Habitat suitability and distribution of harbour and grey seals in the Baltic Sea region

The Baltic Sea region is impacted by pollution, noise, offshore construction, shipping, fisheries, eutrophication and climate change. Seals are important top predators here, but little is known about their at-sea distribution. Using satellite telemetry data from 90 harbour seals Phoca vitulina and 65 grey seals Halichoerus grypus together with haulout counts, we modelled habitat suitability and relative population densities across the region. We used generalized additive mixed models (GAMMs), incorporating a range of static and dynamic environmental variables. Habitat suitability of both species was positively related to proximity to haulouts, explaining 25-30% of the total deviance. Other important variables, explaining 2.5-6% of the deviance in habitat suitability, were increased wave exposure and colder sea surface temperature for harbour seals and wave exposure and higher sea bottom oxygen saturation for grey seals. Harbour seals showed a preference for more coastal habitats than grey seals. The analyses revealed an imbalance between habitat suitability and actual distribution, particularly for grey seals, where some areas with high suitability had low seal densities. This likely reflects the dramatic population fluctuations of seals in the area over recent centuries related to hunting and pollution. Overlap of suitable habitats between the species in the southern and western Baltic indicates interspecific competition that will likely increase as grey seals recolonise their previous range. Our study constitutes the first comprehensive analysis of seal distribution in the Baltic area and is a valuable tool for managers and ecologists assessing pressures and ecosystem interactions in an area where anthropogenic activities are increasing.