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NW Fishletter #337, October 2, 2014

[6] Sea Lion Predation On Salmon Up At Dam

California and Steller sea lions were estimated to have consumed about 2 percent of the spring Chinook run at Bonneville Dam in 2014, says the U.S. Army Corps of Engineers' final pinniped report of the year, released Sept. 16. That added up to 4,746 fish.

The California sea lions were responsible for nearly 3,000 of them, and consumption was higher than the past three years, but still lower than any other year since 2003. But salmon consumption by the Stellers was up again, a trend that began in 2002. "Whether this was due to a lack of sturgeon abundance or switching prey preference to salmonids is unknown," said the report. The adjusted estimate for white sturgeon consumption was only 147, the lowest seen since 2005.

Between four and five California sea lions were observed, on average, in the vicinity of the dam every day during the spring run, higher than the past two years, but lower than any year since 2002. Steller numbers were about twice as high, and were higher than for California sea lions for the fifth year in a row.


Sea lion eating a salmon (courtesy NOAA).

The report noted that daily hazing still had some short-term success at keeping some sea lions away from fishways, but no effect on others. With four traps at the dams, personnel captured 36 different California sea lions, branded 21, and euthanized 15 others that were on the list for removal.

The report said the removal program "may be at least partially responsible" for the overall downtrend of California sea lion numbers at Bonneville. "However, the unusual event of the influx of large numbers of new CSL males showing up at Bonneville Dam tailrace in 2010 and 2013, coupled with the virtual halting of removal actions in 2011, make further analysis of this program more difficult. It is also likely that the reduction in predation on the early Chinook salmon runs is due to the removal of many of the returning CSL that would arrive earlier each year waiting for the arrival of Chinook salmon." The report also noted that more Stellers preying on salmon was continuing to "complicate" the effectiveness of the CSL removal program.

Other research has found the hundreds of sea lions that hang out in the estuary and lower river may be preying on a lot more fish than the relative few who hang out near the dam. Some preliminary results from 2013 found that about 30 percent of the spring Chinook tagged near Astoria, Ore., were never detected at Bonneville, after harvest and turnoffs were accounted for. A presentation on this potential serious source of predation is slated for the November meeting of the Northwest Power and Conservation Council. -B. R.

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