Abnormal tailflukes in harbour porpoise Phocoena phocoena from The Netherlands (Mammalia: Cetacea Odontoceti)

Twentyfour cases of harbour porpoises (Phocoena phocoena) with abnormally shaped tailflukes found dead along the coast of The Netherlands in the period 1934 - 1961 are described and discussed. The phenomenon is not known from elsewere in the world and has not occured earlier or later (as to 1998). Seven abnormal tails (shaped like a ribbon or a string) are kept in three natural history museums in The Netherlands. The other cases are well documented by photographs and/or detailed drawings. Both sources were studied. It is concluded that the abormally shaped tailflukes are caused by extreme regeneration after manmade amputations of the flukes. Most probably, fishermen had cut off the tailflukes of bycatch porpoises and subsequently released the mutilated animals into the sea. All other causes, as given by Slijper (1936) and Van Deinse (1945), are considered highly unlikely.