Résumé
While an Ecosystem Approach to Fisheries management is being promoted by FAO and other bodies, many gaps still persist in understanding the functioning of marine ecosystems and in assessing ecosystem fishing effects. The need for defining new management methods on an ecosystem scale requires the development and the use of ecosystem indicators of fishing effects and new reference points. The present paper aims at evaluating the relevance of some indicators (e.g. slope of size spectrum, mean length of species, biodiversity index...) to quantify fishing effects on the fish community of the southern Benguela upwelling ecosystem. Some objective criteria will serve for evaluating ecosystem indicators, in particular their ecological meaning, their sensitivity to fishing, their exclusiveness or possibility to distinguish fishing effects on fish communities from other environmental effects. To this aim, the multispecies size-based model Osmose (Shin and Cury 2001; 2004; Shin et al. 2004) is used. This individual-based model of trophic interactions lies on the hypothesis that predation in marine environment is an opportunistic process, mainly based on size adequation between a predator and its prey. By using different fishing scenarios, we suggest which indicators may be most informative for quantifying direct and indirect fishing effects on the fish community of the southern Benguela.