Wild collection management and farming of the mangrove oyster (Crassostrea gasar) occurring widely at the Benin (West Africa) coastal zone require knowledge on the feeding ecology to explore energy sources and nutritional needs. Six hundred thirty (630) individuals of C. gasar have been sampled in the rearing site at the Benin coastal lagoon to investigate on the trophic ecology of this cultivated bivalve. The diet analysis revealed that C. gasar is a filter-feeder foraging mainly on phytoplankton (72.70%) and substrate particles (22.95%). This trophic specialization results from anatomical structure, mainly the presence of gills which facilitate the filtering of number of plankton taxa. Dominant phytoplanktons ingested comprised of Diatomophycea (33.52%), Chlorophycae (17.19%), Scenedesmacae (13.80%), Dictyosphaeriacae (3.79%), and Pleurococcacae (2.75%). Eight genuses of phytoplankton, Polycystis, Coelosphaerium, Protococcus, Botryoccocus, Crucigenia, Melosira, Cyclotella, and Gyrosigma dominated the diet of C. gasar with aggregated volumetric proportions reaching 69.06% of the diet. Higher occurrences were recorded mainly for Melosira occurring in 263 (41.75%) stomachs, substrate particles in 211 (33.49%), and Polycystis in 151 (23.97%). C. gasar exhibited a high niche breadths varying from 4.54 to 5.78, suggesting that this bivalve consumed a high variety of food items, thus exhibiting a degree of trophic plasticity. Diet overlaps ( jk ) among different size classes were high and varied from 0.71 to 0.98, indicating an ontogenetic diet shift pattern in C. gasar. Probably, to adapt to the benthic-muddy environment and to increase survival, C. gasar has evolved a specialized feeding mechanism and strategy to retrieve only needed nutrients for growth and to reject awful and nondigestible foods. Also, at the oyster rearing grounds, there is an evidence of shift in the food web structure leading to an increase of the biological productivity at the coastal zone. The output from this study is a valuable documentation for the sustainable development of oyster aquaculture, wild stock management and conservation. However, further scientific knowledge on nutritional needs, phytoplankton toxicity and habitat degradation, and improvement of farming techniques are required for an integrated oyster management.

In tropical Africa, and particularly in Benin, running waters dwell a high and valuable fish fauna that are almost unknown, but under severe degradation. The current study inventoried the fish biodiversity of the Okpara stream, the main tributary of the Oueme River (the largest in Benin), in order to fill the gap of ichthyological data and to improve habitat protection, species conservation and valorization. During 18 months, fish samplings were made monthly with various fishing gears on five (5) locations. A total of 53 fish species belonging to 30 genera, 14 families and 7 orders were recorded among a total of 9552 individual fish collected. The most representative families were Mormyridae with 9 species and Cichlidae with 8 species, then follow Mockokidae, Clariidae, Cyprinidae and Alestidae with 6, 6, 6, 4 species, respectively. Families such as Anabantidae, Bagridae, Claroteidae, Malapteruridae, Polypteridae and Schilbeidae were less represented and individually included 2 species. The Hepsetidae and Aplocheilidae were represented by one (1) species, respectively the African pike, Hepsetus odoe and Epiplatys bifasciatus. A holistic scheme of ecosystem management and species conservation requires a complete knowledge on the fish community structure, ecotoxicology and species ecological status.

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