Tuesday, January 31, 2012

Retreating ice means orcas are increasing threat to narwhals

            The effect of the declining extent of sea ice in the Arctic continues to raise new questions among scientists and generate attention in the media.  The latest stories reveal concerns about killer whales and what impact they will have on prey species like narwhals as disappearing ice provides the orcas with greater access to the region.
            On my first visit to Arctic Canada in 2008, I watched as a pod of about 12 killer whales chased a group of perhaps 200 narwhals through Eclipse Sound in northern Baffin Island.  That’s when I first began to wonder how much of a threat the orcas are to the ice whale.  My guide that week, who has lived his whole life in the region, had never seen a killer whale, so I suspected that they must not be common in the area.  But like most questions about wildlife in the Arctic, answers aren’t easy to come by.
While killer whales are considered resident in the Canadian Archipelago when the water is ice free, their numbers are small and they are seldom observed because they are spread out over a wide area. Despite their small numbers, however, killer whales may be the top predator on narwhals – next to humans – according to many Arctic marine mammalogists, including one who referred to narwhals as “orca candy.”   
“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.
Laidre is one of very few biologists to have observed orcas feasting on narwhals.  In August 2005, while satellite tagging narwhals in Admiralty Inlet in northwestern Baffin Island, she and two colleagues watched as a pod of 12 to 15 killer whales attacked and killed at least four narwhals among a group of several hundred over a six-hour period.  From their observation point at Kakiak Point, they saw what they described as “vigorous surface and diving activity” by the orcas which resulted in a large oiled area on the water, presumably from whale oils released from the dead narwhals, and congregations of seabirds. It appeared that the orcas consumed the narwhals below the surface.  The biologists had tagged several narwhals a few days before the attack, so they were able to monitor the movements of the animals in response to the killer whale aggression.  According to Laidre, the narwhals in the area suddenly moved into shallow water as the killer whales approached, some forming tight groups and others lying still at the surface or moving slowly and quietly.  One narwhal even stranded itself on a beach and thrashed its tail violently for 30 seconds as if to warn its pod mates.  During the attack, most of the narwhals in the area moved as much as 50 miles south and spread out much more than usual.  The animals resumed their normal behaviors within an hour after the killer whales departed the area.
            In a surprising coincidence, given how seldom killer whale attacks on narwhals are observed, another biologist watched killer whales prey on narwhals on the exact same day in Repulse Bay, about 400 miles south of where Laidre made her observation.  Laidre surmised that if the predation level from these two attacks were representative of the daily activity of killer whales in the region, then 200 to 300 narwhals are likely killed on their summering grounds by orcas during the two months of open water in the area.  Coupled with the annual harvest by Inuit hunters and predicted reductions in sea ice, enabling killer whales to hunt narwhals over a longer period each year, this mortality rate raises questions about how these elements will affect the sustainability of narwhal populations.