

One characteristic of essential elements in marine mammal tissues is that they are regulated through homeostasis. The processes leading to homeostatic control include regulation of intestinal absorption and/or biliary and urinary excretion. As a consequence, essential trace elements are generally not bioaccumulated with age. This fact explains the lack of a significant effect of age (ANCOVA, P > 0.05) found in this study on renal Cr, Se and Zn and for hepatic Cu, Fe and Zn, all essential elements (Table 2). Conversely, concentrations of nonessential trace elements have been reported to increase with age in several cetacean species and across various geographical areas, especially the most studied elements, Hg and Cd [START_REF] Honda | Heavy metal concentrations in muscle, liver and kidney tissue of striped dolphin, Stenella coeruleoalba, and their variations with body length, weight, age and sex[END_REF][START_REF] Caurant | Ecological and biological factors controlling the concentrations of trace elements (As, Cd, Cu, Hg, Se, Zn) in delphinids Globicephala melas from the North Atlantic Ocean[END_REF][START_REF] Bustamante | Trace element bioaccumulation in grey seals Halichoerus grypus from the Faroe Islands[END_REF][START_REF] Lahaye | New insights from age determination on toxic element accumulation in striped and bottlenose dolphins from Atlantic and Mediterranean waters[END_REF][START_REF] Lahaye | Biological and ecological factors related to trace element levels in harbour porpoises (Phocoena phocoena) from European waters[END_REF].

Both of them probably accumulate because uptake exceeds the ability of the animal to excrete these elements. Consequently, concentrations will increase with age in the respective storage tissues, liver for Hg and kidney for Cd. However, the kinetics of accumulation seems to differ between Hg and Cd, while remaining similar across species: Hg bioaccumulation occurs throughout the life of individuals, while Cd reaches a plateau after several years, more or less rapidly according to the species [START_REF] Aguilar | Biological factors affecting variability of persistent pollutant levels in cetaceans[END_REF][START_REF] Caurant | Ecological and biological factors controlling the concentrations of trace elements (As, Cd, Cu, Hg, Se, Zn) in delphinids Globicephala melas from the North Atlantic Ocean[END_REF][START_REF] Bustamante | Trace element bioaccumulation in grey seals Halichoerus grypus from the Faroe Islands[END_REF]. Only hepatic Hg showed a lack of a significant age effect on concentration in this study (Table 2); this counterintuitive result could be the consequence of the high number of young individuals in our sample set.

Sex is another biological factor that can have an influence on trace element burdens in marine mammals, and specifically reproductive activities such as pregnancy, parturition and lactation [START_REF] Honda | Heavy metal accumulation and their recent changes in Southern minke whales Balaenoptera acutorostrata[END_REF]. In addition, differences in prey preferences and in feeding areas between females and males may affect trace element burdens [START_REF] Gochfeld | Factors influencing susceptibility to metals[END_REF]. However, the influence of sex on trace element concentrations measured in marine mammals is not wellunderstood and it seems to vary with the element, the tissue and the species analysed [START_REF] Honda | Heavy metal accumulation and their recent changes in Southern minke whales Balaenoptera acutorostrata[END_REF][START_REF] Canella | Differences in mercury levels in female sperm whale, Physeter macrocephalus (Cetacea: Odontoceti)[END_REF][START_REF] Wagemann | Arctic marine mammals as integrators and indicators of mercury in the Arctic[END_REF][START_REF] Aguilar | Biological factors affecting variability of persistent pollutant levels in cetaceans[END_REF]. In this study there was no influence of sex, except male common dolphin showing higher renal Cd concentrations than females (P < 0.05).

Several different routes of entry of trace elements have been suggested in marine mammals: uptake from the atmosphere, absorption through the skin or across the placenta, by ingestion of seawater and ingestion of food including milk for calves (e.g., [START_REF] Caurant | Ecological and biological factors controlling the concentrations of trace elements (As, Cd, Cu, Hg, Se, Zn) in delphinids Globicephala melas from the North Atlantic Ocean[END_REF][START_REF] Wagemann | Arctic marine mammals as integrators and indicators of mercury in the Arctic[END_REF][START_REF] Law | Metals in marine mammals[END_REF][START_REF] Lahaye | Biological and ecological factors related to trace element levels in harbour porpoises (Phocoena phocoena) from European waters[END_REF]. However, the main route seems to be via food (André et al., 1990a(André et al., , 1990b;;[START_REF] Law | Metals in marine mammals[END_REF]. As a consequence of this, the trophic ecology of species also has an important influence on trace element concentrations. In this study, we obtained significant differences (ANCOVA, P < 0.05, Table 2) between species for all trace elements in at least one of the two tissues analysed (for trace elements being above the detection limit). Pilot whale and striped dolphin exhibited the highest Cd concentrations in both liver and kidney (see Table 1). This result is in line with the similar feeding patterns of these species since both species primarily feed in offshore waters and have a high proportion of oceanic squid in their diet [START_REF] Spitz | Dietary plasticity of the oceanic striped dolphin, Stenella coeruleoalba, in the neritic waters of the Bay of Biscay[END_REF][START_REF] Spitz | Prey preferences among the community of deep-diving odontocetes from the Bay of Biscay, Northeast Atlantic[END_REF]Santos et al., 2013a;2013b). Squid are known to accumulate large amounts of cadmium and to be the major source of Cd in top predators [START_REF] Bustamante | Cephalopods as a vector for the transfer of cadmium to top marine predators in the north-east Atlantic Ocean[END_REF][START_REF] Lahaye | Long-term dietary segregation of common dolphins Delphinus delphis in the Bay of Biscay, determined using cadmium as an ecological tracer[END_REF]. Pilot whale, striped and bottlenose dolphins also showed the highest Hg concentrations in liver and in kidney (Table 1). Such Hg accumulation in species having different feeding preferences (fish vs cephalopods) is consistent with the high bioavailability of Hg for upper trophic levels in both fish and cephalopods [START_REF] Bloom | On the chemical form of mercury in edible fish and marine invertebrate tissue[END_REF][START_REF] Bustamante | Total and organic Hg concentrations in cephalopods from the North Eastern Atlantic waters: Influence of geographical origin and feeding ecology[END_REF], and not only on fish prey as was previously thought. Indeed, Atlantic striped dolphins also consumed mesopelagic fishes, such as Myctophids (lanternfish) and Sternoptychids (small deep-sea ray-finned fish) [START_REF] Ringelstein | Food and feeding ecology of the striped dolphin, Stenella coeruleoalba, in the oceanic waters of the north-east Atlantic[END_REF][START_REF] Spitz | Dietary plasticity of the oceanic striped dolphin, Stenella coeruleoalba, in the neritic waters of the Bay of Biscay[END_REF], and these mesopelagic fish from Atlantic waters are well known to also exhibit high Hg concentrations [START_REF] Monteiro | Mercury concentrations in prey fish indicate enhanced bioaccumulation in mesopelagic environments[END_REF][START_REF] Thompson | Seabirds as biomonitors of mercury inputs to epipelagic and mesopelagic marine food chains[END_REF][START_REF] Lahaye | New insights from age determination on toxic element accumulation in striped and bottlenose dolphins from Atlantic and Mediterranean waters[END_REF][START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (north-east Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF].

