Fishery ecological survey of natural coastal lakes of Benin, West Africa

Abstract

Fish assemblages were sampled and water physicochemistry was evaluated in Lake Nokoue and Lagoon Toho-Todougba in southern Benin. Various biological parameters (species richness, species diversity, abundance, relative abundance, ponderal indexes) were evaluated and regressed against environmental conditions to examine patterns of association. Feeding ecology and morphological diversification of the fish assemblages were examined. Multivariate procedures (principal components analysis, euclidean distance, cluster analysis, and canonical correlation analysis) were performed to examine trophic guilds and associations between trophic specialization and morphology. Fish species trophic relationships (food webs) were examined for each fish assemblage. The two fish-, assemblages exhibited low similarity in species composition due to differences in water physicochemical conditions. Among the 35 and 21 species found in Lake Nokoue and Lagoon Toho-Todougba respectively, only a few species dominated. Lake Nokoue was dominated by the perciform Gerres melanopterus and the clupeiform Ethmalosafimbriata, whereas Lagoon Toho-Todougba was dominated by the characiforyn Brycinus longipinis. In Lake Nokoue, the Akadja habitat exhibited high species richness compared to the other local biotas. In general, for the dominant species, small individuals were associated with shallow habitats with or without vegetation. Within assemblages, there was a lack of association between physicochemical conditions and community attributes. In Lagoon Toho-Todougba, the utilization of vegetation (flowers, duckweed, seeds) and aquatic insects was common, but these items were very rare in Lake Nokoue fish diets. Most likely, the destruction of the mangroves in Lake Nokoue has reduced the availability of vegetation and aquatic insects. As a result, Lagoon Toho-Todougba was dominated by detritivorous and insectivorous species, whereas Lake Nokoue was dominated by piscivorous species. About thirteen (I 3) trophic guilds were identified within both assemblages. The detritivore and the piscivore guilds contained more species than the others. Despite the differences in species composition, the two fish assemblages had similar mean values for most morphological traits, so that morphological-based centroids were similar in multivariate analyses. Most species belonging to the same feeding guild had similar morphologies indicating that a degree of diet specialization has accompanied morphological diversification. Further investigation is needed, both spatially and temporally, to assess the existing fish resources, their exploitation, and habitat associations. Also, more data are needed to better understand the roles of interspecific and intraspecific interactions and the overall pattern of community organization with regard to habitat utilization and resource partitioning. In any case, restoration of these ecosystems will require assessments of ecosystem integrity in order to diagnose the relative impacts of human activities.