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West African marine ecosystems are very productive and sustain important fisheries that have developed rapidly in the last decades. The analysis of the fishing impact on exploited resources is usually conducted through single-species assessments. In this study, we propose a complementary approach that enables to account for some ecosystem effects of fishing. In Guinea and Senegal, fisheries have developed relatively recently and at the same time, the collection of landings and surveys data has been carried out. In consequence, the data collection extends from a period where stocks could be considered as non exploited to a situation of overexploitation. This case study is analysed in order to detect shifts in the ecosystem structure in response to increasing fishing pressure. To this aim, trophic spectra and long time series of mean trophic level are examined for demersal fish communities. Trophic spectra display either the distribution of the demersal community biomass or the commercial catches according to trophic level classes. Some substantial and statistically significant changes in the trophic structure of the Senegal and Guinea ecosystems were observed. In particular, the biomass of the high trophic levels decreased whereas the lower trophic levels displayed a relative stability or an increase. This could be linked to a "top-down" fishing effect due to a release of predation on the lower trophic levels of the demersal fish community. In Senegal, the mean trophic level decreased significantly for both the catches and the demersal community biomass. Such a decrease was also observed for the coastal demersal biomass in Guinea. This showed that fishing activities had an impact on the trophic structure of the ecosystem, and a "fishing down marine food web" effect was shown in West Africa for the first time.

Changes in the trophic structure of fish demersal communities in West Africa in the three last decades