Last year they ate about 3.5 percent of the migrating run at a time when salmon numbers were down and demand was up. This year's run begins in earnest in April.
One particular sea lion — named C404 because of a brand applied by a state and federal program — is in a class by himself. He has figured out how to get into fish ladders that help fish past the dam — where endangered salmon and other fish become his easy prey.
The engineers have used everything legal to get rid of him. They have installed gates and tried huge firecrackers, rockets, rubber bullets, and noises sea lions don't like.
But C404 has given them the flipper.
The California sea lion and his kind aren't endangered, but a 1972 federal law protects them. Incorrigibles, however, can be singled out for "lethal removal" through a long, complicated process, said Robert Stansell, a fish biologist with the Corps at Bonneville, about 40 miles east of Portland.
In the 1990s, for instance, a group of sea lions that nearly wiped out a winter steelhead run at the Ballard Locks in Seattle was marked for death, he said. But after a Humane Society lawsuit and President Clinton urged clemency, the worst were packed off to Sea World in Orlando, Fla.
Stansell said C404 has been showing up at Bonneville each year since at least 2003 and has learned to rub it in. Last year he appeared in a window where fish counters keep track of salmon migrating upstream. The data help predict the size of future runs.
"He even rolled over a little so we could get a look at his brand," Stansell said.
The chinook salmon run peaks in about September, but the sea lions head back to southern California breeding grounds around late May when the water temperature in the river rises.
But Stansell said the animals are showing up earlier and in greater numbers, and they are staying later. Now they have begun crawling onto the rocks to rest.
"They're becoming comfortable here," he said.
To shoo the sea lions from the dam, the engineers intend to keep trying the same tactics they've used with little success so far.
They'll also use firecrackers and the like on any newcomers, in hopes of dissuading them from joining the picnickers at Bonneville.
In the meantime, C404 is gaining notoriety for his savvy and liveliness.
"If he were in a litter of puppies," Stansell said, "he's the one you would pick."