Cute But Not So Innocent: Seals Blamed For Porpoise Massacre

Smithsonian's National Zoo, via Flickr. CC BY-NC-ND 2.0

For the past decade, scientists have been trying to solve a curious mass murder mystery case in the North Sea. The victims: hundreds of harbor porpoises, washing up along Dutch coastlines with gruesome, fatal mutilations. The suspects: fishermen, boats, or some large, apex predator. So whodunit? After several years of CSI-style detective work, scientists have finally gathered enough evidence to point the finger at the killer, and it’s not aimed at sharp boat propellers, but those big-eyed, cuddly-looking gray seals.

Gray seals might look cute, but they are powerful predators. Reaching lengths of up to 2.5 meters and weighing up to 250 kilograms in weight, gray seals are the largest predators in the southern North Sea. Despite this, it took almost ten years for scientists to realize that seals were the likely perpetrators.

The lightbulb moment occurred when a group of Belgian researchers examining the slash wounds of torn up porpoises noticed that some of the punctures looked like the teeth marks of gray seals. They decided it was worth pursuing, so they studied photographs and autopsy results of 1081 harbor porpoises that washed up along Dutch shores between 2003 and 2013.

As described in Proceedings of the Royal Society B, they found numerous telltale signs that gray seals had been responsible for some of the deaths. Many of the porpoises had large chunks of calorie-rich blubber missing, suggesting they had been targeted as a food source, and some had five parallel scratches down their bodies, which appeared to be gray seal claw marks.

While the evidence seemed to be slowly stacking up, it’s difficult to blame a gun when you can’t find the bullets. So the researchers set themselves a slightly more ambitious goal: finding seal DNA in the carcasses that the scientists had identified as probable attack victims. This was no mean feat because seawater would have had ample opportunity to wash out bits of predator DNA lingering in the wounds. But the scientists persisted, and their hard work paid off.

In the bite marks on three of the carcasses, they discovered gray seal DNA. Of course, it’s possible that the seals were merely scavengers, taking an easy meal from an already dead porpoise. However, the lesions containing DNA also showed hemorrhages, suggesting they were inflicted during life, shortly before death.

Altogether, the researchers estimated that grey seals were responsible for at least 17% of the cases. This means that predation by these animals is one of the main causes of death in harbor porpoises in this area, alongside fisheries bycatch and infectious disease.

So why have seals started to prey on porpoises when their diet normally consists of fish? The researchers aren’t sure, but they speculate that they could have developed a taste for them when scavenging for dinner in fishing nets if some had become entangled accidentally.

The researchers also point out that bathers in the area should be wary of the seals, as humans may be at risk of attacks.

[Via Proceedings of the Royal Society B, Science and AFP]

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