Haul-Out Site Use and Connectivity of Harbour Seals Between Management Units in Southern Scandinavia

Harbour seals (Phoca vitulina) have a broad distribution in coastal ecosystems across the northern hemisphere. In southern Scandinavia, a lack of spatially-resolved data on harbour seal populations poses a major challenge for developing ecologically informed management frameworks, particularly in Norway, where populations are regulated using county-level administrative boundaries. In this study, we use haul-out data from 26 harbour seals tagged with GPS phone tags during the post-moult period to provide the first assessment of connectivity and movement across management boundaries in the Skagerrak-Kattegat region of southern Scandinavia. Specifically, we examined the frequency and timing of haul-out events relative to management units and quantified spatial networks of connectivity across national and sub-national jurisdictions. We reveal a high degree of spatial connectivity in the region, with haul-outs occurring over a broad, integrated network along the Skagerrak-Kattegat coastline. Generally, harbour seals in the region had a high probability of performing cross-boundary haul-out events, with individuals repeatedly transitioning across distinct Norwegian management units, as well as transnationally between Norway, Sweden and Denmark. This study offers critical insights into harbour seal movement ecology in this data-limited region, whilst also addressing an important topic of applied management. We demonstrate that the current management units in the Norwegian Skagerrak may not adequately reflect the spatiotemporal scales of harbour seal movement. Importantly, these findings can complement forthcoming genetic data and support efforts to redefine management units in the area. More broadly, our study illustrates how telemetry-based assessments of spatial connectivity can provide a powerful tool to inform management frameworks for other wide-ranging marine species facing similar conservation challenges.