Abstract
Dam lakes are often utilized by wintering ducks in Japan. Compared with natural lakes, most ducks which utilize dam lakes are dabbling ducks, and diving ducks such as pochards, Aythya ferina (that consume aquatic plants or benthos) are relatively rare. It is suggested that this is due to the feeding habits of Aythya ducks, which dive and forage in shallow waters, as most dam lakes lack shallow waters suitable for the feeding of Aythya ducks. However, there are a few dam lakes where many pochards winter in the Tohoku district of Japan. These dam lakes are constructed on river terraces that contain relatively large areas of shallow water. In the present study, we investigated the relationship between shallow water area and pochards density in 9 dam lakes in the Tohoku district. We found that the density of pochards was positively correlated with the shallow water (<5m) area and variations in density were positively correlated with variations in the shallow water area. In contrast, we did not find such correlations in dabbling ducks; mallards Anas platyrhynchos, spotbill ducks A. poecilorhyncha, and teals A. crecca. These findings indicate that the density of pochards in dam lakes is strongly affected by the lakes’ shallow water area and this is due to the feeding habits of pochards.
