Herring populations can stabilize at a low or high number, but something has prevented a rebound. Oil likely is no longer a factor, Pegau said.
Responders estimated that as many as 3,000 sea otters died the first year. Hundreds more died in the years after of exposure to oil that persisted in sediment, where otters dig for clams.
Three factors could have had an impact on the otters' ability to survive. Oiled fur loses insulating value. Otters ingest oil as they groom, and researchers years after the spill found blood chemistry evidence consistent with liver damage. Grooming takes time away from feeding.
"One of the lessons we can take from this is that the chronic effects of oil in the environment can persist for decades," said Brenda Ballachey, who moved to Alaska a few months after the spill and spent the next summer dissecting sea otter carcasses collected from beaches and frozen.
The U.S. Geological Survey research biologist is the lead author of a federal study released last month that concludes that sea otters have finally returned to pre-spill numbers.
The pigeon guillemot (GEEL'-ah-mot), which looks like a black pigeon with web feet, is one species that has not recovered. Numbers were declining before the spill. An estimated 2,000 to 6,000 guillemots, or 10 to 15 percent of the population in spill areas, died from acute oiling.
Researchers suspect river otters, mink and other predators targeted guillemot eggs as an alternative to foraging on oiled beaches.
Like sea otters and another bird that took years to recover, harlequin ducks, pigeon guillemot's forage for invertebrates in sediment and likely were affected by lingering oil, said David Irons, a seabirds expert with the U.S. Fish and Wildlife Service.
The decline of its other prey, juvenile herring, didn't help. Numbers continue to decline in both oiled and non-oiled areas. Irons has proposed reducing mink numbers on the heavily oiled Naked Islands, once prime habitat for guillemots, to restore their numbers.
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