Navigating the Waves of Conservation: Spatiotemporal Patterns in Harbour Porpoise Strandings in Swedish Waters

Abstract: Harbour porpoises (Phocoena phocoena) are the only cetacean residents found year-round in Swedish waters. Since in situ monitoring of cetaceans can be difficult, invasive and often costly, strandings can be used as a cost-effective alternative to continuously collect data on these elusive animals. In this study, spatiotemporal patterns, and their possible underlying causes, of harbour porpoise stranding reports in Swedish waters are investigated over the ten-year period of 2014–2023. When making spatial comparisons, for management purposes, the ten-year period is divided into two, 2014–2018 and 2019–2023. Data on 854 stranded harbour porpoises were analysed from the coasts of the Skagerrak, Kattegat, and Baltic Seas. Both significant spatial and temporal patterns could be identified, with strandings peaking in July to September and with hotspots occurring along most of the Swedish west coast, with the most frequent hotspots located around Öresund and especially the area around Kullen peninsula. The spatial patterns of strandings found in this study reflect data on porpoise abundance, prey abundance, and gillnet fisheries effort. The latter is known to be one of the primary causes of porpoise mortality. Furthermore, the coverage of the Swedish stranding network is analysed. While coverage of the stranding network overall has increased over the period, some areas still lack sufficient coverage, including the coast of Falkenberg, southern Gothenburg, northern Halmstad and certain areas around Lommabukten, north of Malmö. With this, we conclude that harbour porpoise strandings in Sweden show distinct spatiotemporal patterns that can be used as baselines for management and monitoring of these small cetaceans.