The dominant species of piscivorous bird does not adversely affect fishery in the lagoons of the southern Baltic Sea

The impact of fish-eating birds on fishery has long been debated. It is therefore important that the arguments in this debate be scientifically based. The Great Cormorant Phalacrocorax carbo is a flagship example of a conflict species that has been well studied. In some areas, however, such as the southern Baltic estuarine lagoons, the Goosander Mergus merganser may be more abundant, exerting a potentially greater impact on fisheries. However, this aspect has not been well studied in this region, so this article is an attempt to fill this knowledge gap. Based on the digestive tract content of 23 Goosander drowned in gillnets in Poland, it was found that the most frequently consumed species was Ruffe Gymnocephalus cernua (70% of biomass), followed by Zander Sander lucioperca (13% of biomass), Perch Perca fluviatilis (9% of biomass) and Roach Rutilus rutilus (8% of biomass). Average Goosander numbers in the Vistula and Odra estuaries in 2011-2018 were 11,000 in winter, 4,500 in spring and 2,800 in autumn. These fish-eating ducks were found to consume 242 t of fish annually during this period, whereas at the same time fishermen caught an average of 4,400 t of fish. The species caught by both Goosander and fishermen were mainly Zander (32 t vs 189 t), Perch (21 t vs 668 t) and Roach (19 t vs 701 t). No negative impact of Goosander foraging in the winter preceding the fishery season was demonstrated for any of the above species (Zander R 2 = 0.022; Perch R 2 = 0.834; Roach R 2 = 0.881).