Otter Decline a Mixed Blessing for Bald Eagles

Oct. 3, 2008 -- A steep decline in the sea otter population in Alaska's Aleutian Islands has forced bald eagles nearby to change their diet, according to a new study.

Study leader Robert Anthony of the U.S. Geological Survey in Corvallis, Oregon, examined the prey remains in bald eagle nests on the islands to evaluate the eagles' diet between 1993 and 1994 and again between 2000 and 2003.

In the first time-frame, sea otters were abundant in the area, after recovering from near extirpation in the 1930s. By the late 1990s, numbers were down more than ten-fold and have not increased since. The second set of measurements corresponds to a low otter population.

Anthony found that between the two sampling times, the eagles' diets shifted from including otter pups, a kelp-dwelling fish called the rock greenling and a fish called the smooth lumpsucker to diets including greater numbers of seabirds and Atka mackerel.

The otters feed on sea urchins in the kelp forest. As long as sea otters are abundant, the urchin population is kept in check, which allows the kelp forest to thrive, said James Estes, of the University of California, Santa Cruz, who was also part of the study, published in the journal Ecology.

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Without the otters, the kelp forest declined, which appears to account for the eagle's switch to more seabird lunches.

The seabirds appear to have been good for the eagles, to the researcher's surprise.

"I would have guessed that we would have seen declines in eagles, but we didn't," said Estes. "The numbers remained constant. If anything, the reproductive success went up a little bit."

They speculate that the higher nutritional content of the seabirds may account for the improvement in the eagles' breeding success.