Scientists were baffled when harbor porpoises (Phocoena phocoena) started washing up on Dutch coastlines with horrible mutilations on their bodies; original suspects included fishermen, collisions with boats or some new type of apex predator but after several years enough data was collected to finally identify the culprit which shockingly was the grey seal (Halichoerus grypus).
A group of Belgium scientists who were studying the porpoise carcasses noticed that some of the puncture wounds appeared to mirror the teeth marks of grey seals; they therefore decided to study photographs and autopsied carcasses of grey seals from 1081 seals between 2003-2013 which had washed up on Dutch beaches.
They ended up finding numerous signs that grey seals had been responsible for the injuries on the harbor porpoises, some had scratches along their bodies that matched grey seal claws and large chunks of blubber had been bitten away; yet visual evidence was not enough to fully suggest that the seals were the cause of the porpoises demise and so the scientists decided to see if they could find seal DNA in the porpoise carcasses to further back the evidence that the seals were the culprit.
Collecting DNA from the carcasses proved a difficult task as seawater would wash away many traces of culprit DNA but eventually DNA was found which matched that of a grey seal. The lesions from where the DNA was collected also showed hemorrhaging suggesting, the injuries were caused whilst the porpoises were alive and that therefore the seals did attack the porpoises and were not scavenging on them prior to a attack by another predator.
The results showed that in at least 17% of porpoise deaths grey seals were the cause, this meant that grey seal predation is one of the top causes of harbor porpoise deaths in this region alongside, by-catch (when they become caught in nets used by fisherman and often suffocate as a result) and disease.
The scientists are still unsure why grey seals have started to prey upon the porpoises but theories include developing a taste for them whilst scavenging fishing nets for by-catch.
What do you think about this finding? Would you have suspected grey seals of this predatory behavior? It is worth noting that at around 2.5m grey seals are the largest predator in the area studied and are therefore capable of taking down large prey items but generally we have always accustomed them to a strict fish diet, until now that is.
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