For Greater Scaup Aythya marila, classified as ‘Vulnerable’ on the European Red List of Birds, the south-western Baltic Sea is one of the most important wintering sites in Europe. In this area, a large concentration of gillnet fishery temporally overlaps periods of the most abundant occurrence of foraging diving birds. The aim of the article is to show how bycatch can impact the population of a diving duck. To assess this, we calculate the Potential Biological Removal (PBR) for the studied Greater Scaup population and we model the population change according to age-structured matrix models. Summing all the available recent figures on Greater Scaup bycatch in north-west Europe yields an estimated mean annual total of 3,991 individuals (2% of the flyway population). For a baseline stable population, an age-structured matrix model indicates that at this bycatch level the Greater Scaup population that winters in north-west Europe will decrease by 36% over the next 30 years, qualifying the status of the population as ‘Vulnerable’ according to IUCN criteria. As this population also experiences decline prior to bycatch, this decrease will be 57%, which qualifies the status as ‘Endangered’. PBR as an indicator of population vitality does not work in our case because the PBR-informed allowable bycatch values have a significantly negative impact on the population. Our results indicate unambiguously that fishery bycatch is among the most important threats responsible for the Greater Scaup’s decline. While recent data suggest that some improvement has taken place in the species’ status over the last 10 years, measures to protect Greater Scaup from bycatch are required. The solution should involve the prohibition of gillnet fishing in selected key sites and the use of mitigation techniques in other areas.