This paper presents a multispecific approach in order to assess the main demersal resources exploited off Senegal (West-Africa). In absence of reliable data on the actual fishing effort, two series of theoretical fishing effort are estimated, using available species abundance indices from 1971 to 1991. These two series correspond to different hypotheses concerning the trends of fleets' fishing power: an optimistic one and a pessimistic one. For each species or taxa, a surplus production model is calculated with landings statistics and effort. The combining of monospecific models provides a multispecific diagnosis. The analysis is completed by the calculation of diversity indices of catches at the equilibrium level, expressed as a function of fishing effort. While considering the uncertainty around this first assessment, we show that Senegalese groundfish resource is globally full or over-exploited, according to the considered hypothesis. Under the pessimistic hypothesis, 9 among the 11 taxa of Pisces were clcarly over-exploited, with fishing effort some-times as much as twice or three times the effort of maximisation (fMSY). The situation is very critical for Arius sp., Epinephelus aeneus, Pagellus bellottii and Pomadasys sp. Using the multispecific approach, pessimistic diagnosis indicates a fishing effort nearly twice fMSY, with a Maximum Sustainable Yield at about 53 000 t for the 11 taxa together. Conversely, in 1991, octopus and cuttlefish appeared under-exploited. The diversity analysis shows that an increase in fishing effort would lead to a decrease in catch diversity. This result is interpreted as a sign of fragility in the exploited ecosystem.

A holistic perspective of the last 150 years of studies in ocean biology reveals two distinct scientific activities. The first involved scientists in many expeditions from the "Challenger" in the 1870s to the Indian Ocean Expedition in the 1960s. These studies ended with what Ray (1970) has called the "Marine Revolution". This was caused by the need for marine biologists to pay much more attention to practical problems of fisheries, pollution, undersea mining and in particular the biology of the 200-mile economic zone which was established by many maritime nations in the 1970s. The new biological science of the oceans moved from largely descriptive ecology into the field of dynamic processes, holistic models, ocean monitoring and networking of different national and international organizations. Further progress in the biological sciences is needed in these areas, particularly in determining the canying capacity of the seas and oceans and the ecological consequences of removing manne species of fish from the oceans as part of the annual industrial harvest. These problems are only likely to be solved by a consideration of the properties of entire ecosystems, which are often different, to that expected, from the sum of the determinate parts.

Using examples, we discuss some aspects of surplus production models. Although the precision of some parameter estimators may appear to be good, the strong asymmetry of confidence intervals and the large impact of the choice of a given formula on those estimators go against this feeling. Models are often formulated with inference to the biological background, but giving a biological meaning to (the estimates of) the parameters may be very dangerous. Fishing effort standardization does not lead necessarily to useful results for management. The fleet dynamics, per se, may not allow equilibrium States. In the context, of general frameworks, where the evaluation of the resource is not the unique objective, surplus production models may be however very flexible tools for fishery analysis, with low parameter requirements.

Pour un stock donné, les estimations de Prises Maximales à l'Équilibre (PME) obtenues avec des modèles globaux peuvent varier notablement. Dans le cas de la pêche d'albacore dans l'Atlantique Est, elles ont fortement augmenté depuis les années 1970. L'hypothèse présentée ici concerne un modèle global de dynamique de population se modifiant en fonction de l'accroissement de la surface de pêche. Cet accroissement se traduit par une diminution d'une quantité de biomasse jusqu'alors inexploitée. Selon les résultats obtenus à partir de la formulation adoptée, l'augmentation de mortalité subie par le stock serait due à l'augmentation de la surface exploitée plus qu'à celle de l'activité de pêche elle-même.

Les modèles globaux utilisés pour décrire la dynamique des populations marines exploitées supposent en général que la production de biomasse peut être décrite par une fonction de la seule biomasse exploitée. Si une certaine quantité de biomasse reste inaccessible à la pêcherie, cette hypothèse n'est pas vérifiée. Nous étudions dans cet article des modèles supposant l'existence d'une telle quantité.Lorsqu'au cours de l'histoire d'une pêcherie cette quantité de biomasse inaccessible a pu se modifier, de tels modèles peuvent être utiles. En utilisant des données d'environnement en Côte-d'Ivoire, cette approche est utilisée ici pour décrire la pêcherie du stock ivoiro-ghanéen de sardinelles rondes Sardinella aurita de 1966 à 1980. Les ajustements réalisés sont satisfaisants, mais ils ne permettentpas de choisir entre deux formulations proposées, qui conduisent à des à conclusions différentes en matière de gestion du stock.