Harbour porpoises are the cetacean stranded most commonly in European waters (IAMMWG et al., 2015), with some bodies found mutilated severely, with parts of blubber and the skin missing. Such injuries were associated initially with strikes from ship propellers (Camphuysen and Siemensma, 2011), fisheries by-catch (Camphuysen and Oosterbaan, 2009; Haelters and Camphuysen, 2009) and scavengers (Camphuysen and Siemensma, 2011). Bottlenose dolphins (Tursiops truncatus) and white-beaked dolphins (Lagenorhynchus albirostris) were also suggested as a possible cause of the mutilations, given that they are known to attack harbour porpoises (Haelters and Everaarts, 2011; Ross and Wilson, 1996). More recently however, studies have shown that grey seals (Halichoerus grypus) are responsible for the mutilations (Haelters et al., 2012; Jauniaux et al., 2014; Leopold et al., 2015a; Leopold et al., 2015b; van Bleijswijk et al., 2014).

Leopold et al. (2015b) carried out retrospective analysis on photographs of 1081 dead harbour porpoises that stranded along the Dutch coastline between 2003 and 2013. Wounds were compared to that of three stranded harbour porpoises found to have grey seal DNA present in wounds, see van Bleijswijk et al. (2014) for results. Of the 1081 dead porpoises, macroscopic assessments of grey-seal-associated-wounds could be carried out on photographs of only 271 animals. Wounds consistent with those made by grey seals were identified in 25% of porpoises, the majority of which were juveniles, with thick blubber layers, that would provide grey seals with a high energy density food source.

The extent to which grey seals predate on harbour porpoises in the North Sea is unknown, but these studies indicate that porpoises are providing a new food source for grey seals, which could be a result of alterations to the ecosystem and prey availability.