Competition between freshwater fish and goldeneyes Bucephala clangula (L.) for common prey.

Results of a descriptive field study showed that goldeneyes preferred lakes without fish to those with fish. Field experiments were carried out in order to investigate whether or not this preference could be ascribed to a higher food supply in the lakes devoid of fish, as goldeneyes and some freshwater fish (perch and roach in the studied lakes) to some extent feed on the same kinds of prey. Changes in the abundance of prey common to fish and goldeneyes and the use by goldeneyes of lakes after experimentally changed predation pressure from fish were studied. Some aquatic insect groups, Cloeon larvae, Odonata larvae, water bugs, dytiscids, and Chaoborus larvae, proved sensitive to predation from fish. Goldeneyes increased their use of an experimental lake after fish were removed, and they used this lake more than an adjacent control lake. It is suggested that fish are able to reduce the availability of foods common to the goldeneyes to such an extent that the selection by the ducks of feeding localities is affected, and that exploitation competition between freshwater fish and goldeneyes may occur.