DIEL AND SEASONAL PATTERNS OF FOOD-INTAKE AND PREY SELECTION BY COREGONUS SP. IN RE-OLIGOTROPHICATED LAKE LUCERNE, SWITZERLAND

Feeding intensity by whitefish Coregonus sp., in oligotrophic Lake Luc erne in Switzerland was high during dusk when the bulk of potential pr ey items were in the depth zone occupied by the fish. Diet composition was fairly uniform throughout the day but changed substantially over the seasons. The fish fed opportunistically; differences between seaso ns reflected changes in prey availability. During the intensive feedin g and growing period (May-September), fish were found in the upper 20 m of the lake feeding primarily on cladocerans. Large and non-evasive species, Daphnia spp. and Bythotrephes longimanus, were the most numer ous and frequent organisms in the diet during the major part of the gr owing season. Smaller (Bosmina spp.) as well as evasive species (cyclo poid copepods) were consumed in large numbers when larger, non-evasive species were rare in the lake. The fish showed strong preference for the least abundant crustacean, B. longimanus, while the most abundant crustaceans, calanoid copepods, were rare in the diet. The fish not on ly selected particular species but, within each species, selected the larger individuals. Diel vertical migration of the prey items in this lake could be, at least in part, attributed to fish predation pressure . The observed selectivity patterns shown by the fish are explained in terms of prey visibility, escape ability, the overlap in distribution of predators and prey in time and space, the profitability of the pre y and the present trophic state of the lake. (C) 1998 The Fisheries So ciety of the British Isles.