Impact of fishing on fish assemblages in tropical lagoons: the example of the Ebrie lagoon, West Africa

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. Effets de la pêche sur l’organisation des peuplements de poissons dans les lagunes tropicales : l’exemple de la lagune Ebrié en Afrique de l’Ouest. Les peuplements de poissons lagunaires sont difficiles à gérer à l'aide de modèles halieutiques conventionnels car ils se présentent souvent sous la forme d'assemblages complexes composés de multiples espèces. Les observations actuellement disponibles sont encore insuffisant