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International audience; Bioaccumulation of toxic metal elements including mercury (Hg) can be highly variable in marine fish species. Metal concentration is influenced by various species-specific physiological and ecological traits, including individual diet composition and foraging habitat. The impact of trophic ecology and habitat preference on Hg accumulation was analyzed through total Hg concentration and stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in the muscle of 132 fish belonging to 23 different species from the Senegalese coast (West Africa), where the marine ecosystem is submitted to nutrient inputs from various sources such as upwelling or rivers. Species-specific ecological traits were first investigated and results showed that vertical (i.e. water column distribution) and horizontal habitat (i.e. distance from the coast) led to differential Hg accumulation among species. Coastal and demersal fish were more contaminated than offshore and pelagic species. Individual characteristics therefore revealed an increase of Hg concentration in muscle that paralleled trophic level for some locations. Considering all individuals, the main carbon source was significantly correlated with Hg concentration, again revealing a higher accumulation for fish foraging in nearshore and benthic habitats. The large intraspecific variability observed in stable isotope signatures highlights the need to conduct ecotoxicological studies at the individual level to ensure a thorough understanding of mechanisms driving metal accumulation in marine fish. For individuals from a same species and site, Hg variation was mainly explained by fish length, in accordance with the bioaccumulation of Hg over time. Finally, Hg concentrations in fish muscle are discussed regarding their human health impact. No individual exceeded the current maximum acceptable limit for seafood consumption set by both the European Union and the Food and Agriculture Organization of the United Nations. However, overconsumption of some coastal demersal species analyzed here could be of concern regarding human exposure to mercury.

International audience; The Nile tilapia (Oreochromis niloticus) was introduced in Southeast Asia, such as in Thailand, in the mid-1960s for aquaculture purposes (Pullin et al., 1997; De Silva et al., 2004). The species was later promoted for aquaculture development in the early 1990s in Lao PDR (Garaway et al., 2000). In Lao People’s Democratic Republic (PDR), like in other countries, this exotic species is well established due to its self-reproduction (De Silva et al., 2004; CABI, 2018) facilitated by its particular life history traits and plasticity (Ishikawa et al., 2013). The species was then suspected to be introduced in the seventies in Laos and subsequently in the Nam Theun watershed where the NT2 Reservoir was impounded in 2008. Population parameters of this alien fish were investigated using the FiSAT II software with the most popular methods of bioparameters assessment to check their interchangeability within a same stock and the same year 2016. The length-at-age data analysis (using otoliths analysis, N = 258) gave slightly different results with the length frequency distribution analysis of fish landings (11 820 individuals). Furthermore, experimental fishing data provided irrelevant parameters due to insufficient representativeness of the sample size. The growth parameter K was estimated to be 0.23 year−1, with asymptotic length L∞ = 52.5 cm based on the length frequency distribution analysis with the fish landing data. According to these landing data, the total, natural and fishing mortality were Z = 1.41 year−1, M = 0.30 year−1 and F = 1.11 year−1. The exploitation rate E = 0.79 is over the Emax = 0.594 obtained by relative yield and biomass per recruit. This estimated stock of 165 tons (more than 700 000 tilapias) was characterized by high mortality (no population growth). These results showed that the population is overfished with too many juveniles caught (L50 = 210.4 mm; 50% mature stages at 295 mm). To maximize the yield per recruit, increase the biomass and sustain this fishery, enlarging the gillnet mesh size of the gill-net is recommended. This example highlights the variability of the parameters calculated from different methods and thus weaken worldwide and even inter-site comparisons. Despite this issue, the Growth Performances Indices (φ′) gathered into the literature can serve as baseline and confirmed the wide phenotypic plasticity of the species due to environmental factors. Analyses revealed difference between fast growing domesticated fish rose for aquaculture with φ′ higher to the tilapia growing in natural and challenging environments.; Le Tilapia du Nil (Oreochromis niloticus) a été introduit en Asie du sud-ouest, notamment en Thaïlande, dans le milieu des années 1960, à des fins d’aquaculture. Ce n’est que dans les années 1990 que cette espèce a été introduite au Laos pour ce même objectif. Cette espèce exotique, de par sa capacité de reproduction naturelle induite par ses traits d’histoire de vie et sa plasticité phénotypique, est bien établie au Laos, ainsi que dans d’autres pays asiatiques. Cette espèce présente dans le bassin versant de Nam Theun aurait ainsi colonisé le réservoir de Nam Theun 2 (NT2) lors de sa mise en eau en 2008, avec également des actions sporadiques d’empoissonnements. Les paramètres de la population du réservoir de NT2 ont été étudiés en recourant à FiSAT II avec les méthodes d’évaluation des bio-paramètres les plus populaires pour vérifier leur interchangeabilité dans un même stock et pour une même année (2016). L’analyse de la relation taille-âge, via l’analyse de 258 otolithes, a donné des résultats légèrement différents de l’analyse des fréquences de tailles issues des débarquements réalisées sur 11 820 individus. Les données issues des pêches expérimentales ont quant-à-elles fournis des résultats non pertinents de par le faible effectif des échantillons. La croissance (k) a été estimée à 0,23 cm an−1 avec une longueur asymptotique (L∞) de 52,5 cm. Selon les données de débarquements la mortalité totale, naturelle et par pêche étaient respectivement de 1,41, 0,30 et 1,11 an−1. Le taux d’exploitation (0,79) était supérieur au taux d’exploitation maximal (0,594) obtenu par le rendement relatif et la biomasse par recrue. Le stock estimé de 165 tonnes (plus de 700 000 individus) était caractérisé par une mortalité élevée. Ces résultats ont montré que la population était surexploitée et que de nombreux juvéniles étaient capturés (L50 = 210,4 mm; 50% mature à 295 mm). Pour maximiser le rendement par recrue, augmenter la biomasse, et soutenir la pêche, il est recommandé d’augmenter la taille des mailles des filets de pêche. Cet exemple met en évidence la variabilité des paramètres calculés à partir de différentes méthodes ce qui sous-entend que les comparaisons avec les autres populations dans le monde, mais également inter-sites sont difficiles. Toutefois les indices de performance de croissance (φ′) relevés dans la littérature peuvent servir de référentiel et confirmer la grande plasticité phénotypique de cette espèce selon les facteurs environnementaux. Les analyses ont révélé que les populations aquacoles présentaient une croissance plus rapide que celle des populations vivant dans les environnements naturels contraignants.

