Scientists confirm that gray seals kill and eat harbor porpoises

by • January 20, 2014 • Dolphin News, Dolphin ScienceComments Off27832

Jan Haelters, a marine biologist at the Royal Belgian Institute of Natural Sciences, has long believed that gray seals are capable of attacking, killing, and eating harbor porpoises. But nobody had ever actually seen a seal attack a porpoise. Until now.

Haelters and colleagues published an article in 2012 with indirect evidence of seal predation; porpoise carcasses found on the beach with injuries that appeared to be causes by gray seal bites and claw marks. This evidence persuaded many in the scientific community at the time. “Scientists involved in seal research in the Netherlands almost immediately accepted this explanation,” suggested Haelters in an email to me.

But not everyone was convinced. Without a direct observation of a seal attack, there was room for alternative explanations as to how the porpoises acquired these injuries. It was still possible that the seals were simply scavenging the already dead porpoise carcasses. Skeptical seal experts at the Dutch Seal Rehabilitation and Research Center (Zeehondencrèche Lenie ‘t Hart) reacted to Haelter’s 2012 article by suggesting that there “is simply no evidence of seals killing and eating porpoises. No one anywhere in the world has seen a gray seal catch a porpoise.”

An article slated to be published in the journal Marine Mammal Science, however, puts this debate to rest. It presents the first reliable observation of a gray seal catching and eating a harbor porpoise. On April 19, 2013,  researchers observing wildlife off the cliffs of Cap Gris-Nez in northern France noticed a juvenile harbor porpoise (under 2 meters in length) swimming close to shore. Suddenly, an adult male gray seal (which can grow to over 3 meters and weigh more than 300 kg) rocketed up from the deep and grabbed the porpoise by the head. A second attack soon followed, and the researchers watched as the seal floated out to sea while gnawing on the porpoise. (Image of attack below)

Gray Seal Attacks Porpoise

This is rather indisputable evidence showing that gray seals do in fact attack and consume harbor porpoises. The question remains as to how unusual this behavior is. The authors note that “observations such as those reported here and elsewhere had not been reported before 2011, despite intense observation efforts since the 1990s. ” Why did it take so long for researchers to finally capture this behavior on film? Is this a new phenomenon?

It’s possible that an increase in both gray seal and harbor porpoise populations in the North Sea in recent decades is resulting in more interactions between these species. Although the authors note that “harbor porpoises and gray seals in the northern North Sea have co-occurred at high levels of abundance” for decades without any observations of predation until now.

It’s also possible that a handful of culinarily adventurous gray seals have only recently developed an appreciation for porpoise flesh.  “It might be a ‘personal’ taste, not widespread amongst the grey seals, as the phenomenon seems to be localised,” suggests Haelters.

With reports from 2009 of seals attacking human swimmers off the coast of Ireland, let’s hope that gray seals don’t acquire a taste for human flesh. In the meantime, researchers hope to investigate just how common these attacks are, and whether they are a significant source of mortality for harbor porpoises.

Thibaut Bouveroux, Jeremy J. Kiszka, Michael R. Heithaus, Thierry Jauniaux, & Sylvain Pezeril (2014). Direct evidence for gray seal (Halichoerus grypus) predation and scavenging on harbor porpoises (Phocoena phocoena). Marine Mammal Science DOI: 10.1111/mms.12111

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