MANAGING FISHERIES INVOLVING PREDATOR AND PREY SPECIES

Several management strategies for ecosystems with biological interacti on are discussed, including predator removal, predator-prey coexistenc e, prey exploitation, overexploitation, and introduction of sanctuarie s. Some case studies related to ecosystem management are briefly prese nted; these describe Lakes Victoria and Tanganyika, discarding from sh rimp trawl fisheries and the development in the North Sea that led to introduction of multispecies analysis. The concept of 'fishing down th e food web' is discussed and the average trophic levels at which the f isheries operate in different ecosystem types are estimated based on q uantified trophic flow models. On a global level, while on average fis heries operate around two trophic levels above the primary producers, still one third of the catch of the 70 major fish species caught in th e world is of piscivorous fish. Using exploitation-predation rate indi ces for different ecosystem types, the amount of finfish consumed glob ally by finfish is roughly estimated to be three times the catches of finfish. Finally some implications for the management of ecosystems ar e drawn up. It makes little difference if short-term prognoses are bas ed on single-species or multispecies considerations. Multispecies mode ls may, however, give the better long-term advice, and adaptive manage ment may facilitate the move towards such long-term goals.