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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
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
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
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
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.