Food habits of great cormorant Phalacrocorax carbo hanedae at Lake Biwa, Japan, with special reference to ayu Plecoglossus altivelis altivelis

Abstract

The recent increase in numbers of individuals of a piscivorous bird, the great cormorant Phalacrocorax carbo hanedae, around Lake Biwa has affected the commercial catch of ayu Plecoglossus altivelis altivelis. The food habits of the bird were studied based on 66 stomachs in 1998 and 2001–2002, and a seasonal variation in food contents was observed. The proportion of ayu in stomachs increased after April, and became most dominant during July and August (0.62 in proportion of weight, 0.85 in frequency of occurrence). In contrast, ayu was not found in the stomachs after late October. The increase of ayu in individuals and density in the surface layer of the pelagic zone of the lake during spring was considered as a trigger for the change of the food habits of the birds. The feeding impact of great cormorant on ayu in Lake Biwa was estimated to be approximately 46 t during April to June, and 310 t in July and August. During the main period for the commercial ayu catch (November to the following June), the estimated mass consumed by the bird is much smaller in comparison with the commercial catch.