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So Far, Only One Sea Lion at Bonneville, Stellers Less Abundant than Past Years by Staff Columbia Basin Bulletin, February 22, 2013

California sea lions have remained scarce so far this year at the lower Columbia River's Bonneville Dam, according to a Feb. 21 status report prepared by U.S. Army Corps of Engineers researchers at the dam.

Through Feb. 20 only one had been observed on three separate days so far this year.

Observers atop the dam have seen no predation on salmon by any of the CSLs. As of Feb. 19 only one adult chinook salmon had been counted so far this year climbing up and over Bonneville's fish ladders.

A total of 141,400 "upriver" spring chinook salmon are expected to return to the mouth of the Columbia on their on their way to hatcheries and spawning grounds above Bonneville in Columbia and Snake river tributaries. Bonneville is located near river mile 146 about 50 miles upstream of Portland.

The research has been ongoing since 2002 to assess the level of pinniped -- sea lions and seals -- predation on salmon and steelhead stocks that are listed under the ESA. California sea lions over the past 10 years have congregated at the dam in springtime in greater numbers than were observed previously. Steller sea lions, even more recently, have also begun to hang out there in greater numbers.

Steller sea lions at Bonneville this year are slightly less abundant than the past two years during the same time frame, according to the weekly Corps status report. The maximum number of SSL seen any day so far this year was 21. Researchers had through Feb. 20 documented approximately 30 different SSLs visiting the dam this early season.

The SSLs had been observed taking one chinook, 27 steelhead and 251 white sturgeon in the waters below the dam. They've also taken 65 shad and 51 fish of undetermined species. Researchers say most of those unknown fish are likely sturgeon, an unlisted species that tends to gather in winter below the dam.

Stellers are listed under the ESA, as well as protected by the MMPA. The sea lion removal authority granted by NOAA Fisheries is for CSLs only. State officials say they expect to begin the 2013 sea lion trapping effort in late March.

California sea lion numbers at Bonneville generally begin to build in March and peak in April and May when fish numbers at their highest. The California sea lions have for the most part exited the Columbia system by the end of May when spring salmon numbers dwindle and breeding duties call.

The entirely male California contingent head south for the early summer breeding season on islands off the Southern California coast and Baja Mexico. When the breeding season has ended, many of the big marine mammals surge north to forage.

The pinniped take of salmonids (chinook and steelhead) has ranged from an estimated 0.4 percent of the run reaching Bonneville during the first year of the study to a high of 4.2 percent in 2007. The vast majority of the salmonids taken each year are chinook salmon. Those predation estimates take into account only observed take in the area immediately below the dam.

In that first year of the research only 30 different sea lions -- all CSLs -- were spotted at the dam. Since then from 82 to 166 sea lions have been counted at the dam. Early on most were CSLs but the population edge has been gained by Stellers in recent years. Stellers focused almost exclusively on sturgeon during the first several years of the study but last year, for the first time, were observed taking more salmonids below the dam than the California sea lions.

Related Pages:
Predators' Toll Below Bonneville Dips; Stellers Take Lions' Share for First Time in Study History by Staff, Columbia Basin Bulletin, 10/12/12