The Extent of Great Crested Grebe Predation on Bleak in Lake Como, Italy

Abstract

Food intake (fish biomass), during the winters of 1997-98 and 1998-99, by Great Crested Grebes (Podiceps cristatus) on a 47 km2 area of the western basin of Lake Como, North Italy, was estimated by analyzing recognizable remains of the fish prey in the stomach contents of birds found dead. Using the size of otoliths and pharyngeal bones, we showed that grebes mainly ate the younger age-classes (2-yr. old and younger) of Bleak (Alburnus alburnus), 80% of all fish prey. Monthly rates of Bleak consumption by wintering grebes were estimated by multiplying daily food intake, extrapolated from the relationship between food biomass intake and water temperature in the different winter months, with the proportion of Bleak in the diet and with the number of wintering grebes present each month. Great Crested Grebes were most numerous in the study area in January-February and there were no significant differences in numbers between the two winters. Size of Bleak eaten increased from November to February and the biomass of Bleak consumed was highest at the end of winter, when water temperature was lowest. Overall in the western basin, grebes consumed between 640 kg (conservative estimate) and 1,000 kg (maximum estimate) of Bleak per winter, which corresponded to 10% to 20% of the annual commercial Bleak harvest in that area. We suggest that grebe predation on Bleak might increase the mortality rates in the younger age-classes of the fish, and thus contribute to a decrease in the stock, but that economic impact of grebe predation on gross annual income from the local fishery is negligible.