The influence of trophic ecology and biological process in trace elements accumulation is confirmed by the discriminant analysis performed (Fig. 3). In fact, harbour porpoise (Pph in Fig. 3) appear distinct from the other species with a focus on the upper left portion of the plot (low LD1, high LD2). This species is the most frequently observed near to the coast and mainly feeds on fish species in the NWIP [START_REF] Santos | The Diet of Harbour porpoise (Phocoena phocoena) in the Northeast Atlantic[END_REF][START_REF] López | Distribution patterns of small cetaceans in Galician waters[END_REF][START_REF] Pierce | Trends in cetacean sightings along the Galician coast, north-west Spain, 2003-2007, and inferences about cetacean habitat preferences[END_REF]. On the lower right portion there are the oceanic and teuthophagous species, striped dolphin (Sco in Fig. 3) and pilot whale (Gme in Fig. 3), which have the highest Hg and Cd concentrations in this study. Finally, common and bottlenose dolphin appear not to gathered out and are in the centre of the individual clouds, coinciding with their intermediate position in terms of diet (fish and cephalopods consumption), and habitat (oceanic and coastal) among the five species in the area [START_REF] López | Distribution patterns of small cetaceans in Galician waters[END_REF][START_REF] Santos | Variability in the diet of common dolphins (Delphinus delphis) in Galician waters 1991-2003 and relationship with prey abundance[END_REF][START_REF] Santos | Variability in the diet of bottlenose dolphin, Tursiops truncatus, in Galician waters, north-western Spain, 1990-2005[END_REF][START_REF] Pierce | Trends in cetacean sightings along the Galician coast, north-west Spain, 2003-2007, and inferences about cetacean habitat preferences[END_REF]. The main elements forcing separation along the canonical axis 1 were renal Cu, renal Se, hepatic Cd and hepatic and renal Cr. Along canonical axis 2, the most important elements determining separation were hepatic Ag, hepatic and renal Cr and renal Cu. These findings reflect distinct trace element accumulation patterns in toothed whales from the NWIP, which can be determined by factors such as diet and habitat use, as well as by different biological processes for the species. Moreover, these results are consistent with the PCB patterns obtained in the first part of this study (Méndez-Fernandez et al., submitted); the pilot whale and the striped dolphin had a higher proportion of less chlorinated congeners than the other three species, which showed a higher proportion of the higher chlorinated PCB congeners. Finally, these results are also highly consistent with the niche segregation among these toothed whales in the NWIP by using Cd, carbon and nitrogen isotopes as ecological tracers [START_REF] Méndez-Fernandez | Ecological niche segregation among five toothed whale species off the NW Iberian Peninsula using ecological tracers as multi-approach[END_REF].

Interelemental relationships

The inter-specific differences in essential trace element concentrations are more intriguing than for the non-essential ones since, as discussed above, they indicate that homeostatic processes probably differ between the species. These different concentrations of essential elements could also be correlated to the concentrations of non-essential elements in the wellknown detoxification processes. For this reason correlations between trace elements known to participate in the Cd and Hg detoxification processes were tested within each species separately (Table 3). Hepatic Hg and Se were positively correlated in the five species while renal Hg and Se were positively correlated in common, striped and bottlenose dolphin. A positive correlation between Hg and Se has been largely described [START_REF] Koeman | Mercury selenium correlation in marine mammals[END_REF][START_REF] Koeman | Mercury and selenium in marine mammals and birds[END_REF][START_REF] Pelletier | Mercury-selenium interactions in aquatic organism: a review[END_REF][START_REF] Cuvin-Aralar | Mercury and selenium interaction: A Review[END_REF] and it is known as a detoxification process that provides protection against high Hg concentrations through the formation of tiemannite [START_REF] Martoja | Identification of tiemannite as a probable product of demethylation of mercury by selenium in cetaceans. A complement to the scheme of the biological cycle of mercury[END_REF]. A 1:1 Hg:Se molar ratio would indicate that almost all available Se is bound to Hg. Owing to the oxy-radical scavenging involvement of Se, tissue ratios close to 1:1 could indicate compromised health [START_REF] Dietz | An assessment of selenium to mercury in Greenland marine animals[END_REF]. Overall we observed that over a concentration of 5 nmol g -1 of Hg values are close to the Se:Hg molar ratio of 1 (Fig. 4). However, below this value of 5 nmol g -1 the variability of the molar ratio is important, indicating an excess of Se compared to Hg and thus also its bioavailability for other functions. In fact, only 2.6 % of the individuals showed a Hg:Se ratio > 1.0. One of these individuals was an immature common dolphin with a high Hg concentration of 303.9 µg g -1 wet wt. Thus, considering Hg:Se ratio, Hg is probably not the main threat for most of the Iberian toothed whales. However, it is important to note the high proportion (~70%) of immature individuals in the sampled population.

In addition, it has been shown that the essential elements Cu and Zn are commonly related to Cd detoxification (Das et al., 2000a) by induction of metallothioneins (MTs) (Wagemann et al., 1988;[START_REF] Teigen | Heavy metal content in liver and kidneys of grey seals (Halichoerus grypus) in various life stages correlated with metallothioneins levels: some metal-binding characteristics of this protein[END_REF]. These proteins, which play a role in the homeostasis of the essential elements (such as Cu and Zn), are induced by Cd as well as other trace elements [START_REF] Engle | Metallothionein and metallothionein-like proteins: physiological importance[END_REF][START_REF] George | Metallothioneins as indicators of trace metal pollution[END_REF]. Here, Zn was significantly correlated with Cd in both tissues of striped dolphins (Spearman correlation, r = -0.509 in liver and r = 0.681 in kidney, P < 0.05, Table 3), and this species was the one exhibiting the highest Cd concentrations (Table 2). There is also a positive correlation between renal Cd and Cu concentrations in striped dolphins (Spearman correlation, r = 0.235, P < 0.05) and between hepatic Cd and Cu in common dolphins (Spearman correlation, r = 0.203, P < 0.05).

However, the high Cd concentrations observed in pilot whales from the NWIP, which are the highest concentrations among the five species studied (Table 2), are probably not sufficient to induce Cu or/and Zn ion displacement from MTs, and consequently leading to coaccumulation with Cd in this species.

Geographic comparison

Data obtained for hepatic Hg and renal Cd concentrations for the five toothed whales in this study are compared with those reported previously all over the world and presented in Table 4. Overall, hepatic Hg and renal Cd concentrations of Iberian common dolphins were of the same order of magnitude than those reported by previous studies on adjacent areas of the NE Atlantic Ocean; however they are smaller than those reported in the Mediterranean Sea, in the western part of the Atlantic Ocean, and in the Pacific and Indian Ocean. The hepatic Hg concentrations of Iberian pilot whales are much smaller than those from northern latitudes of the NE Atlantic Ocean, but also smaller than in the western part of the Atlantic Ocean and than in the Pacific Ocean. For Cd the differences observed among areas are smaller than for Hg concentrations, however the northern latitudes still having greater concentrations. This result is not surprising and confirms results from various studies on the Cd and Hg in marine mammals from Arctic and temperate regions, which indicates that animals from lower latitude display far lower Cd and Hg levels in their tissues, despite a higher anthropogenic influence [START_REF] Paludan-Müller | Mercury, cadmium, zinc, copper and selenium in harbour porpoise (Phocoena phocoena) from West Greenland[END_REF][START_REF] Szefer | Intercomparison Studies on Distribution and Coassociations of Heavy Metals in Liver, Kidney, and Muscle of Harbor Porpoise, Phocoena phocoena, from Southern Baltic Sea and Coastal Waters of Denmark and Greenland[END_REF][START_REF] Bustamante | Trace element bioaccumulation in grey seals Halichoerus grypus from the Faroe Islands[END_REF]Das et al., 2004a). Hepatic Hg concentrations in Iberian porpoises were globally in the same range of concentrations as porpoises from the adjacent waters of the Atlantic Ocean, with the exception of individuals from south England and Irish Sea with means of 22 and 30 µg g -1 wet wt., respectively, compared with the 16.8 µg g -1 wet wt. from the Iberian porpoises. In contrast, lower Hg concentrations have been reported for porpoises from the North Sea, Iceland and Greenland (Table 4). Furthermore, and contrary to Hg, Iberian porpoises showed the highest concentrations from all the areas compared, with the exception of Iceland, and especially, Greenland porpoises. In fact, Cd levels are well-known to be elevated in arctic sediments and organisms (e.g. [START_REF] Macdonald | Cadmium in marine invertebrates and arctic cod in the Canadian Arctic. distribution and ecological implications[END_REF]AMAP, 1998;[START_REF] Dietz | Have Artic marine mammals adapted to high cadmium levels?[END_REF], and especially in cephalopods [START_REF] Bustamante | Cephalopods as a vector for the transfer of cadmium to top marine predators in the north-east Atlantic Ocean[END_REF]. The squid biomass occurring in stomach contents is much higher in porpoises from Greenland than in porpoises from lower latitudes of European waters [START_REF] Santos | The Diet of Harbour porpoise (Phocoena phocoena) in the Northeast Atlantic[END_REF], which could explain the higher renal Cd concentrations obtained in Greenland animals compared to Iberian animals.

