Population shifts in diving seabirds in the southern Baltic: a 35-year analysis

Long-term assessments are essential for understanding whether recent local changes in seabird abundance reflect genuine population recovery or redistribution of wintering birds within a persistently altered system. We analyzed wintering diving seabirds in the Polish Maritime Areas (PMA) of the southern Baltic Sea by combining historical baseline data from 1990 to 1993 with national survey data from 2005 and standardized monitoring from 2011 to 2025. Assemblage-level analyses covered 21 common diving species, including 9 benthic-feeding and 12 fish-feeding species. Species-level analyses focused on six representative species: Long-Tailed Duck ( Clangula hyemalis ), Velvet Scoter ( Melanitta fusca ), Common Scoter ( Melanitta nigra ), Greater Scaup ( Aythya marila ), Great Cormorant ( Phalacrocorax carbo ), and Black Guillemot ( Cepphus grylle ). We assessed long-term changes in abundance, compared patterns between feeding groups, examined species-specific trajectories, and evaluated the changing importance of the study area for biogeographic populations. The abundance of the wintering diving seabird assemblage was approximately 40% lower between 2020 and 2025 than from the 1990 to 1993 historical baseline, with substantial losses in both feeding groups. However, the continuous monitoring series from 2011 to 2025 indicated stabilization or moderate increases in several species and in multi-species indicators, showing that medium-term improvements were superimposed on a much deeper long-term reduction in abundance. Species trajectories were strongly heterogeneous: Long-Tailed Duck and Velvet Scoter remained far below historical baselines, whereas Common Scoter and Great Cormorant increased. At the same time, the proportional importance of the study area increased for several northern-breeding benthic feeders, particularly Long-Tailed Duck and Velvet Scoter, despite declining absolute numbers.