Abstract
Management of the marine environment and conservation of the species that live within it requires accurate information on where animals are, why they are there, and where else they could have been. The harbour seal Phoca vitulina is a widely distributed marine mammal, found in coastal waters of the northern Pacific and Atlantic Oceans, and including the Baltic and North Sea. Data from over 200 GPS tracked harbour seals were used to estimate their distribution at sea in relation to environmental conditions. The model results were supplemented with data from aerial surveys of seals hauled-out on land, to estimate expected density of seals at sea. Seal density was found to be substantially high near haul-out sites and areas of ~30 m water depth, with low mud content. Harbour seals venture further away from the haul-out sites during the winter months (Dec.–Feb.). This seasonal pattern is also supported by independent coastal observations from the Dutch west coast. In addition to their longer trips, seals spend substantially less time on land (~10%) during the winter compared to the spring and summer (>20%). This shift is driven by water temperature, changed prey redistribution, increased requirements of the seals or a combination of these factors. This study shows how the distribution at sea of a population of seals can be estimated by combining detailed individual movement data and data from population counts on land. The model can subsequently be used to study if residual variations in seal densities can be explained by human activities at sea. Furthermore, the model predictions can be used as input variables for individual-based models aimed at estimating the population-level impact of human activities on harbour seals.