Behavioral Responses of Wintering Great Crested Grebes to Dissuasion Experiments: Implications for Management

Great Crested Grebes (Podiceps cristatus) wintering on prealpine lakes of north Italy are considered to cause economic damage to fish, reducing the stock of native Bleak (Alburnus alburnus). We experimentally investigated the effectiveness of three deterring actions (gas-cannon, crackers and shooting) in four study sites at Lake Como, by comparing the time-budget and space-use of wintering grebes in absence and presence of disturbance. All dissuasion methods affected grebe behavior and position on the lake, but some resulted more effective than others. The gas-cannon detonations had little effect, but, crackers and shooting caused significant changes in grebe behavior. Grebes spent less time feeding, resting and preening, but more time moving/swimming during dissuasion than in the control situation. Moreover, birds also moved away from the areas with bleak shoals, resulting in a significant decrease in grebe numbers with either dissuasion methods. Although dissuasion reduced feeding of grebes in areas with shoals of Bleak (Alburnus alburnus), and probably reduced grebe predation on immature fish at localized areas, it involves intensive effort, in terms of man-days