Living on a knife-edge: Unravelling harbour porpoise health through multidisciplinary and cross-border approaches

Harbour porpoises are one of the smallest whales, and a warm-blooded mammal fully adapted to live in the ocean. That does not come without its challenges, particularly with regard to energetics. Porpoises need up to 10% of their own body weight in prey per day. From a metabolic point of view, this gives them little 'leeway': they live on a knife-edge. In the North Sea, harbour porpoises are abundant, but this region is also one of the busiest marine areas in the world. Here, many human activities, often simultaneously, take place. Persistent, emerging and cumulative anthropogenic activities, threaten the conservation of harbour porpoises. A wide social, scientific and political interest into the health and well-being of harbour porpoises had made them the focus species in many environmental monitoring and research programs. However, the majority of these programs have been conducted at the national level, whereas cross-border and multidisciplinary approaches would provide a more accurate assessment of population and ecosystem health. In this thesis, I aimed to advance the development of methods and measurement tools that aid in the determination of the most relevant natural and human-related threats to harbour porpoises. This was done through combining multiple disciplines, including biology, ecology, toxicology, epidemiology and pathology. Most studies included samples or data collected from stranded individuals across the North Sea region. This provides a general understanding of porpoise biology, demographics and ecology, and also gives insight into the health of these animals and threats to individuals and populations.