Sea otters off the Alaskan coast play a pivotal role in marine ecosystems: By dining on sea urchins, the animals help preserve kelp forests that feed a range of species, from barnacles to bald eagles. Now, according to a paper in tomorrow's Science , the sea otter appears to be fighting for its survival--with potentially disastrous consequences for the ecosystem. The researchers suspect that killer whales have begun to snack on the otters, perhaps because their regular diet of seals and sea lions is declining.
Beginning around 1990, a team led by James Estes of the Biological Resources Division of the U.S. Geological Survey and the University of California, Santa Cruz, and others noticed that sea otters were becoming scarcer. Along a necklace of the Aleutians spanning 800 kilometers, Estes's group estimates that otter numbers have plummeted from about 53,000 in the 1970s to 6000 last year. The ecological consequences have been severe, the team reports: On Adak Island, for example, where otters now number about 300, sea urchin are booming and kelp density is down 12-fold.
A clue to this puzzling decline appeared in 1991, when researchers witnessed for the first time a killer whale eating an otter. The whales had been thought to shun otters because the animals provide few calories compared to larger, fat-laden harbor seals and Stellar sea lions. "This reflects real desperation for the orca, says ecologist Paul Dayton of the Scripps Institution of Oceanography in La Jolla, California. "They're eating popcorn instead of steaks."
Estes's group then tagged with radio transmitters 17 otters in a lagoon that orcas can't reach and 37 otters in an open bay; over 2 years, deaths were much higher in the bay. Finally, the team calculated that the dozen observed killer whale attacks on otters, extrapolated to the general population, could account entirely for the observed declines. Remarkably, as few as four whales could be decimating otters along 3300 kilometers of shoreline between the Kiska and Seaguam islands.
The researchers don't know exactly what is prompting the whales to eat otters, but Estes asserts that the chain of events leading to the otter's decline may have been triggered by a boom in commercial fishing in the Bering Sea--which could have sharply curtailed or altered the food supply for sea lions and seals. "It raises the possibility that overfishing can have a wide array of effects on species that we wouldn't expect to be impacted," he says. Other researchers suspect that overfishing alone may not be responsible. "The cause of the killer whale shift is probably very complicated," says Dayton.