Abstract
Based on waterfowl censuses made on 17-18 December 1986 and 9-10 January 1987 on a highly eutrophic lake, Lake Teganuma, Chiba Prefecture, the patterns of distribution of the waterfowl were analyzed in relation to feeding habits and local water pollution. Thirteen species were recorded, with mean daily numbers totalling 1, 988±378 (SD) individuals. Spot-billed Duck Anas poecilorhyncha was most abundant representing 32.3% of total numbers, followed by Northern Pintail Anas acuta 17.6%, Mallard Anas platyrhynchos 16.2% and Common Teal Anas crecca 16.0%. More than 90% of these ducks used the lake as a diurnal resting area. However, although low, Northern Shoveler Anas clypeata, 10.8%, Smew Mergus albellus, 2.5% and Gadwall Anas strepera, 3.9%, fed during the day on food available in Teganuma; 40.7% of Anas clypeata foraged in the shallows where zooplankton, Copepoda Cyclops vicinus and Rotifera Brachionus calyciflorus and Filinia longiseta, occurred at high densities. 40.7% of Mergus albellus on the other hand fed in the central part of the lake, probably on a small fish ("motsugo") Pseudorasbora parva and the shrimp Macrobrachium nipponense, which survived in the polluted water, though they are low in biomass. Anas strepera was found to aggregate at the mouth of the Otsugawa river where there was an inflow of domestic sewage. 41.8% of this species was found feeding on sludge deposits.
