The sea otter population of Prince William Sound, Alaska took 25 years to recover after the Exxon Valdez oil tanker disgorged more than 10 million gallons of crude oil onto the Alaskan coast on  March 25, 1989.

Thousands of otters likely died immediately as they soaked in sludge. For the next two decades, a filthy oil residue poisoned otters as they fed, slowing the recovery process.

In 2013, though, approximately 4,277 sea otters (Enhydra lutris) swam in western Prince William Sound, up from 2,054 in 1993, according to a recent U.S. Geological Survey (USGS) report. Even Knight Island, one of the most heavily polluted areas in 1989, had recovered to pre-spill otter occupancy levels. The otters now abound at near the maximum number the region can support sustainably.

The Other Side of Otters

“Although recovery timelines varied widely among species, our work shows that recovery of species vulnerable to long-term effects of oil spills can take decades,” said lead author and USGS biologist Brenda Ballachey, in a press release. “For sea otters, we began to see signs of recovery in the years leading up to 2009, two decades after the spill, and the most recent results from 2011 to 2013 are consistent with recovery as defined by the Exxon Valdez Oil Spill Trustee Council.”

Prince William Sound sea otters live normal life spans now, too. In a healthy ecosystem, the oldest and youngest otters die more frequently than adult animals in the prime of life, from two to eight years old. However, between 1989 and 2008, biologists counted an abnormally high number of adult otter corpses washed up on the beach, compared to the numbers from before the spill.  However, in the three most recent counts, otters death rates seemed to be returning to a normal range of ages.

Photos: Devastating Oil Spill Disasters

Ongoing adult otter death may have occurred because, after the spill, the surviving otters suffered from near-constant exposure to toxic chemicals as the animals foraged for mussels and other shellfish in the crude-soaked sand and gravel beds of the Sound. USGS scientists found genetic signs in the otters bodies indicating that they experiences chronic long-term exposure to oil residues, compared to otters from other areas.

Although the biologists still observed these genetic warning signs in 2012, the levels had dropped significantly since 2008.

Photo: Sea otter, Enhydra lutris, mother with nursing pup in the Morro Bay harbor. Credit: “Mike” Michael L. Baird, Wikimedia Commons