In the cold waters of the North Sea, a harbor porpoise living near the Dutch coast faces a different set of challenges than one swimming off the coast of Scotland — and now, for the first time, researchers have quantified just how different those circumstances are. An international study led by Utrecht University, in collaboration with the Scottish Marine Animal Stranding Scheme at the University of Glasgow, analyzed the body mass index of 1,700 porpoises that were stranded or found as bycatch across the North Sea between 1990 and 2023. The findings, published in Global Ecology and Conservation, reveal that the physical condition of these small cetaceans varies significantly by region, season, and age — knowledge that could reshape how we protect them.
Previous research into porpoise health was limited to individual countries or short time periods. This study, spanning 33 years and multiple nations, is the first to compile data at such scale. The researchers discovered that porpoises washing ashore in the Netherlands tend to have a lower average BMI than those found in Scotland and elsewhere. Young porpoises showed particularly low BMI readings in summer, while adult porpoises hit their poorest condition in late summer and early autumn. Perhaps surprisingly, animals that died from acute causes like bycatch or predation had a higher average BMI than those that succumbed to illness or long-term health problems — a reflection of the energy reserves an otherwise healthy animal carries before a sudden end.
The implications stretch far beyond the data itself. The study establishes reference values for porpoise BMI across the North Sea, creating a baseline that scientists can use in models predicting how human activities — shipping, offshore construction, fishing — affect porpoise populations. Animals with lower BMI have fewer energy reserves and are more vulnerable to additional stress. Understanding where and when porpoises are in poorer condition could guide policymakers toward smarter, more targeted protections.
Mariel Ten Doeschate, a research associate at the University of Glasgow's School of Biodiversity, One Health and Veterinary Medicine, emphasized that effective conservation requires looking at the ocean as animals experience it, rather than along the national borders humans have drawn. "Because body condition is closely linked to an animal's health and its ability to cope with additional stress, it provides a powerful indicator for identifying where and when porpoises may be most vulnerable," she said. The research was built on earlier North Sea-wide studies of stranding patterns but takes the next critical step: examining the health of individual animals.
Why exactly Dutch porpoises show lower BMI remains an open question — and one researchers are eager to answer. But the study already demonstrates the value of cross-border collaboration. By accounting for regional and seasonal health differences, nations sharing the North Sea can work together to time and place human activities where they'll do the least harm to these protected marine mammals.