Spatiotemporal and demographic variability in body condition of North Sea harbour porpoises

Bioenergetic models play an important role in assessing effects of anthropogenic disturbance on a marine mammal population. However, model application is hindered by knowledge gaps, including population composition, demographics and body condition. The latter can be used as a health and fitness indicator. Body condition, measured by body mass index (BMI), was analysed for 1700 stranded or bycaught harbour porpoises ( Phocoena phocoena ) from five countries around the North Sea between 1990 and 2023. Variation in body condition was examined across regions, seasons, age classes, sexes, and mortality categories. Mortality categories, based on necropsies were: ‘acute’ (including bycatch, trauma and predation), ‘non-acute’ (including infectious diseases or other prolonged processes), and ‘other’ (including live stranding and unknown). All regions experienced seasonal variation in body condition. Juvenile harbour porpoises had the lowest BMI in summer, especially in the southern regions. Adults showed higher regional variability, with the lowest BMI occurring from late summer to early autumn. BMI also differed across mortality categories: individuals classified as ‘acute’ had significantly higher BMI than those in the ‘non-acute’ and ‘other’ categories. For adults, BMI varied by sex, with females in mortality categories ‘non-acute’ and ‘other’ showing lower values than males. These results provide reference values for harbour porpoise body condition in the North Sea, indicating spatiotemporal heterogeneity in body condition. This variability should be considered when assessing the effect of anthropogenic disturbance and informing marine mammal management and conservation in the Anthropocene.