Exposing the grey seal as a major predator of harbour porpoises.

Mardik F Leopold,Lonneke L Ijsseldijk,Lineke Begeman,Judith D L Van Bleijswijk,Andrea Gröne,Harry J Witte

doi:10.1098/rspb.2014.2429

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.

Highlights

Marine mammals strand occasionally with large, fatal wounds
Hundreds of severely mutilated harbour porpoises (Phocoena phocoena) have been found along the southeastern North Sea coastline [6], the cause of the wounding being unknown. This has resulted in controversy among scientists, the fishing industry and conservationists as to whether such mutilations were anthropogenic in origin or naturally inflicted by predators
The estimated frequency of harbour porpoise–grey seal encounters (25% of 721) includes the possible cases of grey seal attacks (‘maybe’ in figure 2: 2%) and animals that probably escaped an attack (6%). These findings suggest that grey seal attacks were the cause of death in at least 17% of the stranded animals

Summary

Introduction

Marine mammals strand occasionally with large, fatal wounds. Suggested causes include ducted propellers [1], fishermen confronted with by-catch [2], and predators or scavengers [3,4,5]. Hundreds of severely mutilated harbour porpoises (Phocoena phocoena) have been found along the southeastern North Sea coastline [6], the cause of the wounding being unknown. This has resulted in controversy among scientists, the fishing industry and conservationists as to whether such mutilations were anthropogenic in origin or naturally inflicted by predators. Acute haemorrhages in the bite wounds and other lesions found during autopsy aid evaluation of the cause of death, and help distinguish between predation of a live animal and post-mortem scavenging. As there is frequently a long interval between death and autopsy of stranded marine mammals, diagnosis of a predator attack by DNA is difficult.

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