International audience; In the current study, ten elements contents (Fe, Zn, Mn, Cu, Cr, Co, Ni, Cd, Pb and Hg) have been measured in muscle and liver of four pelagic fish species (Engraulis encrasicolus, Sardina pilchardus, Scomber japonicus and Trachurus trachurus) from the northwest African coast (South Atlantic Moroccan coast), collected during summer and autumn seasons (July and December 2013, respectively). Significant differences in metal contents were found between the different species (p \ 0.05). Metals levels were also much higher in liver than those recorded in muscle tissues. The concentrations of Fe, Zn, Cd, Co, Cu and Pb were significantly higher in mackerel liver (p \ 0.05).While, in muscle, anchovy presents a higher content of Mn, Cu, Cr, Ni and Pb. A high level of cadmium was recorded in liver of the different species which can be attributed to an anthropogenic source (phosphate industry) and to natural sources (upwelling activities). The main concentration of toxic elements (Cd, Pb and Hg) recorded in the four edible muscles of pelagic fish species, under study, were below the established values by the European Commission Regulations and show that their effect on the consumers health can be considered as negligible.

International audience; The link between trophic ecology and metal accumulation in marine fish species was investigated through a multi-tracers approach combining fatty acid (FA) and stable isotope (SI) analyses on fish from two contrasted sites on the coast of Senegal, one subjected to anthropogenic metal effluents and another one less impacted. The concentrations of thirteen trace metal elements (As, Cd, Co, Cr, Cu, Fe, Li, Mn, Ni, Pb, Sn, U, and Zn) were measured in fish liver. Individuals from each site were classified into three distinct groups according to their liver FA and muscle SI compositions. Trace element concentrations were tested between groups revealing that bioaccumulation of several metals was clearly dependent on the trophic guild of fish. Furthermore, correlations between individual trophic markers and trace metals gave new insights into the determination of their origin. Fatty acids revealed relationships between the dietary regimes and metal accumulation that were not detected with stable isotopes, possibly due to the trace metal elements analysed in this study. In the region exposed to metallic inputs, the consumption of benthic preys was the main pathway for metal transfer to the fish community while in the unaffected one, pelagic preys represented the main source of metals. Within pelagic sources, metallic transfer to fish depended on phytoplankton taxa on which the food web was based, suggesting that microphytoplankton (i.e., diatoms and dinoflagellates) were a more important source of exposition than nano- and picoplankton. This study confirmed the influence of diet in the metal accumulation of marine fish communities, and proved that FAs are very useful and complementary tools to SIs to link metal accumulation in fish with their trophic ecology.

International audience; Subcellular cadmium (Cd) partitioning was investigated in the liver of two marine fish species, the European sea bass Dicentrarchus labrax and the Senegalese sole Solea senegalensis, dietary exposed to an environmentally realistic Cd dose for two months followed by a two-month depuration. Cd exposure did not modify Cd cellular partitioning for either species, refuting the spillover hypothesis. Both species contained most of the Cd in the detoxifying fraction but displayed different handling strategies. Cd was largely bound to heat stable proteins (HSP) including metallothioneins (MT) in sea bass while Cd was more linked to metal rich granules (MRG) in sole. Whole liver concentrations and subcellular partitioning were also determined for essential elements. The greatest impairment of essential metal homeostasis due to Cd exposure was found in sole. These elements followed the Cd partitioning pattern, suggesting that they are involved in antioxidant responses against Cd toxicity. Cd consumption diminished sole growth in terms of body weight, probably due to lipid storage impairment. During the depuration period, the two species showed contrasting partitioning patterns, implying different pathways for Cd elimination from the liver. In sea bass, MT-bound Cd would be excreted through bile or released into blood, crossing the cell membrane via a protein transporter. In sole, MRG-bound Cd would be sequestered by organelles before being released into the blood via vesicular exocytosis. These distinct strategies in cellular Cd handling in the liver might account for differential sensitivity to Cd toxicity and differential Cd excretion pathways between the two marine fish species.

