Who's been eating all those sea otters?

At first, the researchers themselves couldn't quite believe their prime suspect in the case of the missing sea otters.

Killer whales have shared the Aleutian Islands with the otters for millennia, but a group of scientists now charges that the whales have lost so much of their usual prey that they're switching to otters and devastating the population.

"I can't imagine anticipating this would happen," says James Estes of the U.S. Geological Survey. Yet in the October 16 SCIENCE, he and his colleagues propose that the relatively rare killer whales, perhaps only 150 in the region, have driven otter numbers from some 53,000 to only 6,000 in less than a decade.

The population crash could be from as few as four killer whales' switching to an all-otter diet, the researchers calculate. A single whale could consume 1,825 otters per year. That's not likely, Estes clarifies, but the number shows the killing power of the whales.

If suspicions prove correct, the otterwhale tale would provide a rare documented link between goings-on in deep water, where the whales spend most of their time, and shallow kelp forests, where otters rule. Without otters to keep sea urchins in check, the kelp forests are now being devoured, Estes warns. Ecologists study these ecosystems separately, but Mother Nature apparently does not read the texts.

Estes' team has dismissed many proposed explanations for the otter decline. The animals still reproduce at the usual rate and do not seem to be relocating. They look too fat and healthy to be suffering from food shortages, and veterinary pathologists found no signs of an epidemic or widespread environmental contaminant.

Researchers first witnessed whale attacks in the early 1990s. "In one case, a whale just came up and opened his jaws and swallowed the otter," Estes says. Another whale stirred up a wave that washed otters off rocks into the jaws of waiting killers. Other whales leapt from the water and slammed onto otters, stunning them before gulping them down.

Researchers' suspicions grew when they noticed that otter populations dropped rapidly in a whale-accessible area but held steady in a bay that the whales can't reach.

Estes suggests that killer whales were driven to eat otters by the decline in Steller sea lions and harbor seals. Just what caused those species to wane is a matter of debate ranging from overfishing to a slight rise in water temperature.

The whale-otter story sounds reasonable to Help-forest ecologist Michael Foster from Moss Landing (Calif.) Marine Laboratories. "It is startling," he says. However, Californians probably won't see the same attack on their kelp forests, he adds, because species besides the otter seem to control urchins.

Marine ecologist Mark E. Hay of the University of North Carolina at Chapel Hill was also "pretty convinced" by the idea that whales are killing Aleutian otters, even if their ecosystems seem separate. "Many of us think these links may be going on all around us," he says, "but it's hard to see."