Causes of death of harbor porpoises (Phocoena phocoena) stranded on the on the continental coastline of the southern North Sea (Belgium, France, and Dutch coasts) between 1990 and 2007

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The Marine Animals Research & Intervention Network investigated the causes of death of marine mammals stranded on the continental coastline of the southern North Sea (Belgium and France, and selected cases of the Dutch coast). Since the end of the 1990s, a significant rise of harbour porpoise (Phocoena phocoena) strandings occurred in the area, annual stranding average being less than 50 porpoises between 1990 and 1998, and more than 260 between 1999 and 2007. The aim of the study is to present the main lesions and causes of death of 615 porpoises found dead and necropsied, and to find a potential explanation for that dramastic rise. Most frequent observations included net marks on the skin, sub-cutaneous and muscular bruises, emaciation, pulmonary and gastric parasitism, acute pneumonia, and pulmonary congestion and edema. Two causes of death were significant: by-catch in fishing nets and infectious diseases. By-catch was mainly observed in March and April while infectious diseases, mainly responsible for acute pneumonia secondary to parasites infestation, occurred throughout the year. Based on the necropsy, between 1990 and 1998, 20% of stranded porpoises were by-caught and 40% between 1999 and 2007. Only 60% of by-caught animals were healthy, others having evidence of emaciation, pneumonia and parasitosis. There are two possible reasons to explain the recent rise of stranding in the southern North Sea, one could be associated with the increase of capture in fishing net, the other seems to be linked with the southward shift of the population within the North Sea.