Grey seal attacks on harbour porpoises in the Eastern Scheldt: cases of survival and mortality

Abstract

In the southern North Sea, hundreds of mutilated harbour porpoise carcasses (Phocoena phocoena) are found each year on beaches. Recent studies have confirmed that these concern the remains of predation by grey seals (Halichoerus grypus). A retrospective study of post mortem photos helped to further characterise grey seal induced wounds and indicated that grey seal predation is currently one of the main causes of death of harbour porpoises in the Netherlands. In addition to direct mortality, non-lethal interactions also occur. Both outcomes could play a significant role in ecosystem dynamics. The Eastern Scheldt has a resident group of harbour porpoises and also grey seals can be found in this semi-enclosed tidal bay, making this a suitable area for studying interspecific interactions. The Rugvin Foundation collects photographs of harbour porpoises in the Eastern Scheldt in a database for photo-identification purposes. Four individual harbour porpoises within this database presented bilateral tailstock lesions and additional body scarring that matched descriptions of lesions induced by grey seals. These lesions appeared to be completely healed and the sighting of these scarred animals in multiple years demonstrate complete recovery from the attacks. In addition, post mortem research revealed that over the last decade at least ten porpoises found in this tidal bay died from wounds inflicted in grey seal attacks. This apparent predation threat in combination with the scarce food sources available for harbour porpoises imposes significant pressure on the survival of this species in the Eastern Scheldt. Knowledge about the interaction between grey seals and harbour porpoises is currently increasing, mainly through the investigation of stranded harbour porpoises. Our findings demonstrate that the scope of this phenomenon goes beyond direct lethality and that behavioural adaptations in harbour porpoises, aiming to prevent detection, encounter, and, eventually capture by grey seals, are to be expected.