Spatial capture–recapture reveals resource use and declining skipper numbers in Baltic Sea salmon trolling fisheries

Abstract Many populations of Atlantic salmon ( Salmo salar ) native to the Baltic Sea are endangered. A significant group targeting salmon in this region is the recreational trolling fishery. Due to frequent changes in fisheries regulations and conditions, both salmon conservation and fishers are affected. This study aims to enhance our understanding of the spatial extent and population dynamics of salmon trolling boat skippers in the Baltic Sea by estimating their population size and resource use. The study uses participation lists of skippers from recreational fishing tournaments and automatic identification system (AIS) data to track their movements. These lists are formatted as encounter histories for spatial capture–recapture models, while AIS data are integrated as telemetry data to estimate resource selection. The results reveal a 51% decrease in the number of skippers from two time periods: years 2014–2020 and 2021–2023, with the count dropping from 5343 individuals (95% CI = 4622–6178) to 2604 individuals (95% CI = 2273–2983). The decline may be attributed in part to heavy regulations imposed on recreational salmon fisheries in 2022. Furthermore, the resource selection analysis indicates that these skippers target various species outside the Baltic Sea, such as Atlantic Bluefin tuna ( Thunnus thynnus ) in Skagerrak, and endangered Atlantic salmon stocks elsewhere, such as in Lake Vänern. The results of this study suggest that regulations and changes in the Baltic Sea salmon trolling fishery may have broader impacts on seemingly unrelated species and ecosystems.