The tailrace at Bonneville Dam was once a playground and feasting area for dozens of California sea lions. But as states and tribes have worked to remove these salmon-eating pinnipeds, they've slowly been replaced by their larger and much hungrier cousins—Steller sea lions.
Now, with new authority to lethally remove these larger predators, fish managers are hoping the new effort to capture and euthanize Steller sea lions will be as successful as it's been with California sea lions.
According to the U.S. Army Corps of Engineers' annual report on pinniped predation below the dam, the number of California sea lions and their impact on salmon, steelhead and other fish has plummeted.
After reaching a high of 195 in 2015, the estimated minimum number of individual California sea lions dropped to 26 in 2019, and 34 in 2020, when operations to remove them were hindered by COVID-19 restrictions.
Biologists believe that after spending months feeding on the Columbia's salmon runs, the sea lions return to their breeding grounds with news of its bounty and come back with their friends a couple months later. Lethal removal prevents them from spreading that news.
Meanwhile, the number of Steller sea lions, which were not part of the removal program until last year, rose from about 26 individuals in 2009 to between 50 and 89 each year through 2019. Last year the Corps estimates there were at least 45 individual Stellers spending time below the dam, albeit without the benefit of branding or marking to distinguish individuals.
The latest annual report, published on April 14, provides pinniped numbers at the dam in the fall of 2019 and spring of 2020, as well as comparisons of numbers and predation impacts to prior years.
"The number of CSL (California sea lions) has been greatly reduced because of management efforts to remove qualifying animals," the report states.
Data for Steller sea lions is not as complete, since the Corps was focusing on California sea lions, which were authorized for removal. But minimum numbers of Steller sea lions in the report are based on the maximum number seen in one day, it says. Now that California sea lion numbers are greatly reduced, the Corps' focus has shifted to Steller sea lions.
"The number of SSL (Steller sea lions) remains at high levels and impacts from this species during the fall and winter are now being documented," the report says. "White sturgeon and winter steelhead are disproportionately impacted by SSL presence and abundance at Bonneville Dam and SSL now account for more than 90 percent of the Spring Chinook predation events. The recent efforts by management to enact removal authority of SSL may curb these impacts, but the sustained impacts to these fish populations should be noted by fish managers."
Since April 6, when Oregon, Washington and the Columbia River Inter-Tribal Fish Commission began their spring pinniped removal program at Bonneville, five Steller and four California sea lions at the dam have been captured and euthanized, Kessina Lee, southwest regional director for the Washington Department of Fish and Wildlife, said.
"The abundance of Steller sea lions is following roughly the same upwards trajectory through the month of April as last year's Army Corps data," Lee told NW Fishletter in an email. "Last week approximately 20 Stellers were present, and that has increased to approximately 30 Stellers this week. The number of animals hasn't peaked yet, so it's a little early to say how the full April numbers will match up, but based on the current trend it looks to be similar to last year."
Lee also said that the sea lions at Bonneville this spring are again predominantly Stellers. "There is a steady presence of a few California sea lions, but they are fairly sparse and only represent about 10-15 percent of the animals," she wrote.
Last spring, according to the Corps' report, there were few California sea lions from March through May, except for one day at the end of March when 34 subadults briefly visited the dam, but did not stay. Most days fewer than five California sea lions visited the dam, the report said.
Steller sea lions, meanwhile, averaged between 18 and 19 animals a day in April and May, and peaking at 45 on April 29, the report said. In August, states and tribes received an expanded permit from the National Marine Fisheries Service allowing them to lethally remove Steller sea lions as well as California sea lions, and with fewer restrictions. Last fall, they captured and euthanized six sea lions at Bonneville Dam.
At Willamette Falls—another gathering spot for sea lions—one Steller and five California sea lions have been killed under the new permit issued last year, the Oregon Department of Fish and Wildlife reports.
The agency notes that killing the sea lions to prevent upper Willamette steelhead from going extinct follows failed efforts to haze or relocate them.
"ODFW now considers sea lion predation one of the greatest threats to the survival of wild winter Willamette steelhead as well as native Chinook, lamprey and sturgeon," the agency said. "In 2017, the number of wild steelhead that cross Willamette Falls fell to an all-time low of just 512 adult fish, while marine mammals were responsible for taking an estimated 25 percent of the run."
The Corps' report notes that competition between humans and marine mammals for Columbia River salmon has occurred for hundreds of years, leading to persecution of sea lions through bounty programs which led to all-time low population numbers. After the Marine Mammal Protection Act was passed in 1972, populations rebounded, resulting in all-time high levels for the following 30 years.
But while sea lions were known to spend time in the Columbia Estuary, there is no evidence they traveled as far upriver as Bonneville Dam before its completion in the late 1930s or in the six decades after, the report said. California sea lions were first documented there in the late 1980s, and Steller sea lions were first noticed in 2003.
According to WDFW, Steller sea lions received federal protections under the Endangered Species Act in 1990. The population had dropped to about 18,000 animals in 1979, and then grew to more than 70,000 by 2013, prompting the National Marine Fisheries Service to remove protections for the population from southeastern Alaska through Washington and into Northern California. In Washington state, it was listed as a threatened species, but WDFW biologists found the state's population had fully recovered by 2015, and removed them from the state's list.
WDFW has recently examined this status under a periodic review, and recommends that Steller sea lions remain as unlisted, although they will continue to be protected under the Marine Mammal Protection Act. The agency is accepting comments to its recommendation to continue its delisted status.
The Corps notes that Columbia River fish managers have been concerned about sea lions and their impacts to ESA-listed salmon and steelhead for almost two decades.
In addition to work by states and tribes to remove both California and Steller sea lions, the Corps installed metal grating at all fishway entrances at Bonneville Dam, which was effective at keeping the animals out of the fishways, the report said.
It said the average number of Steller sea lions in the fall and winter of 2019 was 40 percent higher than the previous year, and 84 percent higher than the previous eight-year average. Steller sea lions have also been arriving at the dam earlier, the report said.
The report concludes, "The passage of the Endangered Salmon Predation Prevention Act gives management the authority to remove SSL and CSL without requirements of predation, hazing or residency. As shown through fish consumption and CSL abundance data, the removal of CSL over the last decade has contributed to a reduced impact.
"Future management actions for SSL may further reduce the impact to ESA listed salmon and sensitive stocks."