Finally, striped and bottlenose dolphin showed hepatic Hg concentrations in the same order or magnitude as areas from the NW Atlantic however much smaller than dolphins from Mediterranean Sea, Adriatic Sea, SW Atlantic and Pacific Ocean, with some exceptions such as animals from France waters [START_REF] Holsbeek | Heavy metals in dolphins stranded on the French Atlantic coast[END_REF][START_REF] Lahaye | New insights from age determination on toxic element accumulation in striped and bottlenose dolphins from Atlantic and Mediterranean waters[END_REF]. Higher Hg concentrations in Mediterranean organisms are typically explained by high temperature and absence of solar radiation in the deep environment that favors a high methylation rate.

Moreover, in addition to industrial inputs, natural sources of Hg in the Mediterranean Sea may contribute to Hg enrichment through the benthic food webs, as it constitutes the richest natural reserve of this element [START_REF] Bacci | Mercury in the Mediterranean[END_REF]. Concerning renal Cd concentrations, unexpected high values were found in Iberian bottlenose dolphins (5.8 µg g -1 wet wt.). As cephalopods constitutes a major source of Cd for cetaceans [START_REF] Bustamante | Cephalopods as a vector for the transfer of cadmium to top marine predators in the north-east Atlantic Ocean[END_REF][START_REF] Lahaye | Long-term dietary segregation of common dolphins Delphinus delphis in the Bay of Biscay, determined using cadmium as an ecological tracer[END_REF], low Cd concentrations would be expected in bottlenose dolphins as their diet is dominated by fish. Therefore, this result may be due to the higher concentrations of Cd in their environment, since upwelling waters, as for the NWIP, are known to have high inputs of this element [START_REF] Boyle | On the marine geochemistry of cadmium[END_REF][START_REF] Boyle | Cadmium: Chemical tracer of deep water paleoceanography[END_REF]. However, this does not the case for striped dolphin which is a species that mainly feed on cephalopods in the NWIP but nevertheless shows lower values than dolphins from adjacent areas of the NW Atlantic Ocean. Thus, either there is a minor contribution of Cd in upwelling areas as we thought or the different feeding behavior of striped dolphins from Iberian waters and adjacent areas lead to a different Cd exposure for these animals (e.g. [START_REF] Ringelstein | Food and feeding ecology of the striped dolphin, Stenella coeruleoalba, in the oceanic waters of the north-east Atlantic[END_REF][START_REF] Spitz | Dietary plasticity of the oceanic striped dolphin, Stenella coeruleoalba, in the neritic waters of the Bay of Biscay[END_REF]Santos et al., 2013b).

Overall, this comparison suggests that Iberian toothed whale populations are not specially threatened by Hg and Cd exposure in the area. Only porpoises showed slightly higher concentrations of both elements in comparison with other areas of the Atlantic waters but far below the suggested threshold levels of effects in humans and marine mammals for renal Cd (50 µg g -1 wet wt.; [START_REF] Elinder | Cd exposure and health risks: recent findings[END_REF] and hepatic Hg (61 µg g -1 wet wt.; [START_REF] Rawson | Liver abnormalities associated with chronic mercury accumulation in stranded Atlantic bottlenose dolphins[END_REF].

Conclusions

The results of the present study revealed different degrees of exposure to trace elements among the five toothed whale species studied. This is a consequence of the influence of biological and ecological factors such as age, feeding habits and habitat used. However, no relationship was observed between element concentrations and sex. A strong positive correlation between Hg and Se in liver was observed for the five species, though the Hg:Se molar ratio for most of the individuals analysed was less than 1:1, probably reflecting the high proportion of young animals in the sample available for this study and/or indicating that these animals had a good health status. In addition, comparing with other studies world-wide, the element concentrations (Hg and Cd) found in Iberian toothed whales indicates that these populations are not specially threatened by Hg and Cd exposure in the area.

Both parts of this study provide new information and complement the existing database on persistent organic pollutants and trace element concentrations in marine mammals of the NWIP. These data is central in a time when the European scientific community is developing research on large marine vertebrates as indicators of medium and long-term marine environmental change for incorporation under the EU Marine Strategy Framework Directive.

In addition, it is extremely important for the environmental monitoring and for the conservation of these species in an area where surveillance regarding chemical contaminants continues.

Fig 1 .

 1 Fig 1. Map of the study area including the 100 and 200 m isobaths. The 200 m isobath was taken as the limit for the shelf-break. The black line contouring the coast represents the length of the coastline which constituted the sampling zone.

Fig 2 .

 2 Fig 2. Concentrations (mean ± SE, standard error, in µg g -1 wet weight) of mercury (a) and cadmium (b) in the liver and kidney of Dde: common dolphin (Delphinus delphis), Gme: pilot whale (Globicephala melas), Pph: harbour porpoise (Phocoena phocoena), Sco: striped dolphin (Stenella coeruleoalba) and Ttr: bottlenose dolphin (Tursiops truncatus) from the North West Iberian Peninsula. Mercury (Hg) and cadmium (Cd) were chosen since each of them represented one of the two patterns of tissue distribution of the 14 trace elements analysed.

Fig 3 .

 3 Fig 3. Results of discriminant analysis (DA) on trace element concentrations analysed in liver and kidney (data where the concentrations were < LoD are not shown; see Table 1) for Dde: common dolphin (Delphinus delphis), Gme: pilot whale (Globicephala melas), Pph: harbour porpoise (Phocoena phocoena), Sco: striped dolphin (Stenella coeruleoalba) and Ttr: bottlenose dolphin (Tursiops truncatus) from the North West Iberian Peninsula: bi-plot for axes 1-2 (i.e. LD1 and LD2).

Fig 4 .

 4 Fig 4. Molar Se and molar Hg concentrations (nmol g -1 wet weight) in liver tissue of Dde: common dolphin (Delphinus delphis), Gme: pilot whale (Globicephala melas), Pph: harbour porpoise (Phocoena phocoena), Sco: striped dolphin (Stenella coeruleoalba) and Ttr: bottlenose dolphin (Tursiops truncatus) from the North West Iberian Peninsula. The solid line indicate the Hg:Se molar ratio of 1.

Table 1 .

 1 Trace element concentrations (mean ± SD, µg g -1 wet weight) in kidney and liver of common dolphin (Delphinus delphis), pilot whale (Globicephala melas), harbour porpoise (Phocoena phocoena), striped dolphin (Stenella coeruleoalba) and bottlenose dolphin (Tursiops truncatus) from the North West Iberian Peninsula. The total number of samples 'n' analysed per species is indicated, with the values of kidney samples on the left side and those of liver samples on the right side. Age values are expressed as years (mean ± SD).