International audience; Impacted marine environments lead to metal accumulation in edible marine fish, ultimately impairing human health. Nevertheless, metal accumulation is highly variable among marine fish species. In addition to ecological features, differences in bioaccumulation can be attributed to species-related physiological processes, which were investigated in two marine fish present in the Canary Current Large Marine Ecosystem (CCLME), where natural and anthropogenic metal exposure occurs. The European sea bass Dicentrarchus labrax and Senegalese sole Solea senegalensis were exposed for two months to two environmentally realistic dietary cadmium (Cd) doses before a depuration period. Organotropism (i.e., Cd repartition between organs) was studied in two storage compartments (the liver and muscle) and in an excretion vector (bile). To better understand the importance of physiological factors, the significance of hepatic metallothionein (MT) concentrations in accumulation and elimination kinetics in the two species was explored. Accumulation was faster in the sea bass muscle and liver, as inferred by earlier Cd increase and a higher accumulation rate. The elimination efficiency was also higher in the sea bass liver compared to sole, as highlighted by greater biliary excretion. In the liver, no induction of MT synthesis was attributed to metal exposure, challenging the relevance of using MT concentration as a biomarker of metal contamination. However, the basal MT pools were always greater in the liver of sea bass than in sole. This species-specific characteristic might have enhanced Cd biliary elimination and relocation to other organs such as muscle through the formation of more Cd/MT complexes. Thus, MT basal concentrations seem to play a key role in the variability observed in terms of metal concentrations in marine fish species.

This study examines the effects of increasing salinity on fish assemblage structure in the Casamance Estuary, Senegal, using a series of indices. The study data were derived from commercial fishery surveys conducted between April and July 2005. Analysis of within-trophic-group diversity in the Casamance Estuary shows a significant drop in the diversity of apex predators in the upper, more saline reaches of the estuary. By contrast, primary consumers adapted well to salinity changes and exhibited higher taxonomic diversity in the upper reaches of the estuary than in the lower reaches. The findings also indicate decreases in average sizes of the landed species and the trophic levels among fish catches in the direction of the upper reaches. However, the catch per unit effort (CPUE) was globally higher in the upstream area of the estuary as compared with the downstream area. This increasing CPUE trend from the lower towards the upper reaches is attributed to (i) the high primary productivity in the upper Casamance Estuary; (ii) the increasing abundance of euryhaline fish species in an upstream direction; and (iii) ‘telescoping’ of the food chain through the presence of mostly herbivorous fishes in the upper reaches. Such a situation likely facilitated high fish production in the upstream area of the estuary, though multimetric indices indicated unfavourable living conditions for many fish taxa in this area. Keywords : biological indicators, Casamance Estuary, ecosystem monitoring, fishery statistics, inverse hypersaline estuary, Senegal, taxonomic diversity

WOS:000433550600003; International audience; The high demand for fish products has led to important economic issues including the emergence of new markets, especially in developing countries where fishing represents an important economic sector and the main source of animal protein for local communities. The worldwide fisheries have overall passed their peak and many stocks are overfished despite the increasing efforts towards the developpement of fisheries assessment and management tools using global and analytical models. These complex models used for fisheries management, especially in many developing countries, require large amounts of data whose collection and analysis are very costly and expensive. The current studies of the effects of overexploitation on fish communities use an indicator-based approach as an alternative to improve the assessment and management of the fisheries resources. This latter approach is cheaper, simpler and more efficient to establish a link between disruption and its effect on a biotic component. The currently most commonly used indicators have provided successfully results in many studies cases carried out on fish communities. Thus, management approaches based on indicators in the recent years has improved our understanding of the structure and functioning of ecosystems, and therefore, facilitated the interpretation of biological phenomena.

International audience; The presence of tropical species has been reported in Atlantic-European waters with increasing frequency in recent years. Unfortunately, the history of their migrations is not well understood. In this study, we examined the routes and timing of fish migrations in several ecosystems of the East Atlantic Ocean, combining several publicly available and unpublicized datasets on species occurrences. The species studied were those noted as exotic or rare outside their previous known area of distribution. We used sea surface temperature (SST) data obtained from 30 years of satellite observation to define three distinct time periods. Within these periods, temperature trends were studied in six ecosystems: the North Sea, the Celtic Sea, the South European Atlantic Waters, the Mediterranean Sea, the Canary Current, and the Guinea Current. We also incorporated bathymetry data to describe the distribution of species. Measurement across a relatively large spatial extent was made possible by incorporating the capabilities of GIS. While SST increased consistently over time in all of the ecosystems observed, the change in number of species differed among ecosystems. The number of species in the middle regions, such as the South European Atlantic Shelf and the Western Mediterranean Sea, tended to increase over time. These regions received numbers of species from the lower or the upper latitudes according to season. Of all of the species observed in the recent period, seven species from the Canary Current tended to be found in the Western Mediterranean Sea, and six species from these two regions extended their distributions to the South European Atlantic Shelf. Twelve species from the Canary Current moved seasonally to the Guinea Current. In the northern regions, 13 species moved seasonally in the North Sea and the Celtic Seas, and 12 of these species reached the South European Atlantic Shelf. This study presents a picture of routes and timing of species migration at the biogeographical scale. It shows how species have shifted their area of distribution (mostly northward) along the Eastern Atlantic ecosystems or have increased their depth ranges probably to remain within their preferred temperature range in response to long-term and seasonal environmental changes.

