Waterfowl Populations of Lake Mývatn and Recent Changes in Numbers and Food Habits

Waterfowl populations of Lake MSvatn, Iceland, were estimated in 1975-76. Fifteen duck species bred. The most abundant breeding species were Aythya fuligula (about 3000 oCro), A. manila (1700), and Bucephala islandica (700). These species produced about 0.7 fledged young/female in 1975 and about 1.9 in 1976. The difference in production was associated with weather. Males of Anas strepera, A. Penelope, A. fuligula, A. manila, B. islandica, and Mergus serrator from large areas in North Iceland moulted at Myvatn as did about 5-600 non-breeding Cygnus cygnus. Comparison of moulting numbers with spring censuses in the presumed areas of origin of the moulters suggests that spring censuses were highly accurate for diving ducks but relatively inefficient for Anas spp. Wintering waterfowl were mainly B. islandica (over 1000) and smaller numbers of Mergus merganser, Anas platyrhynchos and C. cygnus. Diving ducks probably removed about 1.5-2 g dry wt m-2 yr' invertebrate food from Myvatn and other lentic waters in the basin and about 10-15 g in the upper Laxd river. Densities of breeding ducks were similar at Myvatn (about 125 pairs km-2 water) to other shallow lakes in the region; they were lower than on the Laxd (223) but much higher than in West Iceland (27). Breeding populations of A. maria, Melanitta nigra and Clangula hyemalis have decreased in recent decades. Their decline is associated with changes in food availability. Eurycercus lamellatus (Cladocera) was the preferred food of the ducklings but it declined between 1960 and 1968. The B. islandica population remained unchanged from 1960 to 1976; A. fuligula and M. serrator may have increased. The food of these three species did not change. It is suggested that the diverse waterfowl populations are maintained by long-term temporal heterogeneity, in addition to short-term or spatial heterogeneity.