Seabird bycatch in a Baltic coastal gillnet fishery is orders of magnitude larger than official reports

, ABSTRACT. Bycatch, or the incidental capture of non-target species in fisheries, has been identified as one of the major threats affecting seabird populations worldwide. In the Baltic Sea, a globally important area for wintering seabirds, bycatch in gillnets represents an important cause of human-induced mortality for seabird species whose populations have declined significantly in recent decades. Although countries are required by European law to report official bycatch data, a lack of data on bycatch in small-scale fisheries impedes an assessment of the contribution of bycatch to declines of seabird populations. This study presents data on the total seabird bycatch in the small-scale coastal fishery for an entire country, Lithuania, in the southeastern Baltic Sea, during the 2015–2020 winter period. An average of 19.3% of the total fishing effort in net-meter days (15.5% of fishing days) were observed each winter season, resulting in observations of 909 bycaught birds from 15 species. Two species composed two-thirds of the total bycatch, Long-tailed Duck ( Clangula hyemalis ; 42.1%) and Velvet Scoter ( Melanitta fusca ; 35.4%). Bycatch composition varied with depth, with the majority of bycatch occurring in nets set at depths ≤ 10 m. Adult males dominated the bycatch of benthivorous sea ducks, whereas adult females composed the majority of piscivorous birds caught. Low numbers of juveniles in the bycatch may indicate different wintering sites for young birds. We estimate that between 1500 and 3000 seabirds were bycaught annually in the Lithuanian small-scale coastal fishery in the 2015–2020 period. Because this number is orders of magnitude larger than the bycatch officially reported by the Lithuanian authorities (six birds), our study highlights deficiencies in the country’s current bycatch reporting. In contrast to official statistics based on inadequate data, the unintended capture of seabirds in gillnets remains high, despite financial investments to minimize the impact of fisheries on biodiversity.