Variability of key biological parameters of round sardinella Sardinella aurita and the effects of environmental changes.

We examined growth rates and reproductive characteristics of Sardinella aurita off Senegal and other coastal areas over a 20 year period (1995-2014) to determine how they relate to variations in environmental characteristics of coastal waters. Based on fish length-frequency data and a coastal upwelling index, we found that S. aurita recruitment tends to occur during the periods of most intensive upwelling (March-April off Senegal). Peak reproduction corresponds to periods of low sea-surface temperature (in February or March). The sex ratio was remarkably consistent during the 30 year study period and so was not affected by environmental changes. We hypothesise that S. aurita takes advantage of the higher zooplankton productivity that occurs in coastal waters when upwelling brings nutrient-rich water to the surface (i.e., it increases its growth rate and accumulates energy reserves for spawning). Growth performance appears to be strongly dependent on environmental conditions. The timing of spawning seems to occur when food (zooplankton) is most available for supplying the energy requirements needed by adults for spawning and early development of larvae. Environmental changes seem to have a significant effect on S. aurita growth and reproduction, which endorses their high phenotypic plasticity.

Stable isotope analyses revealed the influence of foraging habitat on mercury accumulation in tropical coastal marine fish.

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.

Trophic ecology influence on metal bioaccumulation in marine fish: Inference from stable isotope and fatty acid analyses.

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.