Sea Lions and Steelhead

Sea lions consumed 9 percent of the adult summer and winter steelhead at Bonneville Dam in 2017, according to a Corps of Engineers report.

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A California sea lion with a salmon. Photo: U.S. Army Corps of Engineers.

Sea lions took a big bite out of the winter and summer steelhead runs at Bonneville Dam in 2017, according to a report by the U.S. Army Corps of Engineers. The final report on sea lion predation on fish – salmon, steelhead, sturgeon, and lamprey – at the dam last year is available here.

“The low run size and high percentage of steelhead consumed by pinnipeds in 2017 is alarming, and warrants particular attention from fish and wildlife managers,” the report warns.

An estimated 322 summer and winter steelhead were consumed (the Corps did not break down the number by species), which equates to 9 percent of the run, nearly twice the impact in 2016. The run totaled 3,241 fish during the study period, which ran from January 1 through June 2, 2017. Last year the run during the sampling period was 5,262 steelhead; 302 were consumed, which was 5.4 percent of the run.

The Corps documented a total of 92 individual California sea lions, 63 Steller sea lions, and one harbor seal in the tailrace areas of Bonneville Dam during the study period. In all, an estimated 5,384 adult salmonids (salmon and steelhead) were consumed by California and Steller sea lions in 2017, which equates to 4.7 percent of the salmonids that arrived at the dam during the study period. That’s less than the 9,525 consumed in 2016 and 10,859 in 2015. But it is a significant portion of the overall run, which was smaller in 2017 than in previous years. Of the 5,384 consumed, Steller sea lions took 3,242, which equates to 2.8 percent of the run, and California sea lions consumed 2,142, which was 1.9 percent of the run. The lone harbor seal was not observed killing salmonids at the dam.

The States branded 92 California sea lions for potential removal if they continue to cause problems for fish, and also branded 12 of the larger Steller sea lions, which also were notably voracious.

“We documented an increasingly high number of Steller sea lions during 2017,” the Corps reported. “Spring Chinook were consumed at similar levels as in 2016, but [in 2017] were primarily consumed by Steller sea lions, the first instance where Steller sea lion consumption was markedly greater than California sea lion consumption.”

The number of Stellers at the dam was 15 percent greater in 2017 than in 2016. The number of Californias at the dam, however, was 38 percent lower than in 2016, and they didn’t show up in numbers until the end of March, when the small and delayed spring Chinook run arrived, according to the report.

Here is a species-by-species breakdown of the fish consumption:

  • Spring Chinook: An estimated 4,951 fish were consumed, which equates to 4.5 percent of the run. Of these, Stellers consumed 2,860, which equates to 2.6 percent of the run, and Californias consumed 2,091, which was 1.9 percent.
  • Summer and Winter Steelhead: An estimated 322 fish were consumed, which equates to 9.0 percent of the combined run. Of these, Stellers consumed 269, or 7.6 percent of the run, and Californias consumed 53, or 1.5 percent.
  • White Sturgeon: An estimated 24 White Sturgeon were consumed. Of these, Stellers consumed 20, and Californias consumed 4. The consumed sturgeon ranged from 2 – 5 feet in length.
  • Pacific lamprey: An estimated 191 lamprey were consumed by both species, and of these Stellers consumed 46 and Californias consumed 145. Lamprey predation was especially high in the tailrace of Powerhouse One, on the Oregon side of the dam.

As in past years, sea lions were hazed from boats and from the shore using non-lethal deterrents, which included cracker shells, rubber buckshot, boat chasing and underwater percussive devices known as seal bombs. It’s not very effective. According to the report, “hazing provides circumspect benefits that merit better evaluation.” Washington and Oregon fish and wildlife agencies trapped and removed 24 of the most problematic sea lions between early April and late May. The most voracious individual, a California, was observed consuming 23 fish in 22 days, which was low compared to other individuals in other years. The highest? A single California sea lion took 198 in 2010.

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