Study: Thousands of harbour porpoises have stranded around the North Sea since 1990

A recent study by researchers of the Utrecht University in the Netherlands has investigated the dynamics of harbour porpoise strandings across the North Sea. Since the species is highly mobile, small, and elusive, studying the animals and their demographics has been fairly difficult. Stranding data is therefore hoped to give a deeper insight into their life history and population structure, especially when analyzed across national borders.

Stranding data between 1990 and 2018 was contributed by dedicated schemes from Scotland, England, Belgium, the Netherlands, Germany and Denmark. Over the entire study frame, more than 16,000 dead harbour porpoises had been found along the coastlines. Seasonal patterns were different across the regions due to the abundance and distribution of the animals, yet they were consistent throughout the 28 years covered by the study. Importantly, a significant rise in death strandings was reported since 2005.

The study further allowed insight into importance areas. For example, a high density of neonatal strandings in the eastern North Sea might indicate its importance as calving site. Additionally, southern parts were found to be population sinks with a higher male dispersion.

Overall, the study is hoped to provide vital knowledge on potentially vulnerable populations and guide managers in establishing conservation measures.

The study area: the North Sea. The colours represent the six regions as assigned in this study, with: region A (in grey) comprised of the Northeast of Scotland from Thurso to St Fergus, including Orkney; region B (in red) from St Fergus, Scotland to Newcastle, England; region C (in dark-blue) from Newcastle, England to Great Yarmouth, England; region D (in light-blue) the rest of the English coast, the Belgian coastline, and the Delta area of the Netherlands; region E (in yellow) the mainland and Wadden area of the Netherlands; and region F (in pink) the North Sea coastlines of Schleswig-Holstein (Germany) and Denmark.

The full study can be found here: IJsseldijk, L. L., ten Doeschate, M. T., Brownlow, A., Davison, N. J., Deaville, R., Galatius, A., ... & Kinze, C. C. (2020). Spatiotemporal mortality and demographic trends in a small cetacean: Strandings to inform conservation management. Biological Conservation, 249, 108733. DOI: https://doi.org/ 10.1016/j.biocon.2020.108733