Impact of Great Crested Grebes (Podiceps cristatus L.) on fish ponds

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