International study reveals differences in the health of porpoises in the North Sea
Porpoises that strand in the Netherlands have, on average, a lower BMI than their counterparts in Scotland and elsewhere. This is according to an international study initiated by PhD student Eva Schotanus and carried out in collaboration with researchers from Scotland, England, France, Belgium and Denmark.
As part of the study, the researchers analysed the Body Mass Index (BMI) of 1,700 porpoises that were stranded or found as bycatch in the North Sea between 1990 and 2023. This is the first time that data from multiple countries has been compiled on this scale. Previous studies on body condition were limited to individual regions.
The researchers found differences in BMI between regions, seasons, age groups, sex and cause of death. Young porpoises had a lower BMI, particularly in summer, whilst the lowest values in adult animals were found in late summer and early autumn. It also emerged that animals that died from an acute cause, such as bycatch or predation, had a higher average BMI than animals that died following illness or other long-term health problems.
Regional differences
The study shows that the physical condition of porpoises is not the same throughout the North Sea. In particular, the lower BMI of porpoises from the southern North Sea suggests that animals in different parts of the North Sea live under different conditions. The researchers therefore conclude that regional differences in physical condition should be better taken into account in research into the effects of human activities at sea.
This is the first time we have been able to compare the condition of porpoises across the entire North Sea on such a large scale. The results clearly show that there are significant regional differences, and this is essential for a better understanding of how these animals live and respond to their environment. Further research is needed to explain why porpoises that strand along the Dutch coast in particular have a relatively low BMI compared to their counterparts in other areas of the North Sea.
Basis for conservation
The results provide reference values for the BMI of porpoises in the North Sea. These data can be used in future models to predict how disturbance—for example, from shipping, offshore activities or other human interventions—affects porpoise populations. Animals with a lower BMI have fewer energy reserves and are therefore likely to be more vulnerable to disturbance.
According to the researchers, the results underscore the importance of international studies and tailored approaches to the management and protection of porpoises. By taking into account health differences between regions and seasons, policymakers can better assess where and when human activities are likely to have the greatest impact on these protected marine mammals.
This research was made possible thanks to close collaboration between various stranding networks, universities and organisations: Museum Vest, Observatoire Pelagis, part of La Rochelle University, the Royal Belgian Institute of Natural Sciences (RBINS), the Scottish Marine Animal Stranding Scheme (SMASS), part of the University of Glasgow, the UK Cetacean Strandings Investigation Programme (CSIP), part of the Zoological Society of London (ZSL), the University of Liège, the University of Copenhagen, and Wageningen Marine Research.
Dutch stranding research is funded by the Ministry of Agriculture, Fisheries, Food Security and Nature. Visit the Stranding Research website or follow our researchers on Instagram for more information.