Death toll of North Sea porpoises revealed

More than 16,000 dead harbour porpoises, including 2600 on UK beaches, have been found on Europe’s coastlines around the North Sea since 1990.

This is according to a new international study led by Utrecht University in the Netherlands and involving the Scottish Marine Animal Stranding Scheme (SMASS), part of Scotland’s Rural College.

The study showed a significant rise in the annual number of standings in areas on the south of the North Sea since 2005, while numbers were relatively stable in northern regions.

The increase in the south corresponds with an increase in sightings of live animals in this region, but it is not entirely clear what has caused the consistent steep increase in strandings.

The study, the findings of which have been published in Biological Conservation, included harbour porpoise strandings from five countries over 28 years (1990-2018), with dedicated stranding schemes from Scotland, England, Belgium, the Netherlands, Germany and Denmark all contributing data.

It was commissioned by Rijkswaterstaat (Dutch Directorate-General for Public Works and Water Management) as part of its Offshore Wind Energy Ecological programme (WoZEP), a research programme addressing the knowledge gaps relating to the effects of offshore wind energy developments on the marine ecosystems of the North Sea.

Lonneke IJsseldijk from Utrecht University, who co-led the research, said: “The harbour porpoise is the most abundant species of cetacean in the North Sea. Most countries hold records and investigate strandings of the species at a national level, but harbour porpoises are a highly mobile species and move independently of these national borders. It therefore makes sense to examine these data at a scale that is ecologically relevant, and collate data from all individual countries for a population-level overview.”

The study examined seasonal and inter-annual variation in stranding numbers across the North Sea area. The seasonal pattern in stranding frequencies was different for different regions, yet was consistent throughout the 28 years covered by the study. Variation in stranding frequencies can be driven by a number of factors including variation in abundance, distribution and mortality of animals. To shed some light on the potential drivers for the different patterns found in the study, researchers looked at biological parameters of the stranded individuals.

Like all cetaceans, harbour porpoises are a protected species in European waters, but are notoriously difficult to monitor.

Mariel ten Doeschate from SMASS and principal investigator in the study said: “Harbour porpoises are one of the smaller and more elusive species of cetacean. Animals most commonly live alone or in small groups and spend very little time at the surface. This means that they are a particularly difficult species to monitor, and gathering information on even the most basic population metrics is very challenging.

“Surveys of live animals are being done to try to estimate abundance and distribution of the species, but these are often logistically restricted. Stranded animals, on the other hand, are being found all around the coastline and are reported year-round, and the schemes contributing data to this study have provided a systematic approach to strandings surveillance in their respective countries.

“Examination of stranded individuals additionally allows us to gather information on parameters like age and sex class, which are indicative of population structure and habitat use - vital data that is very difficult to obtain via surveys of live animals”

IJsseldijk said there was still a lot of uncertainty about the potential effects of offshore renewable energy developments on marine mammals and their habitats.

She added: “To plan and minimise negative impacts of these developments on harbour porpoises, it is essential that consideration is given to vulnerable population groups and spatiotemporal variation in population resilience. Our study has provided valuable insights into baseline variation in stranding rates and population structure, and has therefore been a valuable first step in increasing our understanding of harbour porpoise demographics in the North Sea.”

The research was a collaboration between the Faculty of Veterinary Medicine at Utrecht University the Netherlands, The Scottish Marine Animal Stranding Scheme, the UK Cetacean Strandings Investigation Programme, Department of Bioscience of Aarhus University, Institute for Terrestrial and Aquatic Wildlife Research of the University of Veterinary Medicine Hannover, Royal Belgian Institute of Natural Sciences, Naturalis Biodiversity Center, Cetacean Atlas of Denmark, the Globe Institute, the Natural History Museum of Denmark, and the Fisheries and Maritime Museum Denmark.

Posted by SRUC on 18/08/2020

Tags: Research, Marine
Categories: Research