Hungry Sea Otters Help Keep Oceans Healthy

Sea otters are warding off the accumulation of acidic carbon dioxide in Alaska's waters by preying on sea urchins that feed on underwater kelp beds vital to the oceans' health, according to a study.

Otter-protected kelp beds absorb about 12 times as much carbon dioxide during photosynthesis as thinned-out kelp beds, according to a study published in the September issue of Frontiers in Ecology and the Environment.

Atmospheric carbon dioxide threatens marine environments because the gas, when absorbed into the ocean, increases acidity levels, causing the phenomenon known as "ocean acidification," according to scientists. So by devouring sea urchins, otters allow the kelp to grow and keep the oceans cleaner.

Urchin-eating sea otters produce a significant savings, according to the study's authors, who said it would cost between $205 million to $408 million to offset the carbon that sea otters enable kelp beds to absorb, based on prices used in the European Carbon Exchange.

The study relied on data collected over 40 years from sites between British Columbia, Canada, and Alaska's Aleutian Islands.

Co-author Jim Estes said he hopes the study will help people understand that sea otters have far-reaching benefits.

"On the one side, people like sea otters because they're fuzzy, cool things. On the other side, a lot of people hate them," said Estes, a biologist and sea-otter expert at the University of California at Santa Cruz. Fishermen, including shellfish harvesters, "are, without question, significant competitors" and famously hostile to sea otters, Estes said.

But by preserving kelp forests, otters benefit fishermen because kelp beds provide an important fish habitat, he said.

Sea otters were once hunted nearly to extinction, victims of a commercial harvest that started with czarist Russia's colonization of Alaska. A 1911 treaty ended the commercial hunt and numbers rebounded.

But the population of sea otters from Kodiak, Alaska, to the western Aleutian Islands has dropped sharply in recent years, declining as much as 67 percent since the mid-1980s, according to the U.S. Fish and Wildlife Service.

In 2005, the western Alaska otter population was listed as threatened under the Endangered Species Act, with the steepest population drop in the state's Aleutian Islands.

Estes said many scientists studying Alaska's sea otters believe predation by killer whales is the main reason for the recent decline. Whales have increasingly preyed on otters as Steller sea lions and seals have become scarce, Estes said.

The 1989 Exxon Valdez oil spill also killed thousands of sea otters, up to 40 percent of the population in western Prince William Sound, according to state and federal scientists. The otter population has not fully recovered in spill-affected areas, scientists say.