Two harbor porpoises (Phocoena phocoena) washed up on an eastern Belgian shore on September 24, 2011. The porpoises were still bleeding when they were collected, indicating that their deaths were recent. There were bite marks on the bodies, which scientists compared to the mouths and teeth of potential predators. The prime suspects, based on the necropsies, were gray seals (Halichoerus grypus), researchers reported in 2012 [pdf].
Gray seals, found across the northern Atlantic, are large seals. Bulls can reach a length of up to 3.3 meters and a weight of more than 300 kilograms. They can outweigh a harbor porpoise by a factor of four or more. The gray seal’s preferred diet, though, is mostly fish.And gray seals had never been known to hunt harbor porpoises. Was the 2012 conclusion correct?
Yes, say researchers led by Thibaut Bouveroux of the Observatoire pour la Conservation et l’Etude des Animaux et Milieux Marins (OCEAMM) in Zuydcoote, France, who have caught the seals in the act. The seals do attack porpoises and scavenge long-dead ones, the team reports January 16 in Marine Mammal Science.
In what may read a bit like a gruesome marine police blotter, Bouveroux and his colleagues describe three cases of seal-on-porpoise violence that took place in France in early 2013:
February 16, 2013: An observer performing seabird and marine mammal surveys from Cap Gris-Nez in northern France spotted an adult gray seal about two kilometers off the coast handling a dead harbor porpoise with its flippers. Over a 15-minute period, the seal stripped blubber and skin from the carcass, apparently feeding.
March 16, 2013: A marine mammal observer saw a gray seal in the fishing harbor of Boulogne-sur-Mer “ripping off and eating a large piece of blubber and skin from a dead and putrefied harbor porpoise, stranded in a very shallow area,” the researchers write. The porpoise had been dead for five or six days, based on its state of decomposition.
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April 19, 2013: As in February, an observer spotted seal-and-porpoise action while performing surveys from Cap Gris-Nez. This time, the marine mammals were a mere 10 meters from shore. First, the harbor porpoise, a lone juvenile, was seen swimming slowly close to shore. Then, an adult gray seal attacked from below, grabbing the porpoise on the right side of its head in front of its right eye. The seal released the porpoise and then made another attack, again on the right side. After six minutes, the pair drifted out of view, with the seal still “handling” the porpoise.
That third observation is the first time that such an attack has been documented, and it was photographed. That evidence, combined with other observations led the scientists to conclude that gray seals in this area are indeed hunting, eating and scavenging harbor porpoises.
The seals appear to be targeting the porpoises for their fat and skin, the researchers say. “Fat tissue has the highest energy content and is more easily digested,” they write, “making it unsurprising that we observed gray seal scavenging on even highly decomposed harbor porpoise carcasses.”
It’s not entirely clear why the seals are suddenly finding the porpoises to be so enticing. This is not a behavior that’s been observed prior to 2011, and it’s never been seen among the more-abundant populations of the animals in the northern North Sea. However, the researchers note, those northern gray seals have been experiencing declines in high-energy prey such as the lesser sandeel (Ammodytes marinus), which may be driving those seals south to find other high-energy food, such as the harbor porpoises.