The ecological setting of North Sea fisheries

Fisheries research has traditionally been largely concerned with direct interactions between fishing activ ities and individual fish stocks with a view of optimizing yields. This single species approach has been the basis for the common fisheries policy of the EEC aimed at managing fish stocks by means of catch quota. Although exploitation is undoubtedly a most important factor in the population dynamics of mdi vidual species, there are obvious natural links between different species which may affect thejr respective yields. Extensjve recent studies of interand mntraspecific predation have opened the gate to multispecies assessment, whjch allows partial evaluation of secondary effects of exploiting one species on the poten tja! yields of others. As shown by the pioneering efforts of Andersen & Ursjn (1977) to model the North Sea ecosystem, however, the situation is far more complex, because the dynamics of ind ividual species may be intertwjned through feed-back mechanisms between different groups of organisms jn the food chajn. This contribution is ajmed at a qualitative djscussjon of the varjous ecological links between exploited fish stocks and the bjotjc and abjotjc environment jo the North Sea and of the effect that exploitatjon may have had on different components of the ecosystem.