Traumatic injuries by grey seals on delphinids from northwest European waters

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Abstract
Grey seal Halichoerus grypus populations are increasing across the North Atlantic, where occasional reports describe attacks, predation on other marine mammals and even cannibalism of their own young. Here we report injuries potentially consistent with grey seal-inflicted lesions on 4 delphinids from northwest European coastlines: 1 bottlenose dolphin Tursiops truncatus , 1 common dolphin Delphinus delphis and 2 white-beaked dolphins Lagenorhynchus albirostris . These delphinids exhibited injuries such as puncture wounds and parallel scratches, with varying degrees of tissue remodelling. This indicates that the wounds were not recent but likely occurred days to weeks before death, suggesting opportunistic interactions rather than predation. Ultimately, all 4 delphinids were believed to have died from bacterial infections, including Brucella ceti and Morganella morganii, and it is likely that these delphinids were already debilitated at the time of the seal encounters. In addition, in 2 cases, seal-associated pathogens— Streptococcus phocae and Atopobacter phoca e —were isolated. This study expands the known scope of grey seal interactions to include odontocetes beyond harbour porpoises Phocoena phocoena , highlighting their potential as opportunistic or exploratory top predators. As grey seal populations increase, their interactions with cetaceans may influence individual health and broader ecosystem dynamics. Retrospective and prospective research into marine predator-prey relationships will enhance our understanding of these complex behaviours and their impact on marine ecosystems.

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