Using models to investigate a harbour porpoise bycatch problem in the southern North Sea-eastern Channel in spring 2005

No abstracts are to be cited without prior reference to the author.

At the Belgian coast a remarkably high number of dead harbour porpoises Phocoena phocoena washed ashore in the first half of May 2005. In total 16 were counted between 3 and 16 May. This is a number without known precedent in this area and in such a short period, and it arises concern given the species’ conservation status. The state of decomposition of all porpoises was very similar. They were very decomposed, and it was estimated that they had died at least 2 to 3 weeks prior to their stranding. All carcasses were subjected to a necropsy. Two had been cut open ventrally, clearly indicating that they were bycaught in fishing gear. Bycatch was also the most probable cause of death of the other porpoises. Prior to the stranding in Belgium (and to a lesser extent The Netherlands), high numbers of dead stranded porpoises were reported from North Sea shores of Denmark and the United Kingdom. Given the mortality observed at Danish and British coasts, and the distribution and abundance of porpoises in the North Sea, we had expected that the animals that washed ashore at the Belgian coast in May 2005 would have originated from waters north of the area under Belgian jurisdiction. However, the models developed at MUMM to simulate the drift at sea of objects or oil, indicated that the animals most probably had died between the western part of Belgian territorial waters, and an area offshore Boulogne – Le Touquet off northern France (eastern Channel). The conclusions of this multi-disciplinary research are that (1) necropsies on severely decayed porpoises can in many cases reveal the cause of death of the animals, which in this case was certainly or most probably bycatch; (2) mathematical modelling of the drift of cetacean carcasses can play a useful role in revealing where the animals died and (3) in spring 2005 a porpoise bycatch problem existed in the southernmost part of the North Sea - eastern Channel.