Thursday, May 23, 2013

What preys on narwhals?

                About twice a week for the last three months I have been giving public presentations about narwhals to a wide variety of community groups to promote my book, Narwhals: Arctic Whales in a Melting World. I never get tired of sharing stories about my adventures studying narwhals in the High Arctic and shedding light on their natural history and the threats they face.  Once of the most common questions I am asked relates to the predators that prey upon narwhals.
                It is sometimes difficult to imagine that any animal could attack and kill a whale – even a small whale like a narwhal – but hardly any species is invulnerable. Human hunters certainly cause considerable mortality on narwhals, though quotas are enforced and hunters are closely monitored to ensure that narwhal populations do not decline due to hunting.
               The primary natural predators on narwhals are killer whales, which are virtually absent from the winter range of the narwhal, when the sea unicorn lives near the edge of the ice pack in habitat that is 97 percent covered in ice. When that ice retreats and the narwhals follow it northward, killer whales do, too, where scientists estimate they may kill 200-300 narwhals during the two or three months of open water.
              “My general feeling,” said University of Washington biologist Kristin Laidre, “is that the densities of killer whales in the Arctic are low. Sightings of killer whales are, in general, pretty rare. We do know that they feed on narwhals and belugas, and some killer whale pods, it seems, have evolved to know precisely where narwhals are located in summer, especially in the southern part of their range like in Foxe Basin or Hudson Bay. There they show up pretty regularly, as narwhals are a predictable prey resource that occur in high densities in ice-free shallow waters.” Killer whales are sighted only rarely along the coast of West Greenland, she added, where many narwhals also spend the summer.


              But orcas aren’t the only narwhal predators.  Greenland sharks are known to occasionally feed on narwhals as well. Just days before my arrival at a narwhal hunting camp near Qaanaaq, Greenland, several Greenland sharks were killed by native hunters as the sharks attempted to feed upon a narwhal that had been harpooned and was being transported to shore by the hunters. No one knows with any confidence how many narwhals are killed by sharks, but the number isn’t believed to be large.
             Polar bears, too, have been known to feed upon narwhals, but this probably occurs infrequently and only in certain circumstances. During the icy winter season when narwhals surface to breathe in narrow cracks in the sea ice, polar bears sometimes stake out these openings and wait for narwhals to surface. The bears have been observed to leap onto the back of the whales, dig their claws in and bite at the narwhal’s blowhole, perhaps in an effort to suffocate the whale. One of the whales captured by the Canadian narwhal research team I spent time with in 2010 had the characteristic scratch marks on its flanks and bite marks around its blowhole that indicate a polar bear attack, but clearly that whale survived, probably by diving deeply and forcing the bear to release it.
            These aren’t the only threats that narwhals face, as was noted this week by blogger Isabelle Groc, who visited with the same narwhal research team last year. Climate change and a host of other anthropogenic factors probably will have a greater effect on the whales in the years to come. But in answer to my frequent questioners, it is killer whales, Greenland sharks and polar bears that narwhals have had to steer clear of for many millennia.
  

No comments:

Post a Comment