Killer appetites

Killer whales are eating sea otters in Alaska. This could change Alaska's coastal ecology

 
Published: Saturday 04 July 2015

ALASKA is going through a lot of changes. Few months back, global warming caused the region's ice caps to melt and now, the Alaska's marine ecology is being altered. But this time, its not human induced. It seems that killer orca whales have developed an appetite for Alaskan sea otters. Marine ecologists speculate that a small band of these whales with their voracious appetites have devoured more than 40,000 otters since the early 1990s, almost wiping out otter colonies in parts of the region's Aleutian Islands.

Usually the orcas ignore the otters, preferring seals and sea lions. Not only are these abundant, but these also offer more calories per bite. Paul Dayton, marine ecologist at the Scripps Institution of Oceanology in La Jolla, California, says that to the orcas, otters are like "hairy popcorns". "It hurts the whales when they eat them."

Hairy popcorns or not, the whales seem to enjoy their new delicacy, says University of California's marine ecologist James Estes. Estes and his colleagues first saw an orca attack an otter in 1991. Since then, more than a dozen attacks have been reported.

Estes has also documented a sharp fall in otter numbers in the recent years. About 90 per cent of them have disappeared from a 1,000-km stretch of the central Aleutians. "The magnitude and spatial scale of this decline probably is unprecedented for any carnivore," Estes reported in his recently-published report. He has ruled out disease, toxic pollutants and starvation as causes for the otters' plight. He then noticed that otters were thriving in a bay protected for the orcas (Science, Vol 282, No 1672). "At first I didn't think it was possible, but then we gradually realised that at least some of the killer whales has switched to preying on these sea otters," he says.

Estes estimates that there are about 150 orcas in the central Aleutians, enough to account for the otters' decline. "But it's conceivable, and not Unlikely, that it's one small group of animals," he says. As few as four orcas could have wrecked a havoc of this magnitude. Each of them probably gulped as many as 2,000 otters every year.

The orcas' new delicacy has unleashed other ecological changes. Populations of sea urchins, usually the favoured food of the otters, have exploded. As a result, kelp beds, on which these urchins dine, are fast disappearing. Kelp is the base of the coastal food web and provides habitat for countless varieties of fish, Estes says. Its loss will finally affect the entire food chain, including seabirds, bald eagles, and several other nearshore species.

"This is the kind of thing a lot of us have worried about, says Mark Hay, marine ecologist at the University of North Carolina. "Just a few top predators can alter their habits and have astounding effects on an entire ecosystem."

It is not clear why these killer whales have switched prey, but Estes points to a collapse in the populations of northern sea lions and harbour seals in the region in the last 20 years. Biologists do not know the reason why the collapse occurred, but a 1996 report published by the us National Research Council blames overfishing, warming of the North Pacific and whaling as the most likely causes.

Other experts say Estes' conclusions about the orcas' dietary shift are highly plausible. "I wouldn't put it past them," says John Ford, director of marine mammal research at the Vancouver Public Aquarium in Canada. "They are very adaptable, stealthy and innovative predators."

Orcas have even attacked moose in shallow waters, says Ford, and in Argentina have been known to attack and devour sea lion pups on the beaches.

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