Fish in the diet of the Cormorant and the Yellow-legged Gull breeding near fish ponds [upper Vistula river valley, Southern Poland] - preliminary study

Examination of fish in the diet of Cormorants (pursuit divers) and Yellow-legged gulls (surface plungers) in colonies in an area adjacent to fish-ponds was carried out in 2000-2003. The examined colony of Cormorants consisted of 41-45 nests and that of the Yellow-legged Gulls – 41-90 nests during the study. The diet of Yellow-legged Gulls in freshwater habitats is not well known. The prey of Cormorants consisted of only seven species of fish. Carp was the most numerous prey at 69% of consumed fish. Fourteen species of fish were recorded in the food of the Yellow-legged Gulls with the domination of carp (38%) and roach (34%). Rheophilous fish made up 5-19% of food by number. Both species caught large fish, but generally Cormorants caught on average larger fish. The same pattern was confirmed when carp was compared. The mean prey size of the Cormorants was 20.6 cm in total length (range 7-41 cm) and that of Yellow-legged Gulls 18.5 cm (range 9-47 cm). Fish are the principal food item for both Cormorants and Yellow-legged Gulls in the studied sites in southern Poland.