Spatiotemporal patterns in harbour porpoise (Phocoena phocoena) bycatch in the Swedish gillnet fishery

Incidental capture (bycatch) in fisheries poses a significant threat to many marine mammals, including cetaceans. The harbour porpoise ( Phocoena phocoena ) is a small cetacean that ranges throughout temperate waters in the northern hemisphere which is specifically threatened by passive gear, such as set gillnets. Here, we investigate spatiotemporal patterns in bycatch rates (number of bycaught harbour porpoises bycaught per net-kilometre-day) in the Swedish gillnet fishery, and for the first time we assess harbour porpoise bycatch along the Swedish south coast. Despite that harbour porpoise population abundance is presumed to be higher on the Swedish west coast than along the Swedish south coast, bycatch rates were higher along the southern than the western coast. Although the distribution area of the critically endangered Baltic proper subpopulation includes the southern Baltic Sea, recorded bycatch events in this area occur during the second half of the year when these individuals are expected to have moved further north in the Baltic Sea. Hence, the porpoises bycaught along the Swedish south coast are more likely belonging to the endangered Belt Sea subpopulation, rather than the critically endangered Baltic Sea subpopulation. Our estimates also indicate a declining trend in the number of bycaught individuals, likely reflecting reduced gillnet fishing effort over time, as annual variation in bycatch rate could not be estimated due to data limitations. Overall, our results enhance the understanding of spatiotemporal patterns in harbour porpoise bycatch in the transition zone between the North Sea and the Baltic Sea, providing novel knowledge that may support management actions for this highly sensitive species.