Using movement data of Baltic grey seals to examine foraging-site fidelity: implications for seal–fishery conflict mitigation

Knowledge of the intensity of spatial overlap between aquatic top predators and fisheries is required to efficiently alleviate the negative effects of marine mammal–fishery interactions. We used satellite telemetry to study the movements and habitat use of Baltic grey seals Halichoerus grypus, with special focus on the degree of site fidelity to foraging and haul-out areas and spatiotemporal overlap with coastal fisheries. Most of the tracked seals (14/16 individuals) were ‘residents’, which remained within 120 ± 62 km (mean ± SD) of their capture sites during the open-water season, whereas 2 ‘transient’ seals occupied much larger areas, over 400 km from their capture sites. Residents used on average 4.3 ± 2.5 haul-out sites, indicating high haul-out site fidelity during the open-water season. Residents had active core areas (58 ± 35 km², 50% local nearest-neighbour convex hull [LoCoH], excluding haul-out locations) near river estuaries or at other shallow water areas, indicating foraging-site fidelity to these foraging grounds. They overlapped both spatially and temporally with trap-net fishing. The high site fidelity of grey seals indicates that foraging and haul-out areas should be taken into account in population management. Selective removal of seals overlapping with fishery could be a locally focused method to mitigate seal–fishery interactions. However, removal of individuals in foraging areas may also compromise the conservation needs of the population by targeting the same animals that are hauling out in seal sanctuaries.