At-sea distribution of seals on the Northwest European Shelf: Towards transboundary conservation and management

Quantifying and mitigating transboundary effects of anthropogenic activity is a key challenge in environmental management, particularly for wide-ranging species such as large predators, fish and migratory birds, relying on habitats across multiple national jurisdictions. This challenge is especially complex in marine ecosystems, where the movement of species and impacts across borders is largely unobserved. Central-place foragers, such as pinnipeds and seabirds, exemplify this complexity: abundance is typically assessed on local (regional or national) scales on land, yet at-sea movements and drivers of abundance occur on broader transboundary scales. Resolving this mismatch is critical to effective conservation, especially in areas such as the Northwest European Shelf (NWES), which features globally important predator populations (including two pinniped species) alongside growing anthropogenic pressures and a mosaic of national maritime borders. We model an unprecedented GPS dataset from 236 grey (Halichoerus grypus) and 606 harbour seals (Phoca vitulina) tracked in waters of seven countries across the NWES (United Kingdom, Ireland, France, Belgium, Netherlands, Germany and Denmark). Using regional habitat association models, we generate at-sea distribution estimates for both species at 5 km resolution, scaled to haulout counts, producing country-specific and NWES-wide density maps. Analysis of the extent to which seals making foraging trips from one country occupy the Exclusive Economic Zones (EEZs) of other countries revealed substantial transboundary overlap, particularly for grey seals, and harbour seals in the southern North Sea. A case study apportioning grey seal density within three adjacent offshore marine protected areas in different EEZs revealed that, where total density in a given area is required, overlooking transboundary distribution can underrepresent numbers by an order of magnitude. Synthesis and applications. This study provides the first comprehensive, regionally scalable distribution estimates for pinnipeds across the NWES and its constituent countries. The modelling framework is adaptable to other central-place and migratory species, supporting transboundary biodiversity assessments and international conservation policy. We discuss common limitations and misconceptions of species distribution estimates, highlight priorities for future work and underscore the need for transboundary efforts to manage wide-ranging species, providing a foundation for future ecological modelling and decision-making across shared ecosystems.