25 Years After the Exxon Valdez, Sea Otter Population at Pre-Spill Levels
Nearly 25 years after the Exxon Valdez oil spill injured wildlife off the coast of Alaska, sea otters have returned to pre-spill numbers within the most heavily oiled areas of Prince William Sound, according to a new report issued in February 2014 by the U.S. Geological Survey (USGS).
Sea otters in the path of the oil incurred heavy mortality when 42 million liters of Prudhoe Bay crude oil were spilled in Prince William Sound in March 1989, with an estimated loss of several thousand otters. Through long-term data collection and analysis, scientists found that sea otters were slow to recover, likely because of chronic exposure to lingering oil. Other studies documented persistence of oil in the sea otter’s intertidal feeding habitats.
“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 of the study, Brenda Ballachey, research biologist with the USGS. “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 [a State and Federal partnership formed to oversee restoration of the damaged ecosystem].”
Scientists assessed recovery by estimating the number of living sea otters on the basis of aerial surveys and comparing that number to pre-spill numbers. They also collected carcasses of otters that had died in the spill area. Carcasses were evaluated to determine how old sea otters were when they died. Historically, and before the spill, most dead otters were either very old or very young, but after the spill, more middle-aged otters were dying as well. The ages of dead animals have now returned to the pre-spill pattern.
Recovery also was assessed with studies to detect oil exposure by using gene expression as a biochemical indicator. The most recent genetic evidence suggests a reduction in oil exposure since 2008.
Scientists concluded that the status of sea otters in western Prince William Sound is now consistent with the criteria established for population recovery set by the Exxon Valdez Oil Spill Trustee Council (see "Recovery Objective" on Sea Otters webpage).
The sea otter was one of more than 20 nearshore species considered to have been injured by the spill.
The new publication, “2013 Update on Sea Otter Studies to Assess Chronic Injury from the 1989 Exxon Valdez Oil Spill, Prince William Sound, Alaska,” is available online.
in this issue:
25 Years After the Exxon Valdez, Sea Otter Populations at Pre-Spill Levels