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.
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.
