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

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 fisheries. Due to frequent changes in fisheries regulations and conditions, both the conservation of the species and the fishers are impacted. 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 utilization. The study utilizes 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 5,343 individuals (95% CI: 4,622-6,178) to 2,604 individuals (95% CI: 2,273-2,983). 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 also outside of the Baltic Sea, such as Bluefin tuna (Thunnus thynnus) in Skagerrak, and endangered salmon stocks elsewhere, such as in Lake Vanern. The results of this study suggests that regulations and changes in the Baltic Sea salmon trolling fishery may have broader impacts on seemingly unrelated species and ecosystems.