You know seals as the flippered critters that balance balls on their noses for fish at the marine park. But the cute antics of the pinnipeds betray a deeper, darker secret. They're ruthless murderers and they'll stop at nothing to get a mouthful of delicious porpoise blubber. 

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Mutilated harbour porpoises, washed ashore at Ouddorp, South Holland in 2012. Images: Cees van Hoven.

For ten years, the mutilated corpses of hundreds of harbour porpoises washed up on the shores of the North Sea's Dutch coastline. The lifeless bodies were missing huge chunks of skin and blubber from the head and tail, and had deep wounds on their flippers. They often had parallel scratches running perpendicular to the length of their bodies. The mysterious circumstances behind the cetacean deaths pit local fishermen against scientists and conservationists.

The former blamed natural predators for the porpoise killings, while many in the latter group thought the injuries could have been anthropogenic in nature (in other words, somehow caused by humans). Perhaps the wounds were inflicted by boat propellers. Perhaps the porpoises had been caught up in fishing gear and had been cut up by fisherman trying to remove them from their nets. Or, possibly, the porpoises had died from some other cause and their injuries were simply the result of scavenging. 

If other animals had eaten the porpoises, then evidence should be present in their wounds. When a terrestrial animal like a zebra gets eaten by a predator, the specific identity of the killer – perhaps a lion or a hyena – can be determined using the DNA evidence from saliva left in the bite wounds. But when the victim is a marine mammal like a porpoise, seawater has a tendency to flush out whatever saliva might be deposited in the animal's wounds, making the identification of a possible predator much harder. DNA is most likely to be found in marine mammals just after death in an animal that died rather quickly. Unfortunately, there is often a long delay between a kill, the discovery of a corpse and a necropsy to determine the cause of death.

Still, sometimes researchers get lucky. Earlier this year, researcher Mardik F. Leopold and colleagues were fortunate enough to examine three freshly killed harbour porpoises ... and in their wounds they discovered grey seal DNA.

"All three animals," Leopold writes, "were in good nutritional condition and had fed shortly prior to death as shown by the presence of partly digested prey in their stomachs. The mutilations were considered fatal and [severe loss of blood] was the most likely cause of death." That's not enough to necessarily pin a decade of dead porpoises on grey seals, but it was enough to raise a few eyebrows and cast some doubt on the boat propeller theory.

Armed with the knowledge that grey seals were capable of inflicting severe damage on harbour porpoises, Leopold turned to the necropsies that had already been conducted on 1,081 harbour porpoises that stranded on the Dutch coastline between 2003 and 2013.

Leopold compared photos of porpoises with unknown killers to the wounds from the three porpoises whose killers he had identified. If the wounds were similar enough, he could be pretty sure that grey seals caused the other deaths as well.

Putting all the clues together, Leopold assigned grey seals the blame for 17 percent of harbour porpoise deaths, with an additional six percent of porpoise mortalities "probably" due to seals as well. He thinks the true proportion of porpoise deaths attributable to seals is probably higher since "mutilated carcasses with an opened abdominal or thoracic cavity are likely to sink rapidly and decay, therefore going unrecorded." He adds that animals that initially escaped an attack could have died later from complications resulting from their seal-inflicted injuries.

Since most of the dead porpoises were healthy juveniles in prime condition, the researchers think that the seals were probably after their valuable, energy-rich blubber.

“So have seals been attacking porpoises all along, without anybody really noticing? Or is this a relatively recent innovation in seal hunting tactics?”

So have seals been attacking porpoises all along, without anybody really noticing? Or is this a relatively recent innovation in seal hunting tactics? It isn't clear. But the researchers offer one plausible scenario – and it doesn’t reflect well on fishermen.

Porpoises are often caught and drown in fishing nets as bycatch, and seals are well known for their opportunistic scavenging on helpless animals caught in those nets. While just five of the more than 1,000 necropsies showed evidence of entanglement, the researchers speculate that "harbour porpoises entangled in such nets may have triggered grey seals to turn from scavenging to attacking live animals." In other words, attacking weak, helpless, drowning porpoises caught in nets may have provided the right conditions for seals to learn how to safely hunt their healthier, stronger counterparts.

Whether or not seals have recently learned to take on porpoises, these findings will force researchers to alter their understanding of harbour porpoise behaviour, ecology and population dynamics. Since most of the animals eaten by seals are juveniles, they're being removed from the population before breeding age, which threatens the stability of future porpoise populations.

In addition, the porpoises may learn to put on less weight, keeping them more agile and better able to escape attacking seals. That's a pattern seen elsewhere in porpoises subject to predation by bottlenose dolphins. Unfortunately, the gains made by evading predation may be undone by losses due to starvation. That trade-off would also have negative consequences for the health of future porpoise populations. 

And if that wasn't bad enough, the researchers note that the mutilated porpoises usually wash up on beaches favoured by human bathers and surfers. And that means humans could potentially be at risk from grey seal attacks as well, they warn. 

Top header image: Andrea Schieber, Flickr