From the 1990s, seals and harbour porpoises washed up around the North Sea with a distinctive spiral laceration running along the body. Ducted propellers on offshore vessels were the leading explanation, and the pattern was widely described as propeller damage.
The explanation shifted after direct observation. Grey seals were filmed attacking and consuming harbour seals in the Netherlands and elsewhere, and forensic examination of wound morphology matched grey seal predation better than propeller strike. Grey seals have since been documented preying on harbour porpoises as well, and on grey seal pups. The behaviour appears to be carried out mainly by particular large males rather than the population at large.
How common it is remains unclear. The proportion of North Sea harbour porpoise mortality attributable to grey seals varies between studies and regions, and disentangling grey seal predation from scavenging of animals that died of other causes is not straightforward when only a carcass is available.