International audience; To preserve ecosystem services of tropical reservoirs that provide food and income for local populations, it is necessary to monitor fish population temporal dynamics, stock, and yield fishing. The aim of this review is to provide information to who monitor fish populations in tropical reservoirs. The topics, investigative methods, observation scaling, and metrics used in publications since 1960's were inventoried. Temporal and spatial variations in fish population and fish production were the primary topics. Since 2000, studies on trophic structures and fish biological parameters have appeared. Experimental fishing with gillnets has been the technique most often used, but others include hydroacoustics for fish stock assessments and modeling to characterize trophic structures. Annual and seasonal scaling approaches dominated, as well as longitudinal/transversal gradient sampling, which permitted the characterization of fish colonization processes and the detection of halieutic resource overexploitation. For modeling approaches, the entire reservoir was considered. The calculated metrics varied depending on the investigation method. Species richness, number of individuals, and biomass per unit effort were the most common metrics for the different topics. Trophic level, effectiveness, and consumption rate were specific to trophic structure. To monitor reservoir fish populations, it is necessary to have an eco-systemic approach.

International audience; We analysed the genetic diversity of nine populations of two subspecies of Sarotherodon melanotheron using allozymes in relation to the existence of environmental pressures. The genetic variability of Sarotherodon melanotheron melanotheron in the Ivorian ecosystem (A = 1.23, He = 0.04 and P-(95%) = 11.11) was lower than that of Sarotherodon melanotheron heudelotii of the estuaries in Gambia and Saloum (A = 1.68, He = 0.08 and P-(95%) = 22.23). The analysis of allelic diversity revealed that the individuals of S. m. melanotheron subspecies from a much polluted environment (lagoon of Grand-Lahou or Ebrie) presented a relatively higher frequency of allele GPI-2*80 than those of a less polluted environment (Aby lagoon and Ayame lake). The allele IDH-2*90 was expressed only in the specimens of S. m. heudelotii found in the Saloum estuary, a very saline environment compared to the estuary of Gambia. A deficit of heterozygosis was observed with the tilapia Sarotherodon melanotheron. This study confirmed the possibility of using allozymes as biomarkers and bioindicators of environmental disturbances.

International audience; The Canary Current Large Ecosystem is characterized by a major, nutrient-rich up-welling of deep, cold oceanic waters, which stimulates high biological productivity that results in an abundance of both pelagic and demersal fishery resources. In this area, the Moroccan coast is also under direct anthropogenic activities influences such as phosphates industries. Several observations indicate that variations of the upwelling activities observed in the recent years would generate the remobilization of the contaminants. Our study deals with the metallic concentrations in the otoliths of 3 fish species of economic interest ( Merlucius merlucius, Pagellus acarne, Sardina pilchardus) along the Moroccan coast. The fishes were collected in markets in june 2012 for the northern part of Morocco (during the maximum of the upwelling activity) and in November 2012 for the southern part. Concentrations were measured by ICP-MS on transects from nucleus to the edge of otoliths to characterize environmental variations or accumulations linked to ecophases and growth. For each species, 5 fishes were analyzed in 7 locations from North to south of the Moroccan Atlantic coast. First results indicate that the concentrations are highly variable in hake transects, and on the other hand, transects in seabreams seem very stable regardless of location. Transects in sardines indicate different patterns depending on the locations and, some of them, clearly show the upwelling signature.

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International audience; Présenter les défis scientifiques et les enjeux sociétaux des aires marines protégées a permis de mettre en évidence certaines problématiques inhérentes à la multiplication sans précédent de ces aires marines protégées. L’ambition de cet ouvrage était d’exposer, de la manière la plus objective possible, les avantages,inconvénients et éventuels risques de ce développement d’espaces naturels protégés en mer.À l’instar des aires protégées terrestres, il est toujours tentant de chercher à comparer les zones protégées à celles qui ne le sont pas et de mettre en opposition les zones marines classées avec celles ne bénéficiant d’aucune protection. L’une est-elle systématiquement mieux protégée que l’autre ? Les première sont-elles vocation à servir de laboratoire expérimental pour les secondes (AUBERTIN et RODARY, 2008) ? En tout état de cause, la protection des socio-écosystèmes nécessite à la fois des recherches issues des sciences naturelles et d’analyses en termes de gouvernance. Les travaux présentés dans cet ouvrage ont permis de démontrer que des controverses et difficultés d’application existent, mais qu’elles ne doivent pas diminuer l’importance des aires marines protégées, notamment pour la résilience des écosystèmes face aux pressions anthropiques et au changement climatique. Car, au-delà de l’écosystème concerné par la réglementation de l’AMP, la mise en place d’aires marines protégées a de nombreux autres effets qu’il importe de souligner.

