"We thought they were playing," says marine biologist Sebastian Fuhrmann of the environmental consulting firm IBL-Umweltplanung, whose photos of the killing of a young harbor seal will appear in the March 2015 issue of the Journal of Sea Research. "It looked really cute, but in just a few seconds, it was over."
The triumphant hunter of the harbor seal was, astonishingly, a gray seal.
These soulful-eyed animals have long been thought to subsist on lowly
creatures such as cod. But now the gray seal seems to be morphing into
the most murderous killer of the southern North Sea.
New eyewitness accounts and forensic evidence have
implicated the sumo wrestler-size marine mammals in the bloody
mutilation and death of harbor seals and harbor porpoises across the region. Some of the latter apparently succumb after being ambushed and held underwater until they suffocate. (Watch a video of harbor seals hunting under the waves.)
The gray seal "has the image of a nice, cuddly, friendly animal that eats fish," says marine biologist Mardik Leopold of the Institute for Marine Resources and Ecosystem Studies in the Netherlands.
But mounting evidence is suggesting otherwise.
A grey seal (Halichoerus grypus), Germany's biggest predator, attacking a young harbour seal (Phoca vitulina) on the shore of Heligoland in the North Sea, Germany.
Photograph by Sebastian Fuhrmann
Surprisingly Dangerous
When large numbers of mangled harbor porpoise carcasses
began to wash up along the southeastern region of the North Sea a decade
or so ago, no one suspected gray seals.
Porpoises can easily outswim them, and the seals hadn't been seen dining on any other creature bigger than a duck.
But clues slowly began to accumulate that gray seals in some areas are more fearsome than scientists had realized.
In 2013, a wildlife watcher saw a gray seal near the French
coast suddenly pop up next to a harbor porpoise and clamp its jaws onto
the porpoise's head, according to a study published in October 2014 in Marine Mammal Science.
The same year, a gray seal in German waters was seen
whirling a helpless harbor seal around by the neck; a half-eaten harbor
seal carcass washed ashore the next day, according to the upcoming Journal of Sea Research study.
Gray seal DNA has also been found deep inside bite marks on
the bodies of badly battered harbor porpoises, according to a pair of
studies published in 2014 in PLOS ONE and Marine Ecology Progress Series.
Thanks to DNA analysis, scientists can trace certain
porpoise injuries to gray seals. That's because the seals' handiwork
often causes large areas of missing skin and blubber, as well as three
to five parallel scratches on their prey's skin, according to a November 2014 study in the Proceedings of the Royal Society B.
Though scientists have ruled out a few other culprits for the mangled
corpses, including other predators such as Greenland sharks, some
biologists are still skeptical that gray seals are primarily
responsible. (See "Slow Sharks Sneak Up on Sleeping Seals [and Eat Them]?")For instance, biologist Dave Thompson believes that many of the harbor seals thought to be eaten by gray seals have actually been torn apart by ship propellers, and that the gray seals scavenge them after death.
"The propeller-damaged carcasses seem to be turning up
wherever we look, so the problem is very widespread and still massively
under-reported," said Thompson, of the University of St. Andrews in
Scotland.
Even so, it's clear "we have a new top predator in the North Sea,"
especially for harbor porpoises in the last four years, concludes Thibaut Bouveroux of South Africa's Nelson Mandela Metropolitan University.
"The question is why."
Cute ... and Deadly
Scientists have several theories. It's possible gray seals
recently developed a taste for porpoise meat after preying on porpoises
snared in fishing nets. Or, the fish that gray seals normally savor are
growing scarcer.
Another idea is that these marauders of the waves have simply
returned to their old habits: The mammals have recolonized the North
Sea's southern stretches after being mostly wiped out in the region due
to overhunting. (Also see "How a Leopard Seal Fed Me Penguins.")
Whatever the reason, other animals should keep a wary eye
out for the appealing seal with the huge, liquid eyes and clownish
flippers.
"Just because they're cute doesn't make them less of a predator," says biologist Abbo van Neer
of Germany's University of Veterinary Medicine Hannover. "Yes, it's
bloody. Yes, it's gruesome. That is just the way nature is."
