Harbour porpoises (Phocoena phocoena) are one of the smallest marine mammals in the world. They are widespread and distributed in cool temperate coastal waters of the Northern hemisphere, with sightings most common in waters <200 m deep.
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).
Haelters et al. (2012) compared teeth marks on lesions of two freshly dead harbour porpoises collected along the Belgian coast with the teeth structure of 52 common seal (Phoca vitulina) skulls and 87 grey seal skulls, collected from the North Sea. The bite-mark lesions, loss of skin and blubber, and signs of drowning (grey seals can stay under water for up to 30 min., whilst harbour porpoises up to 5 minutes (Thompson and Fedak, 1993; Westgate et al., 1995)), indicated that grey seals were likely responsible for the attacks. Further evidence was presented by van Bleijswijk et al. (2014) and Jauniaux et al. (2014) who analysed DNA left in wounds on three and five dead harbour porpoises respectively, finding that DNA matched that of grey seals.
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.
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