Het raadsel van de Bruinvismutilaties: extreme verminking en frequente strandingen van Bruinvissen in Noord Nederland, winter 2008/2009

From early November 2008 until mid March 2009, a total of 167 dead Harbour Porpoises were found on the Dutch coast. At least 60 of these animals (36%) were mutilated, that is, fins or flippers were cut off or significant parts of the skin was loosened with clear-cut, straight cutting wounds. Since few beachcombers report any details and because many other carcasses were petrified, only 13% of the reported strandings were definitely not-mutilated. For the entire coastline, the percentage of carcasses reported as ‘mutilated’ ranged from 5.9% in November 2008 (n= 34), 30.3% in December 2008 (n= 33), 22.6% in January 2009 (n= 31), 68.0% in February 2009 (n= 50) and 36.8% in the first two weeks of March 2009 (n= 19). The strandingsfrequency along the entire coastline and over the entire winter period amounted to 1.2 carcasses day-1 and peaked in February 2009 (1.8 day-1). Most mutilated carcasses were found at Texel (10% in November, December 58%, January 57%, February 92%, early March 60%; in all 27 out of 47 stranded individuals) and along the mainland coast of Noord-Holland (IJmuiden-Den Helder; Feb 82%, Mar 38%, in all 17 out of 36 recorded carcasses; no mutilated carcasses known Nov-Jan). The photos illustrate some of the characteristics of the mutilations: clear-cut, straight wounds, fine physical condition (blubber), fresh carcasses, frequent rope marks around the tail stock, and sometimes lineimprints in gum (beak), around flippers, fins, flukes or tailstock. Most animals had bright red eyes, another feature that is common in cetaceans that have drowned. All mutilated animals are probably bycatches, given all the characteristics that point into that direction, but pathological research is currently underway to confirm or reject that suggestion. The frequency of strandings and mutilations are exceptional and in fact un-precedented for the area and for the season, and it is hoped that research will reveal the cause of the mutilations, so that the problem can be solved.