	Table 1. (Continued)
		Liver	< 0.07 a	< 0.07 a	< 0.07 a	< 0.07 a	< 0.07 a
	Se	Kidney	2.7 ± 1.1	2.9 ± 1.6	3.0 ± 0.95	3.2 ± 1.4	4.6 ± 3.1
		Liver	5.0 ± 5.8	16.9 ± 30.1	8.0 ± 11.6	12.3 ± 17.2	10.8 ± 13.0
	V	Kidney	< 1 a	< 1 a	< 1 a	< 1 a	< 1 a
		Liver	< 1 a	< 1 a	< 1 a	< 1 a	< 1 a
	Zn	Kidney	18.8 ± 8.7	22.6 ± 5.4	19.1 ± 1.7	24.2 ± 7.5	19.6 ± 5.4
		Liver	40.5 ± 19.5	42.3 ± 14.5	32.7 ± 7.8	53.0 ± 21.1	33.8 ± 8.6

a Values were less than the limit of detection (see

Materials and Methods)

Table 3 .

 3 Spearman rank correlation coefficients (r) between several non-essential and essential elements in the liver (L) and kidney (K) of common dolphin (Delphinus delphis), pilot whale (Globicephala melas), harbour porpoise (Phocoena phocoena), striped dolphin (Stenella coeruleoalba) and bottlenose dolphin (Tursiops truncatus) from the North West Iberian Peninsula. Significant values are in bold.

		HgL/SeL HgK/SeK CdL/CuL CdK/CuK CdL/ZnL CdK/ZnK
	Delphinus delphis	0.854*** 0.511*** 0.203*	0 .051	-0.178	0.072
	Globicephala melas	0.929** 0.616	0.380	0.183	0.262	0.316
	Phocoena phocoena	0.956*** 0.193	0.506	0.211	0.103	0.246
	Stenella coeruleoalba 0.915*** 0.816*** -0.046	0.235*	-0.509*	0.681**
	Tursiops truncatus	0.964** 0.976**	-0.071	0.405	0.179	0.238
	*0.01 < P < 0.05
	**0.001 < P < 0.01
	***P < 0.001

Table 4 .

 4 Mean ± SD of hepatic Hg and renal Cd concentrations (µg g -1 wet weight) of the five toothed whale species from all over the world. Sample size "n" of each species and area is in brackets. The average or ranges of age (years) and length (cm) are given when available.

	Species	Area	Age/Length	Total hepatic Hg	Renal Cd

± 30.0 (14) 2.3 ± 5.3 (12) This study

	NW Spain	<1 -6	1.2 ± 0.3 (3)	0.1 (2)	Lahaye et al., 2007
	France	0 -26	17.9 ± 21.9 (13)	1.4 ± 1.4 (13)	Lahaye et al., 2007
	Celtic Shelf	0 -23	10.7 ± 14.4 (8)	0.6 ± 0.7 (8)	Lahaye et al., 2007
	France (English channel)		2.6 ± 3.2 (4) a	0.3 ± 0.6 (4) a	Das et al., 2003b
	France (English channel)	0 -26	10.7 ± 14.3 (8)	0.6 ± 0.7 (8)	Lahaye et al., 2007
	Irish Sea	0 -11	30.0 ± 48.8 (12)	0.6 ± 0.6 (11)	Lahaye et al., 2007
	East Scotland	110 -170	6.0 ± 5.9 (6)	2.7 ± 2.8 (6)	Falconer et al., 1983; Wells et al., 1994
	NW Scotland	0 -12	12.4 ± 11.8 (5)	6.4 (2)	Lahaye et al., 2007

Acknowledgments

The authors gratefully acknowledge the assistance of volunteers from the Galician (CEMMA) and Portuguese (SPVS) stranding networks. They also thank C. Churlaud and M. Kalombo for running some of the metal analyses. This work was supported through the PhD grant of PMF from the Portuguese Foundation for Science and Technology of the Government of Portugal (SFRH/BD/36766/2007). GJP acknowledges support from the EU under the ANIMATE project (MEXC-CT-2006-042337). MBS was supported by the Spanish Ministry of Education, Programa Nacional de Movilidad de Recursos Humanos de Investigación (PR -2010-0518) and the LOTOFPEL project (Plan Nacional de I + D + I, CTM 2010-16053). FLR acknowledges support from the EU under the ECOSUMMER project (MEST-CT-2005-020501).

Delphinus delphis

Globicephala


Raymond Laë

Jean-Marc Ecoutin

Asber Mendy

Jean Raffray

Jean-Yves Weigel

Ousmane Djobe

Effects of a targeted shrimp (Penaeus notialis) exploitation on fish catches in the Gambia estuary

Keywords: Artisanal fisheries, Gambia estuary, Fish statistics, Finfish, Shrimp, Mono-exploitation

Results of an annual survey of fishing activities and catches in the Gambia estuary (200 km long), carried out from June 2001 to May 2002, indicated that fishing effort targets mainly (66% of fishing trips) shrimps. As a result of the globalisation of fish markets, the fishery has become a quasi mono-exploitation of shrimp for exporting. At the opposite, fishing activities focusing on fish were low and carried out with large and medium mesh drift nets, gill nets and surrounding nets (17% of fishing trips), handlines and longlines (15%). The annual catch, estimated at 2350 tonnes for the estuarine part of the River Gambia, consisted of 1800 t of fish and 550 t of shrimp. Stownets were the most efficient gear accounting for 50% of the total production (550 t of shrimp and 600 t of fish) while other significant techniques were mainly drift nets (21%), longlines (11%) and gillnets (5%). Fish catches were composed with 55 fish species among which 16 species accounted for 90% of the annual catch. The average catch length of these first sixteen species was 295 mm, clearly demonstrating that fishing for fish was targeting large fish. The effects of the targeted shrimp exploitation resulted in a low fishing effort for fish leading to low fish landings and consequently to an under exploitation of fish resource in the Gambia estuary.

Introduction

In West Africa, most of the estuaries have been affected by habitat degradation due to the exploitation of mangrove forests and/or reduced freshwater inflows mainly resulting from dam construction and drought periods. These different perturbations have been mentioned in Senegal, in the Saloum a Corresponding author: lae@ird.sn estuary [START_REF] Kebe | Étude des systèmes de production et de commercialisation dans le secteur des pêches au Sine Saloum[END_REF][START_REF] Diouf | Les peuplements de poissons des milieux estuariens de l'Afrique de l'Ouest : l'exemple de l'estuaire hyperhalin du Sine Saloum[END_REF][START_REF] Bousso | La pêche artisanale dans l'estuaire du Sine Saloum (Sénégal)[END_REF] and the River Casamance [START_REF] Albaret | Les peuplements de poissons de la Casamance (Sénégal) en période de sécheresse[END_REF] where severe hydrological modifications were characterized by hyperhalinity and reversed salinity gradient ranging from 36 psu at the mouth to more than 150 psu upstream. In the same region the Gambia river, located to the south of the Sine Saloum estuary and to the north of the Casamance estuary, was less affected by the different drought periods that affected the Sahelian region since the last thirsty years. This estuary did not suffer these natural perturbations because it has a much larger catchment extending to the Guinea uplands that are considered as the water tower of West Africa [START_REF] Lamagat | Monographie hydrologique du fleuve Gambie. ORSTOM; Organisation pour la Mise en Valeur du fleuve Gambie (OMVG)[END_REF]). Hence although water discharge significantly decreases during the dry season, it is still sufficient to maintain an upstream-downstream salinity gradient in the estuary, with seasonal variations due to water input in the wet season. As industrial activities and farming are also reduced, this estuary seems to be an exception in the sub region and could be qualified as healthy environment.

