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Which characteristics define the prey species constituting the diet of a given predator? Answering this question would help predict a predator's diet and improve our understanding of how an ecosystem functions. The aim of this study was to test if the diet of common dolphins, Delphinus delphis, in the oceanic Bay of Biscay reflected prey availability or a selection shaped by prey energy densities (ED). To do this, the community of potential prey species, described both in terms of relative abundance and energy densities, was compared to the common dolphin diet in this area. This analysis of a predator's diet and its prey field revealed that the common dolphin selected its diet on the basis of prey energy densities (significant values of Chesson's index for ED > 5 kJ g − 1 ). High-energy prey were positively selected in the diet [e.g. Notoscopelus kroeyeri, ED = 7.9 kJ g − 1 , 9% of relative abundance in the environment (%Ne); 62% of relative abundance in the diet (%Nd)] and low-energy prey disregarded (Xenodermichthys copei, ED = 2.1 kJ g −1 , 20%Ne, 0%Nd). These results supported the hypothesis that common dolphins selected high energy density prey species to meet their energetically expensive life style and disregard prey organisms of poor energy content even when abundant in the environment.

Summary 1. The development of ecosystem approaches to environmental management implies the need to account for multiple pressures on ecosystems. Trends in multiple metrics that respond differently to changes in major environmental pressures need to be combined to evaluate the impacts of fishing and environmental changes on fish communities. 2. An exploited fish community is viewed as a three‐level food chain in which the two upper levels, or functional groups, are targeted by fishing fleets, while the lowest level is subject to environmental variation. Qualitative modelling is used to predict changes at the two upper levels, that is, top‐down vs. bottom‐up perturbations. Abundance and length metrics are calculated from survey data for 14 Mediterranean and East‐Atlantic groundfish shelf communities at both population and functional group levels. The joint likelihood of time trends in metrics is used to evaluate the evidence for different causes of changes. 3. A wide diversity of impacts is found to have equal evidence at the population level within each community. Consistency between the impacts identified and changes in pressures known from independent information is found at the functional group and community level. The results suggest that there is some compensation between species within functional groups. 4. Synthesis and applications. The method can be used to conduct an integrated assessment of community dynamics subject to multiple pressures. Joint trends in metrics provide evidence of which known pressures are having an impact on the community, and thus, which management actions should be taken to mitigate these changes.

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Marine populations are distributed heterogeneously in space and time because of the diversity of habitats and the requirements of species life cycles. Human exploitation of these resources also varies as a function of space, time and the type of fishing activities performed. These three factors determine fishing strategy at different levels of integration. The purpose of this study was to describe and analyse, with respect to different time scales, the relations between the modalities of resource exploitation and the biological or demographic characteristics of the species involved. These investigations relate to the more general task of acquiring the basic knowledge needed for spatialised management of fishing effort. A fleet of trawlers from La Rochelle, operating in the Bay of Biscay, was studied over a 15-year period (1979-1993), which led to the development of a reference resource exploitation scheme for these vessels. The degree of stability of this scheme over time was determined from landing profiles of the 18 most important species fished (94% of landings). An annual cycle for the species composition of landings was determined by multiple factor analysis. Two factors account for more than 55% of the inertia of the data. The first, of biological origin, is closely related to the breeding activity of species and associated migrations between the coast and the open sea. The second is spatial in nature, corresponding to the distribution and availability of resources according to a bathymetric gradient. This organisational scheme persisted over the first 13 years, but showed signs of change toward the end of the study period. Analysis of multi-year trends indicated four periods marked by sustained levels of landings per unit of effort for some species (decreasing for sole and wedge sole, or increasing for Norway lobster, striped red mullet, rays and the smallspotted catshark). These changes are attributable to restrictions on resource access imposed on the fleet (regulations and/or competition among the fishing gears for occupation of space), variations in the abundance of traditionally fished populations (hake, anglerfish) and changes in the professional behaviour of fishermen.