International audience; Sous la pression de nombreuses organisations conservationnistes, le postulat de l’efficacité biologique des aires marines protégées s’est imposé : elles réduisent lamortalité des espèces cibles, améliorent l’abondance et la qualité des peuplements,favorisent la reproduction et le recrutement et vont même jusqu’à repeupler le milieuextérieur. Nombre de ces synthèses globales sur les AMP tendent à comparer dessituations très hétérogènes et sans vouloir remettre en cause l’apport positif des AMP, le chapitre de Laë et al. vise, sur des bases scientifiques rigoureuses, à mieux décrire lesprocessus de restauration, à les resituer dans le temps et dans l’espace

International audience; A comparative study of fish assemblages of a marine protected area and an exploited area was carried out in the Sine Saloum estuary (Senegal, West Africa). Data were collected at these two sites between 2008 and 2011. The two areas are morphologically similar, and comparison of the physicochemical parameters (salinity, transparency, temperature and percentage of saturation of dissolved oxygen) showed that the water masses are similar. The bio-ecological indicators and ecological, trophic and size structures were also compared. Given the environmental similarity of the two sites, their differences in bio-ecological characteristics and fish assemblage organization can be explained by the protection status of the MPA. The species richness and fish abundance per sampling unit were higher in the exploited site, but the mean trophic level and the observed maximum size of individuals were higher in the MPA. Although slightly higher for the MPA, the differences observed for biomass and average size between the two sites were not significant. The beta diversity in the MPA was higher than in the exploited site. The fish assemblage was dominated in terms of abundance by two pelagic herbivores Ethmalosa fimbriata (80%) and Sardinella maderensis (12.3%) and in terms of biomass by E. fimbriata (37.4%) and a benthic predator, Arius latiscutatus (23.7%). The assemblage of the MPA had a marine affinity characterized by the presence of piscivorous or generalist predators; large individuals were not uncommon. In the exploited site, the fish assemblage was relatively stable, dominated in abundance by E. fimbriata (33.4%) and S. maderensis (32.8%). In terms of biomass, a species of mullet, Liza dumerili, ranked first (26.2%), followed by E. fimbriata (22.4%) and S. maderensis (14.9%). The assemblage in the exploited site had a more estuarine affinity, dominated by detritivorous or phytophagous herbivores. Individuals observed there were mostly of small or medium size. This study confirms that the establishment of MPA is an effective tool for restoring marine biodiversity and trophic structure of fish assemblages.

We assayed the mtDNA phylogeography [196 base pairs (bp) of the cytochrome b] and population structure (n = 680) in the estuarine fish, Ethmalosa fimbriata, from its whole distribution range: 14 locations along the West African coasts were sampled. Specifically, we considered Pleistocene glaciations as well as the hydrodynamics and climatic conditions of the estuarine environments in order to identify the main evolutionary forces that have shaped the genetic variation in mtDNA, i.e., the contemporary or the historical gene flow. There was an overall significant population differentiation among estuaries (Fst = 0.10). Although E. fimbriata showed a significant pattern of isolation by distance over the entire sampled range, this genetic structure did not mirror contemporary gene flow but the colonization sequence of the present distribution range. Finally, the mtDNA genetic structure traced the past historic dispersion that occurred at the end of the Pleistocene glaciations. The central part of the present distribution area was probably the species origin and due to difference in the historic migration rate northward and southward, isolation of a South group occurred first, 110,000-190,000 years ago, before the divergence of the North group 47,000-82,000 years ago.

Lagoon fish communities often consist of complex assemblages of numerous species, difficult to manage with conventional stock assessment models. Useful data are still lacking for evaluating the importance of man-made disturbance and reference situations are missing, especially in developing countries. As a consequence, by analysing data collected 20 years ago in two of the six sectors of the Ebrie lagoon (Ivory Coast), this study aims to evaluate the impact of fishing effort on fish assemblages. These two sectors (V and VI), located far from the inlet, have similar physical, chemical, biological and fish fauna characteristics. The major difference lies in the fishing intensity: sector VI has a low fishing intensity (fishing for personal consumption using only individual gear), whereas sector V is heavily fished (professional fishing with both individual and collective gear, particularly beach seines which result in a considerably higher fishing effort). Comparisons between the two sectors were based on two complementary scientific approaches: a 3-year commercial fisheries survey (1978-1980) and a 1-year experimental survey (1981). The impact of fishing on fish assemblages is analysed through the main characteristics of fish populations and communities. The results show that there were major changes including an increased catch yield (37.5-189 kg ha –1 y –1 ), a lowering of fish diversity in catches, of fish biomass (100-20 kg ha –1 ), of average catch length (22.6-14.6 cm) and of trophic level of catches (26-58.5% of herbivore/detritivore species in total catches). Such results are quite unusual because they occurred even in non-overfished ecosystems: the fish assemblage was deeply modified in sector V compared to the lightly fished adjacent sector VI, even though fishing effort in sector V was only high but at a reasonable level. These data must be completed by similar studies in tropical lagoons with variable levels of fishing intensity in order to understand fish assemblage re-organization when submitted to stresses of different intensity.