Usually, estuaries are a focus of human activities such as fishing, farming, manufacturing, shipping and among these ecosystems, tropical and subtropical estuaries are one of the most exploited in the world. Most tropical estuaries are vitally important as a source of fish to the local people, and fishing plays an important role in the economy [START_REF] Blaber | Tropical estuarine fishes, Ecology, Exploitation and Conservation[END_REF]. Although commercial fisheries are rare in these areas, subsistence and artisanal fisheries have evolved and are now threatened by over-exploitation resulting from a huge increase in fishing effort and a use of illegal fishing gear or techniques. As a consequence, even in developing countries, estuaries are generally heavily exploited by artisanal fisheries as it is the case for the Sine Saloum and Casamance estuaries (Le [START_REF] Reste | La pêche crevettière artisanale en Casamance[END_REF][START_REF] Bousso | La pêche artisanale dans l'estuaire du Sine Saloum (Sénégal)[END_REF]). In the Gambia, in the absence of any regular monitoring of the fisheries, it was difficult to obtain reliable estimates of fishing effort and catches in the estuarine area. No time series were available and only some exceptional studies were carried out [START_REF] King | A review of the state of the fisheries in the Gambia[END_REF][START_REF] Lesack | Distribution of effort and structural aspects of the artisanal fishery in the Gambia[END_REF][START_REF] Lesack | Transport of carbon, nitrogen, phosphorus and major solutes in the Gambia River, West Africa[END_REF][START_REF] Dorr | Studies on adult, juvenile and larval fishes of the Gambia River, West Africa, 1983-1984[END_REF][START_REF] Lesack | Estimates of catch and potential yield for the riverine artisanal fishery in the Gambia, West Africa[END_REF][START_REF] Njai | Traditional fish processing and marketing of the Gambia[END_REF]. Nevertheless, from this work, it emerged that the situation could be different from the adjoining estuaries, because fishing activities mainly involved foreigner fishermen and concentrated on shrimps at the detriment of fish fishing. Catches were usually estimated at 1200-1500 t y -1 , half of which comes from the estuarine part of the river. The number of fishermen was estimated to be about 3000, 83% of whom operated in the estuary. Seasonal shrimping occupied more than 1000 people at the height of activity. Difficulties in marketing and the lack of fish processing and preservation infrastructure were considered to be major limiting factors for fishing in the Gambia.

Facing this unusual situation for a developing country, our purpose, as shrimp activities seemed to be the most developed, was to understand what could be the effects of a targeted shrimp exploitation on fish fishing. One preliminary condition was to collect reliable data on artisanal fisheries through an annual survey carried out in the Gambia estuary from June 2001 to May 2002. This included an assessment of the status of the fishery, exploitation levels, fishing methods and seasonal variability.

Materials and methods

Study area

The Gambia river is located at the western edge of West Africa (Fig. 1). It originates in the Fouta Djalon plateau of northern Guinea and flows 1200 km through southern Senegal and the Gambia to the Atlantic Ocean. The total area of the Gambia river basin is 78 000 km 2 , but the estuarine subdivision is limited to the part from Kaur to Banjul. From this area to the mouth, the entry of marine water gives rise to saline floodplains covered with a dense mangrove composed mainly with Rhizophora mangle. The mangrove of the Gambia have very big trees because of large freshwater and marine input [START_REF] Marius | Mangroves du Sénégal et de Gambie, écologiepédologie-géochimie-mise en valeur et aménagement[END_REF].

The river flows into the Atlantic Ocean through a 13 km wide mouth which together with a maximum tidal amplitude of 2 m allows mass entries of sea water into the estuary. The mangroves play an important role for fish in the Gambia estuary as they form feeding and breeding grounds for very many species, and they are a major source of trophic enrichment [START_REF] Thayer | Utilisation of the red mangrove prop root habitat by fishes in south Florida[END_REF][START_REF] Vance | Use of a mangrove estuary as a nursery area for postlarval and juvenile banana prawns, Penaeus merguiensis de Man, in northern Australia[END_REF][START_REF] Ronnback | Distribution pattern of shrimps and fish among Avicennia and Rhizophora microhabitats in the Pagbilao mangroves, Philippines[END_REF][START_REF] Blaber | Tropical estuarine fishes, Ecology, Exploitation and Conservation[END_REF].

The climate (Albaret et al. 2004) is dominated alternately by the dry harmattan air, which originates in the Sahara and a south-westerly monsoon of humid oceanic air. Rains occur from June to October, with the greatest precipitation in August. There is also a marked hydrological seasonality in the Gambia River leading to the definition of three main seasons: 1) flow from July to October, 2) fall from November to February and 3) low water from Marsh to June. As a result of freshwater input, salinity is low (0-10) in the upstream and middle area from August to January (wet season). Then in February, salinity increases progressively until July when it reaches its highest value, close to that of sea water at Tendaba (dry season).

Fisheries sampling procedures

The study site was the lower river of the Gambia estuary, which means the area subjected to marine influence (200 km upstream during the dry season). Information on fish resources a n dfi s h i n ge ffort was collected from studies of fishermen activities. Data were obtained by sampling small-scale fisheries during a complete annual cycle from June 2001 to May 2002. The total number of fishermen, estimated to be 2500, included full-time (exclusively fishermen) and part-time (fishermen/farmers or fishermen/craftsmen) fishermen (on the basis of the time they spend fishing), with their numerous assistants.

As the definition of fishermen and assistants is vague in developing countries, the fishing unit used was the active fishing canoes making a real contribution to fishing effort.

The study period spanned one hydrological cycle of the estuary, which also corresponded to one biological cycle for fish and the fishery. The monitoring was based on two stratifications:

1) geographical stratification of the estuary into three homogeneous sectors (Fig. 1) corresponding to variations in salinity and fish community composition, 2) stratification of the fishery based on two different classes of fishermen (shrimpers and fish fishermen).

The mouth and middle sectors included four sampling villages each, respectively Albreda, Barra, Pirang, Toubakolong, and Bintang, Kerewan, Tankular, Tendaba. The third sector included three villages: Balingo, Jappeni and Kanikunda. Each of these villages was sampled over a ten days period per month. Monthly census, daily capture and fishing effort data were collected independently at every sampling site:

• A monthly census was carried out in order to assess the migrations of fishermen inside and outside the estuary. The 11 villages sampled represent between 60 and 80%, according to the season, of the fishing population living and working in the estuary. • Fishing effort was computed from a daily survey of the activities of 10 fishing units randomly chosen in the same village; 1150 questionnaires were collected each month, 33% from shrimpers and 67% from fishermen targeting fish. • Estimates of fishing effort, total catch, fish composition and length structure were collected from returning fishing canoes sampled at the landing stage with an average of 625 surveys being made per month. In all, 3200 surveys were carried out at the mouth, 3000 in middle sector and 1300 upstream.

Fish landed were sorted by species when possible (70% of sampled species) or by genus (30%) when field identification was difficult. Then for each taxon, individuals were counted and weighed and 10 were measured. All species combined, more than 130 000 fishes were measured during the monitoring of the fisheries (Table 1).

Statistical analysis

In order to understand the abundance distribution of the main fish species in the Gambia estuary, the 55 fish and crustacean species observed in canoe landings have been grouped into ecological classes. The composition of the assemblages was discussed using the species classification proposed by [START_REF] Albaret | Les peuplements des estuaires et des lagunes[END_REF], which defines eight bio-ecological categories ordered on two gradients from a central point, the strictly estuarine species (Es). The gradient of marine affinity comprises four categories: the estuarine species from marine origin (Em), the marine estuarine species (ME), the marine species accessory in estuaries (Ma) and the marine species occasional in estuaries (Mo). The gradient of freshwater affinity comprises the estuarine species from freshwater origin (Ec), the freshwater species with estuarine affinities (Ce), and the freshwater species, occasional in estuaries (Co). Four of these categories (Ec, Es, Em and ME) compose the fundamental lagoon and estuarine community.

