By-catch and drowning in harbour porpoises (Phocoena phocoena) stranded on the northern Dutch coast

Abstract

Stranding data and recorded post-mortem findings were studied for 153 harbour porpoises (Phocoena phocoena), which were collected by the Seal Rehabilitation and Research Centre (SRRC; Pieterburen, The Netherlands) in the period 1984–2006. Special consideration was given to ‘by-catch’ listed as a major cause of death. A distinct increase in the numbers of strandings of porpoises along the Dutch coastline has occurred in the recent years of the studied period. This corresponds to the number of porpoises observed in Dutch waters in the same period. Although strandings occurred throughout the entire year, they were most frequent during the January to July period. By-catch and drowning were noted most frequent in the winter and spring seasons (December–April). By-catch and drowned porpoises were found along the entire Dutch coastline. The numbers of animals per area varied depending on the collection efforts. At post-mortem investigation, three probable causes of death were identified most frequently: pneumonia, emaciation and by-catch/drowning. The by-catch and drowning rate was calculated to vary between 7% and 19%. Overall, the percentages obtained from this study appeared to be in line with those established by others in neighbouring countries. Varying fishing techniques are used in Dutch waters. A careful study of the fishing methods involved and close co-operation with fishermen are required to determine effective measures to reduce the by-catch of porpoises.