Sea lions raid Columbia, grow taste for sturgeon
Sea lion predation on sturgeon in the Columbia River at Bonneville Dam increased for the sixth straight year in 2011 with more than 3,000 estimated killed.
Monitors for the U.S. Army Corps of Engineers observed for 3,315 hours this winter and spring and saw 1,353 sturgeon caught by sea lions.
When adjusted for hours not observed and other factors, the estimated predation is 3,003 sturgeon lost to Steller and California sea lions.
That compares to 2,172 in 2010.
“There hasn’t been a letdown in the annual increase,’’ said Brad James, a Washington Department of Fish and Wildlife biologist.
The Corps observes from January through May on weekdays.
Monitors also observed for 28 hours at night in 2011, determining that only about 1 percent of the predation occurs after dark.
The data also showed all size of sturgeon are being killed, but fish between 2 feet and 4 feet are hit the hardest.
Steller sea lions start showing up at Bonneville Dam as early as October. The Corps’ annual report says its predation estimates should be considered minimums.
Stellers appear to be hardest on sturgeon. Observers saw only three sturgeon killed by California sea lions this year.
Predation on sturgeon dropped substantially during the last week of March when spring chinook salmon arrived at the dam.
Overall, salmon and steelhead made up 48.7 percent of the predation observed by the Corps, sturgeon were 30.1 percent and shad were 2.1 percent. Slightly more than 18 percent of the fish killed could not be identified.
It is estimated 3,298 spring chinook were lost to marine mammals, which was 1.2 percent of the run including jacks.
Spring chinook were 91.9 percent of the salmonids taken and steelhead 8.1 percent.
In the past few years, Steller sea lions have been observed swallowing steelhead whole, suggesting they can eat steelhead and spring chinook jacks entirely below the water’s surface.
Two California sea lions were documented upstream of Bonneville Dam in mid-April, having passed through the navigation lock.
One was trapped in mid-May and released downstream of the dam. The other has eluded capture and been seen as far upstream as The Dalles marina.
Washington and Oregon factor in a kill of 11,000 sturgeon annually to sea lions in their lower Columbia River population model.