The catch per trip of the main fishing gears in the Gambia estuary was presented as a scatter plot of data as function of both month and spatial distribution (mouth, middle and upstream estuary). As these values are both dependant on natural variability of the ecosystem and availability of fish, their values are widely distributed (standard deviation equivalent to mean). As a consequence, for comparison purpose, we decided to smooth the curve along the points. A lowess (locally weighted regression) curve [START_REF] Cleveland | Robust locally weighted regression and smoothing scatterplots[END_REF]) was fitted by least squares ("Gaussian") to the data to summarize the distribution patterns. The smooth curve locally minimizes the variance of the residuals or prediction error. Only the points in that vicinity determine the curve value at each particular location along the x-axis. The span (i.e. the value between 0 and 1 controlling the amount of smoothing) was fixed to 0.5. The calculation and graph was done using the R statistical language. In addition, as the variances of the different gear samples were not homogeneous, a non parametric test was realised. A Kruskal-Wallis test for several independent samples was used that is equivalent with a one way ANOVA for normal quantitative data [START_REF] Scherrer | Biostatistique[END_REF]. A cross analysis was performed testing both the effect of gears and those of spatial and temporal distributions.

Fishing effort data and fish landing data collected by the survey network were extrapolated to each of the three areas of the estuary using descriptors of the fishery, such as the number of fishing units classified as shrimpers or fish fishermen, which had been estimated during a preliminary census of the whole estuary (Fisheries Department 1997). The extrapolation was given by: The extrapolation was made in order to assess the total shrimps and fish catches in the estuary. It could be done in this way even with migration of fishermen, because the monitoring of the fishery produced monthly information about fishermen census, fishing activities, CPUE and catch per species for each fishing gear in the three zones of the Gambia estuary.

C = Σ 1 Z Σ 1 G Σ 1 M [UP • J (s/p • j)CPUE] C =

Results

Fishing places and fishing units migration

The annual census of the Gambia estuary (Fisheries Department 1997) counted 106 fishing villages. Among the latter, 46% have less than 5 canoes, 90% less than 15. As most of the total canoes (900) were non-motorized (89%), it clearly appeared that fishing activities were concentrated in a few commercial places, the others conducting subsistence fishery activities.

Our monthly census of 11 sampled villages (Fig. 2), showed that the number of fishing units was quite stable throughout the year in some villages (Pirang, Bintang, etc.), decreasing from April to August in some villages located downstream in the estuary (Barra, Toubakolong, Albreda), and at the same time increasing in villages located upstream in the estuary (Tendaba, Jappeni, Kanikunda). These migrations mainly concerned shrimpers and were dependant on water salinity and shrimp abundance: 1) from March to August, this number in the 11 sample villages ranged from 331 to 217, 2) the number of shrimp fishing units in the estuary varied from 432 in December 1999 to 596 in June 2000. In fact, most of the shrimpers are Senegalese people settled in the Gambia in big villages at the mouth of the estuary, moving from downstream to upstream when shrimp abundance varies. These shrimpers are reinforced by migrant Senegalese shrimpers, moving to the different estuaries of the sub-region (Sine Saloum, Gambia, Casamance, Guinea) and working in the Gambia estuary for shorter periods of three months concentrated on high shrimp abundance.

Fishing effort

The Gambia estuary fisheries are typically multi-gear. Some thirty-eight different types of fishing gear and capture methods have been described. The main types were stownets (fixed nets for shrimps), drift nets, gillnets, surrounding nets, cast nets, handlines and longlines. Among the drift nets, surrounding nets and gill nets, 40% of the nets were large mesh ones (>50 mm side) and 55% were medium mesh ones (30 mm < side < 50 mm). Only 5% of the nets were small mesh ones (side < 30 mm). All the gears were adapted to the habitat characteristics of the estuary, to the fish species and to the targeted fish sizes as many artisanal fisheries in developing countries [START_REF] Welcomme | River fisheries[END_REF][START_REF] Ecoutin | Exploitation des stocks[END_REF]. Some different gears could be used during one fishing trip as it was generally for stownets and lines.

According to the one-year survey carried out in the estuary, the percentage of daily fishing trips of stow nets (53%) was generally higher than those for other gears: lines (17%), drift nets (17%), gill nets (7%), surrounding nets (6.5%), cast nets (2%). The percentage for stow nets was high and stable for all the seasons and in the three areas. For the other gears, the percentage was rarely greater than 10% and varies according to area. For this reason, shrimp activities can be considered as professional ones (full time activity) whereas hooks (longlines and handlines combined) and nets are only occasional ones (part time activity).

On an annual basis (Table 2), considering the number of shrimp units (500) and fish units (300) and their daily activity, the fishing gear most frequently used were stownets (66%) and large and medium mesh gill nets (17%). In addition, fishing activities decreased from the mouth (62% of fishing trips) to the middle area (26%) and upstream area (12%).

Although fish communities varied strongly during the dry and wet seasons in terms of diversity, abundance and species composition (Albaret et al. 2004), fish fishing activities did not reflect this natural variability because fish fishermen mainly targeted top predators. As a consequence, fish effort targeting fish appeared to be quite stable both in spatial distribution and intensity (3835 fishing trips during the wet season versus 4400 during the dry season). As a first conclusion, fishing activities in the Gambia estuary are highly specialized and mainly targeting shrimps.

CPUE

First analysis of the average annual catches per fishing trip shows that catches exceed 23 kg (SD = 27 kg) for all gear combined. The values ranged from more than 35 kg for surrounding nets, drift nets and cast nets to 23 kg for hooks and gill nets (Fig. 3). The case of stownets must be considered separately as catches were composed both of shrimp and fish: small fish mainly caught with stow nets were discarded and large fish provided from handlines used by shrimpers when waiting for the turn of the tide. Although this CPUE is the lowest registered (14 kg), the 30% of shrimp that composed the capture gives a high trading value to the catch and make profitable the use of stownets.

A seasonal variation in catch (fish and shrimp) existed for all the three zones and all the gears (Fig. 3), even if catch per trip in the mouth seemed to be more regular. The CPUE were greater between September and March in the middle area, and from January to April in the upstream area. These values are significantly different as attested by a cross Kruskal-Wallis test on estuarine zones, seasons and gears (Table 3). Concerning shrimp, 2600 canoes were sampled in the three zones during the three seasons. CPUE values ranged from 3.1 kg trip -1 during the fall in the mouth to 10.9 kg. trip -1 during low water in upstream area. According to Figure 4, Penaeus notialis was concentrated in the mouth and middle areas during the whole year (with low abundances during the fall) whereas they were very abundant in the upstream area only during the low water season and at the beginning of the flow (May to August) when saline water moved upstream. The values per season for the three areas are significantly different as attested by a Kruskal-Wallis test, but only for the low water season when they are calculated per area.

Catch per species showed that the fish landings were composed of 55 species (both fish and crustaceans), of which 16 species accounted for 90% of total CPUE (Table 4). The most abundant species was Polydactylus quadrifilis, but Arius spp. and Penaeus notialis were also very abundant. In terms of ecological classes, estuarine species of marine origin dominated all three sectors although estuarine species of freshwater origin and freshwater species were present in middle and upstream sectors during the wet season. Twenty-nine species were of marine origin, twenty-one were estuarine, and only five were of freshwater origin. Occasional marine species were recorded at the mouth of the estuary and freshwater species mainly in the upstream area. All other species (Marine accessory, Marine estuarine, Estuarine marine, Estuarine) were recorded at all landing sites depending on season and salinity.

The analysis of CPUE values for individual species (Table 5) showed that the different fishing techniques were highly specialized and targeted mainly one species: Sarotherodon melanotheron for cast nets (65%), Penaeus notialis for stow nets (47%), Polydactylus quadrifilis for large (67%) and medium mesh (59%) drift nets, Cynoglossus spp. for large (33%), medium mesh (26%) gillnets and large mesh surrounding nets (40%), Arius spp. for handlines (33%) and longlines (50%). For each fishing technique, one to three species accounted for 70-80% of catches.

Total catches

In 2001-2002 the total estimated catch for the artisanal fishery was 2350 tonnes. Stow nets were the most productive gear with respectively 550 and 600 t of shrimp and fish per year. They accounted for 50% of the total production, the other half production being provided mainly by drift nets (22%), hooks (15%), surrounding nets (5%), gillnets (6%) and cast nets (2%).

