Killer Whales Thinning Otter Population

A group of ecologists says it has solved the mystery of Alaska's missing sea otters. Since 1990, otter populations off the Aleutian islands have plummeted by as much as 80 percent. Ecologists say killer whales are eating the otters. But not everyone is buying the theory.

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A group of ecologists says it solved the mystery of Alaska's missing sea otters. Since 1990, otter populations off the Aleutian Islands have fallen by as much as 80 percent. Ecologists say killer whales are eating the otters, but as NPR's John Nielsen reports, not everyone buys that theory.

JOHN NIELSEN reporting:

People raised on movies like "Free Willy" tend to think of killer whales as well-meaning, fence-jumping giants who want nothing more than to live in perfect harmony with all the other creatures in the ocean. But sea otters living in the Bering Sea off Alaska probably think differently. Terry Williams, a marine biologist at the University of California at Santa Cruz, says that's because the killer whales eat them.

Ms. TERRY WILLIAMS (University of California at Santa Cruz): It's a bizarre way of hunting, where these killer whales have learned to swim upside down, below a sea otter that's resting on the water's surface. These giant pectoral fins come up on either side of the sea otter. The mouth comes up, takes the sea otter, grabs it and gulps it down. It's gone.

NIELSEN: Williams is part of a team that first observed these sneak attacks seven years ago when they were studying the otter die-off along the Aleutian Islands. As they documented more attacks, they began wondering why a 10-ton orca would waste its energy on a 65-pound sea otter. Williams says the answer turned out to be a simple one. Orcas were eating otters because the really big whales they used to eat were nearly wiped out by commercial whalers back in the 1950s. When the big whales got hard to find, Williams says, the orcas switched to big, stellar sea lions and then to medium-sized seals.

Ms. WILLIAMS: Followed lastly by sea otters, which, you know, still give you calories, but it's going to take you a bit to fill up on them.

NIELSEN: In the current issue of the journal Ecology, Williams and her colleagues show just how big the killer-whale bite out of the otter population is. First, Williams determines that the average orca needs to eat the equivalent of five to seven sea otters a day just to get by. Then she multiplies five to seven by 170, which is the estimated number of mammal-eating orcas living near the Aleutians. Then she multiplies that number by 365 days.

Ms. WILLIAMS: And you end up with a whopping 300,000 sea otters in a single year potentially eaten by these animals.

NIELSEN: Williams says it's hard to overstate how much these findings upset people who all but worship killer whales.

Ms. WILLIAMS: We all want to think they're smiling at us and happy and they'll eat a few dead fish, and we're out there saying, `Well, gosh, no. You know, they're killing cute, little, fuzzy sea otters.' And a lot of people don't like hearing that.

NIELSEN: But it's not just whale-huggers who hate the orca hypothesis. Serious scientists, like Doug DeMaster, don't think much of it either.

Mr. DOUG DeMASTER (Director, Alaska Fisheries Science Center): This hypothesis is too simplistic, and it's not ready for the textbooks yet. It's still waiting to be tested.

NIELSEN: DeMaster is director of the federal government's Alaska Fisheries Science Center. He says the authors of the orca paper greatly overestimate the number of sea otters being eaten by the killer whales, partly because he's sure these killer whales are also eating lots of other things, like fish. He also says the original idea that these orcas once ate the great whales can't be proven.

Mr. DeMASTER: The people who really know killer whales, who spend their lives following them around and studying them--they just don't find any evidence that large whales are an important part of their diet.

NIELSEN: Environmental groups say the orca paper does a disservice to the cause of conservation by diverting attention from what they say is the real problem in the Bering Sea: overfishing. If there were more fish, they say, marine mammals and orcas would have plenty to eat.

For the moment this debate is largely academic; nobody is seriously talking about killing orcas to protect otters, seals and sea lions. But Jim Hasties, a federal biologist and a major proponent of the orca theory, says there is one solution that almost everyone can agree on: continuing efforts to rebuild the region's stocks of great whales. John Nielsen, NPR News, Washington.

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