Two indicators of fish exploitation, catch per unit effort (CPUE) and yield per unit area, were used with surveys of small-scale fishery landings to compare two African reservoirs that differ in their level of fish exploitation, being high for Selingue Reservoir and low for Manantali Reservoir, both located in West Africa. The heavy exploitation in Selingue Reservoir induced a decline in catches per fishing trip (9.5 kg trip−1), whereas the catch remained high at Manantali Reservoir (17.0 kg trip−1). Moreover, the annual fish yield per unit area was lower for Manantali Reservoir (19 kg ha−1) than for Selingue Reservoir (79 kg ha−1). There was no relationship between the Morphoedaphic Index and catches for the two reservoirs. Rather, only the fishing effort and annual water-level variations defined the reservoir fish yields. Based on the results of the present study, it appears that CPUE and fish yield per unit area represent simple and robust indicators for describing the impacts of fishing intensity in aquatic ecosystems.

International audience; Marine Protected Areas (MPAs), in addition to their important role in the conservation of critical habitats and vulnerable species, are now also used in a context of ecosystem-based fisheries management. The aim of this study was to assess the observed changes in the fish population structure in the years following a fishing ban. The experiment took place in the Bamboung bolong, a small bay in the Sine Saloum estuary in Senegal, West Africa. The protection of this zone was undertaken with local fishermen support, and a formal MPA was created in 2004. The survey of the fish assemblages was initiated in 2003, the year before the fishing ban, and went on until 2007, with 12 sampling sites being monitored three times per year.In this paper, we describe and analyze the evolution of the fish assemblage during this five years' experimental period. Some changes have been highlighted on which the effect of the fishing ban was shown. After the fishing ban, an increase of total biomass and of maximal fish length has been registered. In the same time, the number of species has increased and the percentage of large and/or iconic species targeted by fishing has also increased. The community structure has been modified, with more small fish, more big fish (new large species and more large individuals in the original species) and fewer medium sized fish. The contribution of marine affinity species has increased in depend of the estuarine part of the assemblage. Finally, the trophic structure has been modified with an overall increase of the mean trophic level, resulting from an increase of the percentage of generalist or piscivorous predators and a sharp decrease of herbivorous and detritivorous low trophic level species. Marine predators which numbers and size were reduced by fisheries are again important components of the system. We argue that despite the intrinsic variability of the tropical estuarine environments, the introduction of a total ban on fishing in such estuarine zones can substantially improve the health status of the fish assemblage.

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The factors affecting fish yields in lagoons were studied through the stability, species composition and size structure of the yields at high and low fishing intensities, respectively, in Togo and the Ivory Coast. Heavy fishing was followed by the development of a plateau phase resulting from a reorganization of species assemblage. In such a state, exploitation produced only a few dominant species for which average catch lengths were small; which were generally herbivorous and exhibited a continuous reproduction, sometimes a lowering of the age at first sexual maturity, permitting a faster turnover. Even if total catches did not collapse, fish biomass and catch per unit effort decreased when fishing effort increased. One option for management could be to locate the level of fishing effort on the plateau such that fishermen can be at least financially independent and provide sufficient fish for local subsistence.

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International audience; no abstract

L'histoire recente des peuplements ichtyologiques du Delta Central du Niger au Mali est marquee, entre autres, par deux periodes de secheresse survenues en 1973 et 1984, un accroissement important de l'effort de peche depuis les annees 1960 et la construction d'un barrage hydro-electrique mis en service en 1984. A des degres divers, ces evenements sont venus perturber le cycle biologique des poissons qui reposait sur une adaptation des especes aux cycles hydrologiques des fleuves Niger et Bani. La secheresse provoque ainsi une reduction des surfaces en eau et une baisse de la duree d'inondation qui divisent par quatre les surfaces moyennes inondees dans l'annee. De 1968 a 1989, la diminution des captures est evaluee a 45000 tonnes passant ainsi de 90000 a 45000 tonnes. Dans le meme temps, malgre la baisse des prises totales, on observe une augmentation des rendements a l'hectare qui passent de 40 kg en 1968 a 120 kg en 1989. Ce phenomene est a rapprocher de l'abaissement de l'âge moyen des poissons (69% des individus peches ont moins d'un an) du a une augmentation de la mortalite par peche et de la mortalite naturelle plus forte en cette periode de secheresse. Cette augmentation de productivite s'accompagne egalement d'une rarefaction de certaines especes comme Gymnarchus niloticus, Polypterus senegalus, Gnathonemus niger dont les reproduction sont infeodees aux zones d'inondation et d'especes comme Cithorinus cithorus et Clarotes laticeps qui se distinguent par une frequentation intensive des plaines d'inondation. Dans le meme temps, on assiste a un developpement de certaines familles comme les Cichlides et les Clariides qui presentent dans les deux cas de fortes resistances a l'anoxie. D'une maniere generale, les especes presentant des âges de premiere maturite faibles (inferieurs a 1 an) et plusieurs periodes de reproduction dans l'annee sont celles qui sont actuellement les mieux representees dans les peuplements