The total catch was divided into 1800 t of fish and 550 t of shrimps. Fish catch was similar and equivalent to 650 t in the mouth and middle sectors of the estuary. Shrimp catch was higher (360 t) at the mouth where fishing activities can continue throughout the year than in the middle area (78 t). The upstream sector was less productive for fish (460 t), but provides a seasonal shrimp production equivalent to middle sector (94 t). On an estuary scale, shrimp catch was higher from April to September while fish production was better from January to May (Fig. 5).

Two taxa represented half of fish landings: Polydactylus quadrifilis (26%) and Arius spp. (20%), which are mainly caught by handlines, longlines and drift nets. According to Albaret et al. (2004), among Arius two species were abundant: A. latiscutatus and A. parki, the third one A. heudeloti being less frequent. Then, Pseudotolithus elongatus accounted for 9% of fish catches and the six following taxa ranged from 3t o4 % :Liza grandisquamis, Pentanemus quinquarius, Ethmalosa fimbriata, Cynoglossus spp., Ilishia africana and Pseudotolithus senegalensis (Fig. 6).

Catch lengths

Analysis of the shrimp lengths (15 000 individuals measured during one annual cycle) showed significant differences between the three geographical areas (ANOVA, p < 0.0001). Large average lengths were registered at the mouth (mean±SD, Table 4. Annual distribution limits of fish and shrimp species recorded in the catch of the estuarine artisanal fishery (expressed as number of individuals caught: * rare, ** frequent, *** abundant; Cat: ecological categories described in the text). Percentage in brackets represents the species composition of annual CPUE (kg fishing trip -1 ) for the main species. 121 ± 31 mm) and upstream areas (136 ± 27 mm) and smaller ones in middle area (113 ± 31 mm). The differences between the hydrological seasons were not significant at the contrary of the interactions between hydrological seasons and zones (p < 0.0001): shrimps were present and large in upstream area only during the low water season (133 ± 29 mm) and the beginning of the fall (137 ± 26 mm). In the middle area, the fall (109±27 mm) and the flow (113±29 mm) were mainly characterized by small shrimps. Last, in the mouth the average length of shrimps were quite stable during all the year (values ranging from 120 to 124 mm).

For fish, the average length for the first sixteen species (90% of total fish landings) was 295 ± 175 mm. The species with the highest maximum length had a high average catch length: Polydactylus quadrifilis (639 ± 299 mm), Arius spp. (306 ± 66 mm), Pseudotolithus elongatus (243 ± 52 mm), Cynoglossus spp. (417 ± 75 mm) and Pseudotolithus senegalensis (359 ± 156 mm). According to these observations, fishing for fish is targeting large fish with high average lengths (Fig. 7).

Discussion

Spatial distribution and exploitation of shrimps

According to [START_REF] Lhomme | Biologie et exploitation de la crevette Penaeide au Sénégal[END_REF], post larval recruitment of Penaeus notialis in the Sine Saloum, Gambia and 1) caught in the Gambia estuary. Numbers in bold character represents the observed maximum length.

Casamance estuaries depends on adult stocks located in the sea from the frontier between Senegal and Guinea Bissau. Juveniles and sub-adults live in the estuaries where their length of stay is around 3 months [START_REF] Lhomme | Biologie et dynamique de Penaeus (Farfante Penaeus) notialis (Perez Farfante 1967) au Sénégal[END_REF]) but mainly depends on water current and salinity [START_REF] Reste | Étude des variations annuelles de la production de crevettes dans l'estuaire de la Casamance (Sénégal)[END_REF][START_REF] Reste | Variations spatio-temporelles de l'abondance et de la taille de la crevette Penaeus notialis en Casamance (Sénégal)[END_REF][START_REF] Kebe | Étude des systèmes de production et de commercialisation dans le secteur des pêches au Sine Saloum[END_REF]: values of 30 psu favour a long stay until an average length of 13 cm. In the Gambia estuary, salinity is always favourable in the mouth for the occurrence of shrimp and fishing activities are possible throughout the year, but shrimp length is either small or large depending on the season and water currents. In the middle area, salinity and currents are always favourable, even if they are not optimum for shrimps. This is an intermediate situation where yield and shrimp length allow the fishery to be permanent. In upstream areas, shrimp resources vary greatly from zero abundance during the wet season to high abundance during the dry season. As a consequence there is no shrimp activities during the wet season but in contrast, an incoming migration of Senegalese fishermen during the dry season, with the certainty of obtaining high yields of large shrimps.

According to previous studies [START_REF] Leendertse | Management Aspects in Estuarine Shrimp Fishing in the River Gambia[END_REF] and using comparative analysis with some estuaries of the sub-region, it is possible to conclude that shrimp fishing effort in the Gambia is still at a sustainable level for three main reasons:

1) The number of shrimpers is quite low: 430 during the wet season and 600 during the dry season when it was 600 in [START_REF] Bousso | La pêche artisanale dans l'estuaire du Sine Saloum (Sénégal)[END_REF].

3) The annual shrimp catch is estimated to be 550 t compared with 1600 t in the Casamance estuary for the period [START_REF] Lesack | Estimates of catch and potential yield for the riverine artisanal fishery in the Gambia, West Africa[END_REF][START_REF] Diouf | Plan d'action forestier, pêche et aquaculture continentales[END_REF][START_REF] Reste | Variations spatio-temporelles des prises et de la taille des crevettes Penaeus notialis dans l'estuaire sursalé du Saloum (Sénégal)[END_REF].

As an early diagnosis fishing effort seems to be sustainable but it may be increasing because of political instability in Guinea Bissau and Casamance regions which led to Senegalese fishermen leaving to find safer areas. In addition, irrespective of fishing in the estuarine area, the shrimp stock is dependent on fishing effort applied to adult stages in the sea [START_REF] Garcia | Stock recruitment relationships and the precautionary approach to management of tropical shrimp fisheries[END_REF]. This is worrying because overfishing is yet a reality in marine waters leading the Senegalese authorities to establish a two months biological rest in order to preserve fish resource [START_REF] Chavance | Pêcheries maritimes, Ecosystèmes et Sociétés en Afrique de l'Ouest : Un demi-siècle de changement. Actes du symposium international[END_REF]).

Current state of fish fishing in the Gambia estuary

Fishing for fish in the Gambia estuary is based on the use of very selective gears (medium mesh nets and hooks) that target large fishes. The daily rate of use of these fishing gears is low as is the number of fish fishing units (300). Consequently, the effective fishing effort is low in comparison with that of stownets. The CPUE is high for this type of artisanal fishing and especially for gear targeting large fish. Fishing activities are mainly concentrated on a few species that are commercially profitable: Polydactylus quadrifilis, Arius spp., Cynoglossus spp. Annual catches of 1800 t are relatively low for such a large estuary and average catch lengths are high ranging from 300 to 600 mm for the main species captured. These findings suggest low levels of fish exploitation. When comparing these values with the other sub-regional estuaries, it appears that total catch in the Senegal basin was estimated to be 18 500-25 900 t year -1 in 1974 [START_REF] Reizer | Définition d'une politique d'aménagement des ressources halieutiques d'un écosystème aquatique complexe par l'étude de son environnement abiotique, biotique et anthropique[END_REF] and 8000-10 000 t year -1 in the late 1980s [START_REF] Diouf | Plan d'action forestier, pêche et aquaculture continentales[END_REF][START_REF] Bousso | La pêche artisanale dans l'estuaire du Sine Saloum (Sénégal)[END_REF]). In the Sine Saloum, from 1990 to 1993 fish production was estimated to be 10 000 t per year [START_REF] Bousso | La pêche artisanale dans l'estuaire du Sine Saloum (Sénégal)[END_REF] and 14 000 t in the Casamance estuary in 1984 [START_REF] Diadhiou | La pêcherie artisanale de poisson en Casamance[END_REF]. Fishing parameters in the Gambia estuary (Table 6) are always clearly lower than those recorded in the other subregional estuaries: fewer fishermen of all socio-economical classes, fewer fishing units, fewer total catches and even less annual catch per canoe. This last point can be explained by 1) a high specialisation of fishermen on shrimp fishing (lower yield) and by 2) the low level activity of the other ones only fishing for subsistence purposes. As a consequence, even if CPUE are high, the annual catch per canoe is relatively low.

