A sea otter swims in Alaska's Prince William Sound, where a 1989 tanker accident spilled millions of gallons of oil, harming wildlife. (Mark Thiessen/Associated Press)

Nearly 25 years after a huge oil spill off the southern coast of Alaska, sea otters in the most affected parts of Prince William Sound have recovered to their pre-spill numbers, a federal report released last week showed.

Several thousand sea otters died in the aftermath of the Exxon Valdez oil tanker running aground and leaking nearly 11 million gallons of crude oil into the sound in March 1989, according to the U.S. Geological Survey, which issued the report.

The report’s findings show how long it takes for many species to recover from oil spills, U.S. Geological Survey research biologist Brenda Ballachey said.

“Our work shows that recovery of species vulnerable to long-term effects of oil spills can take decades,” Ballachey said.

The slow pace of recovery for the otters was probably because of their continued exposure to oil when digging into sediment on the sea floor, the study found.

Sea otters were among more than 20 near-shore animal species hurt by the spill.