IMPACT OF GREAT CRESTED GREBES (PODICEPS-CRISTATUS L) ON FISH PONDS

Because Great Crested Grebes (Podiceps cristatus L.) mainly feed on fi sh, fish-farmers sometimes claim that economic losses are caused by gr ebes (EIFAC, 1988). Such claims were not based on scientific research. Therefore, a study was carried out to measure the impact of Great Cre sted Grebes on a fish-farm, managed by the Organization for the Improv ement of Inland Fisheries (O.V.B.) in Valkenswaard (The Netherlands) d uring the period 1986-1988. The O.V.B. provided fish data for each pon d including fish density and fresh fish weight for each fish species. The number of grebes was counted at least once a week. Activity patter ns, including nocturnal activity, were determined both for the period with and without young. No nocturnal foraging activity was observed. F urthermore, fish biomass consumed per 1000 s foraging time by adult bi rds and chicks was calculated. In some fish-ponds grebes ate coarse fi sh, and chicks especially were fed these fish. The average daily fish consumption was 222 g for adults and 108 g for juveniles. Based on ava ilable fish data, it can be concluded that grebes consumed fewer fish than the number expected to die, and only marginally influenced the fi sh populations in the aquaculture ponds studied.