International audience; The utility of taxonomic and functional approaches in assessing the structure of fish communities is tested in the hypersaline estuary of the Casamance river using data from surveys of commercial fisheries conducted between April and July of 2005. Both taxonomic and functional diversity decrease from downstream to upstream regions of the estuary. In terms of species composition, marine-estuarine species (33.3-46.3%, depending on the site) and estuarine species of marine origin (29.3-41.7%) dominate the exploited population in the Casamance estuary. In contrast, the proportion of strictly estuarine species observed upstream is twice that observed downstream. Quantitative analysis based on biomass landed distinguishes two groups in the population: (1) a group of species that is dominant downstream, containing primarily terminal predators and secondary consumers categorised as marine species that are occasional or accessory in estuaries, estuarine marine species, and estuarine species of marine origin; and (2) a group of species characteristic of the upstream region, dominated by a few species (Sarotherodon melanotheron, Tilapia guineensis, and Mugil cephalus) mainly of strictly estuarine and/or herbivorous categories and Elops lacerta, a carnivore fish. The outcomes of the two approaches are similar, and both indicate that the fish community in this estuary is under the influence of strong environmental disturbance. However, the scales at which the specific and functional approaches most reliably reflect environmental conditions are different. The taxonomic approach, i.e., the use of specific biomass is more appropriate at the ecosystem scale and therefore more accessible to local human communities, whereas the functional approach is better suited to regional and sub-regional studies because of the change in species composition from one environment to another.

Evaluation pluridisciplinaire de l'efficacite d'une AMP emblematique : le cas du bolong de Bamboung au Senegal

National audience; The dam anti-salt of Maka was set up as 1998 on the river Casamance (Senegal) to restore the fresh water conditions for the benefit of the rice production activity of the local populations. It separates two environments of salinity contrasted: a downstream salted part under influence of the inverse estuary and an upstream part of fresh water. The comparative study of the diets of two Cichlidae Sarotherodon melanotheron heudelotii and Tilapia guineensis living on both sides on the anti-salt dam of Maka was made on the basis of samplings over three different periods in 2006 and 2007: a warm wet season (in October), a cold dry season (in February) and a warm dry season (in May). The diets of both species are approached by a classic method of analysis of the stomach contents by noting the occurrences of preys. Compared with the Ndour et al. (2011) J. Sci. Halieut. Aquat., 4: 120-133 121 populations of S. m. heudelotii and of T. guineensis who live upstream to the dam, those living downstream to the dam present more important occurrences of preys. S. m. heudelotii tends to ingest of advantage of plant fragments to the detriment of the mud when the salinity becomes high. On the other hand, T. guineensis who has a wider trophic spectrum, tends to consume more vase (bivalves) than of plant fragments, when the salinity increases. The comparison of the diets of this two Cichlidae on both sides of the dam, shows an evolution of their diets according to the environmental conditions.

International audience; Trophic indicators were used to compare two Malian freshwater reservoirs whose main differences are based on their different fishing pressures. Data were collected from a scientific survey of small-scale fishery landings conducted in 2002/2003. The trophic levels of fish species caught by artisanal fisheries are estimated from observations of scientific fishing or from the metabase Fishbase. Important differences exist in the trophic structure of both reservoirs. In Selingue (with high fishing pressure), very few top predators are found in the catches while the low trophic level fishes increase in total catches. In Manantali (with low fishing pressure), the top predators contribute twice as much to catches compared to Selingue. Hence, the mean trophic level of catches in Selingue (2.80) is lower than in Manantali (2.97). When comparing these results with those of study made in 1994/1995, it clearly appears that the effects of the fishing pressure in Selingue are obvious through a decrease of 0.12 in the mean trophic level while in Manantali this mean level has increased by 0.33 due to a recent strategic targeting of top predators. Trophic spectra seem to be relevant tools to characterize exploited fish communities from multi-specific and multi-gear small-scale fisheries catch data

International audience; To investigate the changes in the fish assemblage of the Sine Saloum estuary (Senegal) over a 10-year period, it was surveyed during a complete hydrological cycle (three principal hydro-climatic seasons) first in 1992 and then in 2002-2003. The sampling protocol for the two surveys was identical, using the same sampling technique, the same collection periods, and the same sampling stations. The Sine Saloum is an inverse estuary in terms of its salinity gradient. It is affected by the intense drought that has occurred in this biogeographic region for more than 50 years. The estuary is also subjected to high fishing pressure. The second data-collection period followed a few years of higher recorded rainfall (approximately 35% higher than in 1992) and was characterized by increased fishing pressure (over 50% higher than in 1992). For the two study periods, the same set of indicators were calculated, including fishing indicators (catches, density, yields), size-based indicators (size structures, mean length, maximum observed length, size spectra), ecological indicators (richness, species diversity, K-dominance models, ABC curves, ecological categories) and trophic indicators (mean trophic level, trophic composition of catches). Overall, the main changes in the estuary's fish assemblage between 1992 and 2002 were (1) a loss in total biomass (40% less) for an equivalent species richness (approximately 55 species); (2) a decrease in the maximum observed lengths for many species (mean decrease of 17%); and (3) a decrease in the mean trophic level (more than 0.11 units). Analysis by bio-ecological and trophic category showed that the main species concerned were benthophagous species and, to a lesser degree, generalist predator species from marine origin that inhabit the estuary more or less permanently

