More than 16,000 dead harbour porpoises have been found on coastlines around the North Sea over almost three decades, according to a new study.
The research, led by Utrecht University in the Netherlands, also shows a significant rise in the annual number in southern areas since 2005.
It included harbour porpoise strandings from five countries between 1990 and 2018, with dedicated schemes from Scotland, England, Belgium, the Netherlands, Germany and Denmark all contributing data.
Lonneke IJsseldijk, 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.
About 2,600 were on UK beaches.
Seasonal patterns were different across the regions, yet they were 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.
The study also involved the Scottish Marine Animal Stranding Scheme (SMASS), part of Scotland’s Rural College.
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.”
She added: “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”