A sea otter, an animal found along the Pacific coasts of North American and Northeast Asia, swims in kelp. Otters were exposed to lingering oil in beach sediments after the 1989 Exxon Valdez oil spill.

(Benjamin Weitzman, U.S. Geological Survey)

Nearly three decades after the 1989 Exxon Valdez Oil Spill, some wildlife species are still recovering, according to a recent study. Scientists continue to examine the animals to figure out how long it has taken for them to get past the pollution's effects.

“Because wildlife species in the spill area vary so much in terms of what they eat, habitats that they useand their ability to rebound after a drop in numbers, ,” lead author Dan Esler said in a release on the study. “Some species were barely affected, others such as bald eagles, rebounded quickly, and other species took much longer to recover, such as sea otters.”

The scientists were able to identify several ecological factors that affected the wildlife’s ability to rebound.

“A key finding was that for some species, such as harlequin ducks and sea otters, chronic oil over the long term than acute effects of the spill,” wrote the researchers.

They also discoveredanimals that preyon invertebrate species, such as crabs and worms, living on or in contaminated sediments were more likely to be affected by the spill than animals that feed on fish or plankton living in water, according to the release.

“Many seabirds experienced direct and indirect effects of the spill, population trajectories of some piscivorous birds, including pigeon guillemots and marbled murrelets, were linked to long-term environmental changes independent of spill effects,” wrote the researchers.

Species with low reproductive rates were also said to have a limited capacity for recovery. Orca whales, in particular, have still not managed to get back to their pre-spill numbers, according to the release.

“The observed variation in mechanisms and timelines of recovery is linked to species' specific life history and natural history traits, and thus may be useful for predicting population recovery for other species following other spills,” wrote the researchers.

When the incident occurred in 1989, and spilled itscontents on more than 1,300 miles of Alaska's coast. The 11 million gallons of oil killed an estimated 250,000 birds, 2,800 otters, 300 seals, 250 bald eagles, more than 20 killer whales and countless other creatures.