Unexpected Porpoise Predators Revealed As Formidable Gray Seals



Your typical gray seal, Halichoerus grypus, a hunter of small cetaceans? (Source)

Following the regular discovery of mutilated harbor porpoise carcasses on Dutch beaches since 2003, a group of biologists began a ten year investigation into the situation's cause.¹ The usual suspects of boat propellers or hostile fishermen were dismissed after the deaths regularly continued their toll, and the enigma continued until a group of Belgian researchers came to a startling conclusion.¹ In 2012, these researchers took note of apparent bite marks present in some of the wounds inflicted upon the thousands of porpoise carcasses.¹ These matched the canine teeth of an unexpected yet certainly capable mammalian predator: the gray seal Halichoerus grypus. With bulls reaching up to almost eleven feet in length and weighing as much as 310 kilograms this was no huge surprise, especially considering their being relatives of animals like the formidable leopard seals. Further examination of the carcasses showed the marks of pinniped claws and signs of the seals having gone after the nourishment of a porpoise's blubber¹, yet the proposition was still subject to some debate.

Harbor porpoise carcass with wounds indicative of gray seal predation. (Source)

The real clincher came on April 19, 2013.² A series of attacks on a juvenile harbor porpoise by an adult male gray seal were observed by wildlife spotters from the cliffs of Cap Gris-Nex.² The pinniped apparently bit down onto the porpoise's head in an attempt to drown it, attacked it in such a manner a second time, and then carried the carcass out to sea.² This was duly photographed, the images of which are reproduced below. While this event brought confirmation that gray seals can and do prey upon such cetaceans, the question still lingered as to whether the aforementioned Dutch carcasses were definitely the result of such an occurrence. The Belgian biologists took to analyzing any DNA samples left in the wounds, although the possibility of salt water having degraded such genetic material was strong.¹ In the deep bite marks left on three porpoise bodies, the scientists found their jackpot: gray seal salivary DNA.¹ With these recently published findings, an excellent case of zoological forensics work was concluded. However, an enigma still remained as to what caused these gray seals to adopt such a unique prey item. The researchers from Belgium suggest that this habit began as a result of porpoises being caught in the wider sea nets recently employed by Dutch fishermen.¹ Gray seals notoriously steal fish from such nets, and it has thus been suggested that the pinnipeds stumbled upon the better-nourishing porpoises and began actively preying upon them as a result.¹ There is worry not only that the new predatory habits may lead to ecological unbalance, but also that human bathers and surfers may be in danger as well.¹ Although it may sound to be a stretch, the Belgian researchers warn that the seals' hunting of larger prey may very well lead to an accident between humans and a curious individual.¹


The first confirmed act of gray seal predation upon a harbor porpoise individual. (Source)

As mentioned previously, I'm not particularly surprised in regard to this discovery considering these often impressive mammals are essentially "sea bears" sharing a common ancestry with modern day Ursidae. Nonetheless, this finding certainly contributes new light to be shed on the bizarre and unexpected habits of modern day fauna, and gives a shocking reminder to never underestimate the capabilities of large mammal species!


References:

1. You, Julia. "Gray Seals May Be Becoming the Great White Sharks of Dutch Beaches." Science/AAAS. N.p., 25 Nov. 2014. Web. 26 Nov. 2014. http://news.sciencemag.org/biology/2014/11/gray-seals-may-be-becoming-great-white-sharks-dutch-beaches
2. Gregg, Justin. "Scientists Confirm That Gray Seals Kill and Eat Harbor Porpoises | Justin Gregg | Science Writer." Justin Gregg. N.p., 20 Jan. 2014. Web. 26 Nov. 2014. http://justingregg.com/scientists-confirm-that-gray-seals-kill-and-eat-harbor-porpoises/#!prettyPhoto