Relationship between biological, pathological and toxicological parameters and the cause of death in harbour porpoises (Phocoena phocoena) stranded on the coast of Belgium and northern France

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The Marine Animals Research and Intervention Network (MARIN) investigated the cause of death of marine mammals stranded on the coastline of the southern North Sea (Belgium and northern France) or as bycatch delivered by fishers. The aim of the study was to compare biological, pathological, and toxicological parameters involved in the cause of death (traumatic or infectious) of harbour porpoises (Phocoena phocoena) collected between 1990 and 2008. In total, 520 porpoises were necropsied and sampled. Two causes of death appeared to prevail: first, infectious diseases: 191 animals had severe parasitosis and pneumonia, and second, bycatch in fishing nets: 120 had died of trauma, mostly as a result of bycatch in fishing gear. Most strandings and bycatches occurred in winter and there were more males than females. The majority of stranded animals taken as bycatch were juveniles. Compared with porpoises taken as bycatch, animals that had died of an infectious process had a thinner blubber layer (emaciation) and the histological investigation demonstrated a marked lymphoid depletion (spleen, thymus, and lymph nodes). Nevertheless, many porpoises taken as bycatch that could serve as a ”control“ population also presented evidence of emaciation and infectious diseases, thus confirming the need to carry out a complete necropsy to evaluate their health status. The total PCBs was determined in the blubber of selected individuals: it was higher in males than females and higher in adults than juveniles. Finally, the animals that had died of infectious disease were more contaminated than porpoises taken as bycatch. Our observations are very similar to results previously reported from neighbouring countries.