Effects of foreign demand on structuring estuarine fisheries in the Gambia

At the beginning of the 1980s, fishing was mainly conducted by small local fishing units (380 fishermen) and foreign units accounted for only 15% of the total (equivalent to nine companies targeting shrimps and using less than 200 stownets) (Fisheries Department 1983). Twenty years later, estuarine fishing is mainly carried out by about a hundred fishing companies involving 800 fishermen (more than 80% of them foreigners) using more than 1750 stownets. As a result of the globalisation of fish markets, in 2002 the Gambia exported 150 t of shrimp to West Africa and 350 t to Europe. This globalisation was expressed by the appeal of European markets and in this case, the differences between the local and European prices of shrimp led to a re-allocation of fishing effort to valuable species: the cost of one kg of shrimp is three time higher (30 dalasis) than those of fish. As a consequence, the fishery has become a quasi mono-exploitation of shrimp for exporting.

Conclusion

The consequences of shrimp specialization in the Gambia estuary for fish exploitation are multiple. The period before shrimp exploitation in the estuary was characterised by the trade in processed fish to local and sub-regional markets, whereas the present period is mainly characterised by the attractive flow of fresh shrimps from the freezing factories of Banjul (87% of fresh products and 70% of total trade products). Mono exploitation has led to a re-allocation of fishing effort on shrimps and has prioritised European consumers rather than local ones. This has led to a change in the traditional valuation of fish: fresh shrimp has now replaced artisanal processing (drying, smoking) of fish. It really appears that during the last two decades, mono-exploitation of shrimp in the Gambia estuary has strongly affected and reoriented estuarine fisheries leading to an underexploitation of fish resource.

In addition, when thinking about the real problem of bycatch and discards, the best example is often given by shrimp trawling which is characterized by the highest ratio of bycatch to shrimp catch, generally from 5 to 10:1. This worrying angle has not been studied in this paper for the artisanal shrimp fisheries although many discards have been registered. Even if discards, when referring to fixed nets, seem to be lower than for trawlers, the quantities are consequent and this particular problem would deserve to be examined in order to correctly assess the impact of the shrimp fisheries on fish resource.

Fig. 1 .

 1 Fig. 1. Stratification of the Gambia estuary into three geographical strata (mouth: grey, middle: white and uspstream: black) based on the spatial and temporal variations in salinity. Black spots are the 11 sampled villages during the survey, 2001-2002.

  Total catches, UP = Number of fishing units in the sector, J = Number of days of the month, s = Number of trips during the sampling period, p = Number of sampled fishermen, j = Number of sampled days, CPUE = Catch per unit effort for one fishing gear in one area for one month, Z = Number of zones, G = Number of types of gears, M = Number of months.

Fig. 2 .

 2 Fig. 2. Monthly census (number of fishing units) in the sampled villages of the Gambia River from June 2001 to May 2002. The number of units per village was stable (white), decreasing from April to August (grey), increasing at the same time (black).

Fig. 3 .

 3 Fig. 3. (a) Mean annual catch and (b) spatial and temporal variability of catch (using the lowess curve in Cleveland 1979) of the artisanal fisheries of the Gambia estuary. Catch include shrimp and fish.

4.

  Spatial and temporal distribution of shrimp catch (kg) per fishing trip for the estuarine artisanal shrimp fishery, 2001-2002.

Fig. 5 .

 5 Fig. 5. Monthly distribution of annual catch (shrimp and fish) of the artisanal fisheries of the Gambia estuary, 2001-2002.

Fig. 6 .

 6 Fig. 6. Species distribution (code in Table 1) of annual catch (including shrimps) of the artisanal fisheries of the Gambia estuary, 2001-2002.

Fig. 7 .

 7 Fig.7. Catch length (average and standard deviation) of the main fish species (code in Table1) caught in the Gambia estuary. Numbers in bold character represents the observed maximum length.

Table 1 .

 1 Codification and number of fish and shrimp species measured per geographical area in the Gambia estuary.

	Species	Code	Mouth	Middle	Upstream	Total
	Arius spp.	Ari	9993	10 700	790	21 483
	Cynoglossus spp.	Cyn	3945	3193	253	7391
	Ethmalosa fimbriata	Efi	911	4034	716	5661
	Ilisha africana	Iaf	1807	1839	80	3726
	Liza grandisquamis	Lgr	1539	4436	346	6321
	Penaeus notialis	Pdu	9656	4522	1087	15 265
	Pseudotolithus elongatus	Pel	6822	11 879	2940	21 641
	Pomadasys spp.	Pom	2624	201	86	2911
	Polydactylus quadrifilis	Poq	191	4653	3289	8133
	Pentanemus quinquarius	Pqq	5959	2026	44	8029
	Pseudotolithus senegalensis	Psn	2949	905	397	4251
	Sarotherodon melanotheron	Sme	31	641	998	1670
	Total		46 427	49 029	11 026	106 482

Table 2 .

 2 Spatial distribution of fishing effort (number of fishing trips) for the fishing units sampled during the one year survey.

	Fishing gear	Code	Mouth	Middle	Upstream	%
	Cast nets	CastN	4	75	107	0.7
	Stownets	StowN	12 014	3132	1468	66.3
	Drift nets large mesh		57	1121	427
	Drift nets medium mesh	DriftN		207	426	9.0
	Drift nets small mesh				4
	Gill nets large mesh		85	260	25
	Gill nets medium mesh	GillN	462	23	74	3.7
	Gill nets small mesh			1
	Surrounding nets large mesh		652	9
	Surrounding nets medium mesh	SurN	5	281	194	4.7
	Surrounding nets small mesh			13	14
	Longlines		262	1338	274	7.5
	Handlines		2019		2	8.1
	Total		15 560	6460	3015

Table 3 .

 3 Results of a cross Kruskal-Wallis test on spatial and seasonal catch per trip (all gears combined) and between gears (code in Table2) independently of estuarine zones and hydrological seasons. H 0 = the sample group is homogeneous. Table gives the value of H c and one asterisk represents a probability lower than 0.05, two asterisks a probability <0.01 (alternative hypothesis H1 of disparity for at least one population is accepted).

Table 5 .

 5 Percentage of catches for the main species (code given in Table1) caught by the gear used in the Gambia estuary (the difference to 100% is composed on numerous species which individual percentage is lesser than 10%).

		Sme Pdu Ari Poq Cyn Pom Lgr
	Cast nets	65	11
	Stownets	47 14
	Drift nets large mesh	67
	Drift nets small mesh	59
	Gill nets large mesh	24 33
	Gill nets medium mesh	23 26
	Surrounding nets large mesh	20	40
	Surrounding nets medium mesh	11	32
	Handlines	33	11
	Longines	50 32

Table 6 .

 6 Characteristics of some estuaries in the South Senegal region[START_REF] Guiral | Les écosystèmes à mangrove[END_REF] and quantitative data on fishing activities (adapted from[START_REF] Ecoutin | Aménagement technique du milieu[END_REF]. Shrimp fishing is only based on the use of 13 mm mesh stownets, catching medium size juveniles (average length of 121 mm) in the central course of the river when in the Casamance river and the Sine Saloum, shrimpers also use drift nets and baskets (killis) trawled by two men near the banks, mainly targeting small shrimps

	Saloum	Gambia	Casamance

Acknowledgements. This study was conducted as a joint program between IRD (UR RAP, Dakar) and the Gambian Fisheries Department.