(Newser) – It took 25 years, but sea otters have finally recovered from the Exxon Valdez oil spill in Alaska. A federal study of Prince William Sound sea otters affected by crude oil spilled from the Valdez in 1989 has concluded that the marine mammals have returned to pre-spill numbers. One big problem was that sea otters feed on clams, and clams suffered, too, because oil from the spill remained in sediment for years.

"One of the lessons we can take from this is that the chronic effects of oil in the environment can persist for decades," says the federal scientist who authored the new report. After the supertanker leaked 10.8 million gallons of crude oil, responders recovered nearly 1,000 sea otter carcasses. The estimated number of immediate deaths attributed to the spill ranged from about 1,000 to 3,000. Many of those that survived initially suffered lingering problems from oil-soaked fur. (Read more sea otter stories.)

