In this article, we present the population trends of waterfowl breeding at Aasla (60°17’N/21°56’E), an island in the Archipelago Sea, southern Finland, in 1975–2015. A total of 49 648 breeding pairs were counted during the four-decade period. The peak in breeding pair numbers was recorded in the 1990s, mainly due to an increase in the Eider population. The average number of breeding waterfowl pairs was very even during the three Finnish bird atlas periods (1974–79, 1986– 89, 2006–10). However, population sizes of many of the studied species changed considerably during these periods. Twenty-two waterbird species have been surveyed and their trends recorded. The most abundant species during the 40-year survey period was the common eider Somateria mollissima, followed by the common goldeneye Bucephala clangula, mallard Anas platyrhynchos, goosander Mergus merganser and the great crested grebe Podiceps cristatus. While the total pair number (all species pooled together) re
mained constant, 13 species showed a negative trend (10 of them significant) and nine a positive trend (seven of them significant). As a general rule, the smaller species have decreased and the larger ones (species the size of or larger than the eider or barnacle goose Branta leucopsis) have increased during the study period, with the Gadwall Mareca strepera and the red-necked grebe Podiceps grisegena being the exceptions. The population increases were at least partly due to eutrophication, a period of mild winters and decreased hunting pressure, at least locally. The subsequent decline was probably due to over-eutrophication of the sea and an increase of predators (white-tailed eagle Haliaetus albicilla and the European pine marten Martes martes). Other factors at the local, but also the large scale, may also have affected the local waterbird breeding population: e.g. changes in local land use by cessation of cattle grazing and climate-driven changes and hunting pressure at the flyway scale. During the last 15 years, the status of six of the studied species changed from Least Concern (LC) to Endangered, and three from LC to Vulnerable (VU) in the Finnish Red List. The population trends at Aasla were in general agreement with these national trends, the most notable exception being the goosander, whose population has been stable during the entire study period. The Eider was also more abundant than at the beginning of our study period, but its decrease since the 1990s justifies concern.