Les pecheries du delta central du Niger font l'objet d'une exploitation intensive depuis de nombreuses annees. L'analyse des debarquements de poisson pendant les deux dernieres decennies laisse apparaitre de fortes fluctuations dans la production qui decroit regulierement de 87 000 t en 1969–70 a 37 000 t en 1984–85. Deux causes peuvent etre evoquees pour expliquer ce phenomene : l'apparition de la secheresse en Afrique et/ou l'augmentation de la pression de peche. Dans le premier cas, les captures annuelles et l'indice d'inondation des plaines, calcules en fonction du cycle hydrologique du delta (de juillet a juin), sont bien correles, ce qui s'explique a posteriori par la composition des captures qui s'effectuent a 69 % sur des poissons de moins de 1 an. Dans le second cas, certains elements suggerent qu'il y a eu une intensification importante de la pression de peche sans que celle-ci puisse etre chiffree. La chute des captures totales ne semble pourtant pas due a une mauvaise exploitation des ressources. Les difficultes ressenties par les pecheurs : reduction des prelevements individuels qui passent de 1 900 kg/an en 1966 a 740 kg/an en 1989, necessite de diversifier les activites en pratiquant l'agriculture ou de migrer vers les lacs de barrage ou a l'etranger, seraient donc dues a des phenomenes naturels (secheresse) ayant entraine par la suite des complications d'ordre sociologique et economique. De fait, la reduction des zones en eau provoque une diminution des sites exploitables et l'emergence de conflits entre villages ou familles pour l'acces a la ressource. Par ailleurs, un certain relâchement du controle des autorites traditionnelles lie a un accroissement demographique important aurait provoque une augmentation du nombre de pecheurs en vingt ans de l'ordre de 20 000 personnes (43 000 en 1966 contre 63 000 en 1989) conduisant a un partage difficile des captures par ailleurs en baisse.

International audience; The adaptive mechanisms on the early life stages of fishes to hypersaline stress are still poorly understood and probably determine the resistance of a population to disruption, compared with other less plastic species. The Casamance River in Senegal is an ideal location to test the adaptation to salinity as a dam was built in 1998 to exclude saline water intrusion. This lowered the salinity from 70 to 5 upstream and 60 downstream. The salinity influence on the growth in the early life of three West African fish species (Ethmalosa fimbriata, Sarotherodon melanotheron, and Tilapia guineensis) was studied using the width of microstructures in the otoliths and the individual migratory behaviour analysed from strontium (Sr) to calcium (Ca) ratios in the otoliths. The Sr:Ca ratio was quantified along individual transects measured from the posterior edge of the otolith to the core. The fishes were sampled on both sides of the dam that separated water with low salinity upstream from metahaline and hyperhaline water downstream. The results showed that salinity has different influence on the growth of each species. Ethmalosa fimbriata has the highest growth during the first 180 days in the freshwaters, indicating growth inhibition in the hyperhaline areas. For the two other species no growth difference were found. The Sr/Ca ratio varied widely, in Tilapia and Sarotherodon from below the dam. Individual life histories were more heterogeneous than upstream and showed a crossing of the dam for some individuals which could reach half of the fishes analysed. On the contrary in E. fimbriata, despite the large range of salinity, identical Sr/Ca profiles were found both upstream and downstream. This indicated that Sr/Ca ratio was not appropriate to evaluate life history patterns linked to salinity for this specie.

Fish communities living within the artificial reservoirs of Manantali and Selingue in Mali are compared after the monitoring of small-scale fishery landings over 13 months (May 2002–June 2003). The fishing pressure constitutes the main difference of these two similar reservoirs. The effects of fishing on the structure of the fish communities are analyzed by using fish size spectra and derived indicators. In Selingue Reservoir, the fishing gears mostly target smaller species (mean length of 165 mm) than in Manantali Reservoir (mean length of 210 mm). Unlike in Selingue, the size structure does not change between seasons in Manantali. In Selingue, fishing practices that target the fish recruitment of the year constitute a structuring factor of the fish sizes observed in the catches. In spite of similar values of the slopes of the global fish communities’ size spectra between the two reservoirs, they clearly display an intense exploitation at Selingue. Then, size-based spectra indicators represent a potential tool for assessing the impact of fishing on fish communities in small-scale fisheries.

A comparative study, combining three different assessment methods (fish gillnet sampling, artisanal fisheries surveys and hydroacoustics) was conducted in Mali where two man-made reservoirs (Selingue and Manantali) are particularly suited for investigating the impact of fishing effort on the fish assemblage. These two ecosystems have relatively similar areas, edaphic and environmental properties but are subjected to different levels of fishing exploitation (low at Manantali, high at Selingue). The comparison is based on two indicator parameters: the abundance indices and the size spectra distributions, obtained by the three methods at two contrasting hydrological seasons (April and October). The results were compared first between the two seasons, and then between the two lakes. The present work is based on two main hypotheses: (1) that there is a higher fish abundance in October associated with smaller overall sizes, after spawning; (2) a lower abundance and smaller sizes in the Selingue reservoir than in Manantali, because of the much higher fishing pressure in Selingue. The relevance of each method to the selected indicators is discussed. On the one hand, the three methodologies on the whole gave similar conclusions and they also complement each other. On the other hand, some results do not match the hypotheses because of biases due to difficulties and technical limitations of each method in such ecosystems (shallow water with vegetation and stumps of former forests).