title: Exposing the grey seal as a major predator of harbour porpoises.
abstract: Harbour porpoises (Phocoena phocoena) stranding in large numbers around the southern North Sea with fatal, sharp-edged mutilations have spurred controversy among scientists, the fishing industry and conservationists, whose views about the likely cause differ. The recent detection of grey seal (Halichoerus grypus) DNA in bite marks on three mutilated harbour porpoises, as well as direct observations of grey seal attacks on porpoises, have identified this seal species as a probable cause. Bite mark characteristics were assessed in a retrospective analysis of photographs of dead harbour porpoises that stranded between 2003 and 2013 (n = 1081) on the Dutch coastline. There were 271 animals that were sufficiently fresh to allow macroscopic assessment of grey seal-associated wounds with certainty. In 25% of these, bite and claw marks were identified that were consistent with the marks found on animals that had tested positive for grey seal DNA. Affected animals were mostly healthy juveniles that had a thick blubber layer and had recently fed. We conclude that the majority of the mutilated harbour porpoises were victims of grey seal attacks and that predation by this species is one of the main causes of death in harbour porpoises in The Netherlands. We provide a decision tree that will help in the identification of future cases of grey seal predation on porpoises.
authors.affiliation: Department of Ecosystems, Wageningen IMARES, PO Box 167, 1790 AD, Den Burg, Texel, The Netherlands mardik.leopold@wur.nl.
authors.forename: Mardik F
authors.identifier: http://orcid.org/0000-0002-4540-9841
authors.initials: MF
authors.lastname: Leopold
authors.affiliation: Department of Pathobiology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, 3584 CL, Utrecht, The Netherlands.
authors.forename: Lineke
authors.identifier: http://orcid.org/0000-0001-8856-9135
authors.initials: L
authors.lastname: Begeman
authors.affiliation: Molecular Biology Laboratory, Department of Biological Oceanography, Royal Netherlands Institute for Sea Research, PO Box 59, 1790 AB, Den Burg, The Netherlands.
authors.forename: Judith D L
authors.identifier: http://orcid.org/0000-0002-4748-8168
authors.initials: JD
authors.lastname: van Bleijswijk
authors.affiliation: Department of Pathobiology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, 3584 CL, Utrecht, The Netherlands.
authors.forename: Lonneke L
authors.identifier: http://orcid.org/0000-0001-7288-9118
authors.initials: LL
authors.lastname: IJsseldijk
authors.affiliation: Molecular Biology Laboratory, Department of Biological Oceanography, Royal Netherlands Institute for Sea Research, PO Box 59, 1790 AB, Den Burg, The Netherlands.
authors.forename: Harry J
authors.initials: HJ
authors.lastname: Witte
authors.affiliation: Department of Pathobiology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, 3584 CL, Utrecht, The Netherlands.
authors.forename: Andrea
authors.initials: A
authors.lastname: Gröne
mesh_terms: D000818:Animals; D005260:Female; D020387:Food Chain; D008297:Male; D009426:Netherlands; D009659:North Sea; D050741:Phocoena / Q000502:physiology*; D011235:Predatory Behavior; D012189:Retrospective Studies; D046048:Seals, Earless / Q000502:physiology*
keywords: DNA; bite mark; decision tree; marine mammals; mutilation; predation