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Report: Sea Lion Abundance Drops, But 'Sustained Impacts' to Fish Remain by K.C. Mehaffey NW Fishletter, March 13, 2023

An annual report on sea lion predation at Bonneville Dam concluded that while the lethal removal of pinnipeds appears to be reducing their numbers, fish managers should take note of the sustained impacts from years of predation on fish populations, including spring Chinook, steelhead, white sturgeon and chum.

The Feb. 17 report by the U.S. Army Corps of Engineers analyzes impacts on fish from Steller and California sea lions below the dam from July 2021 to June 2022. It is the Corps' second report since the federal Endangered Salmon Predation Prevention Act gave Northwest states and tribes authorization to kill Steller sea lions in the lower Columbia River. Prior permits only authorized lethal removal of California sea lions.

The report notes that during the 2021-2022 study period, state and tribal agencies trapped and euthanized 33 Steller sea lions and 14 California sea lions. Managers also lethally removed three California sea lions at Willamette Falls during that period, according to the Oregon Department of Fish and Wildlife.

Under the lethal removal permit, states and tribes are no longer required to mark and observe individual sea lions prior to killing them, so the Corps doesn't know how many different sea lions visited Bonneville Dam during the study period. Through observations, the agency reported a peak of 62 Steller sea lions at the dam. Their average daily count was 39 percent lower than the previous fall and winter, and 10 percent lower than the previous spring. The average count during both seasons was also lower than the previous 10-year averages. "The decline in SSL is a new result and likely the result of management's newly enacted authority to lethally remove SSL," the report states.

In addition, except for the occasional sighting of one individual, Steller sea lions largely stayed away from the dam for eight weeks during this study period as opposed to a six-week absence the previous year. "This is the first time since 2011 that SSL presence at the dam has gone down," the report stated.

The numbers of California sea lions have already been reduced through lethal removal. A maximum of 20 California sea lions were counted below the dam in the spring, which was similar to the previous spring. The report said a 40-percent increase in average abundance of CSLs relative to the spring of 2021 was due to short periods of high abundance. But the count averaged just three individuals per day, a 57-percent decrease compared to the 10-year average, the report says.

Sea lions have visited the mouth of the Columbia River for hundreds of years, but have only traveled to Bonneville Dam to forage for about 20 years, the Corps reported. All the Steller and California sea lions in the Columbia River system are males who leave their breeding grounds to find food.

California sea lions -- which have been lethally removed from the dam for several years -- only spend a few months each spring foraging for fish below Bonneville Dam. But the significantly larger and more aggressive Steller sea lions now spend most of the year in the lower Columbia River, leaving only for a two-month period in the summer. Steller sea lions are now the dominant pinniped species at Bonneville Dam.

Currently, the Corps only monitors predation by sea lions when 20 or more sea lions are present, and the report's fish consumption estimates apply only to the periods and tailraces being observed, the report notes. Like previous years, all three tailraces were sampled in the spring, and only the Powerhouse 2 tailrace was sampled in the fall and winter.

In the spring -- despite the presence of nearly three times as many Steller sea lions compared to California sea lions -- the Corps calculated that CSL consumed about 1.5 percent of the run, while SSL consumed 1.6 percent of the run. "Despite their low abundance, CSL consumed as many spring Chinook Salmon as SSL," the report stated.

That could be because Steller sea lions are stealing fish captured by California sea lions, or because California sea lions are more motivated to kill spring Chinook since they are only at Bonneville Dam for a few months, the report noted.

Additionally, consumption by Steller sea lions could be underreported if they are eating fish too quickly for observers to notice or consuming them underwater where observers cannot see them.

"[A]dult SSLs can swallow sizeable spring Chinook Salmon almost whole in a matter of seconds, whereas adult CSLs typically stay at the surface and break the fish into smaller pieces," the Corps report noted. This difference likely influences the ability and confidence of observers to document prey eaten by Steller sea lions, which may be higher than observations indicated, it said.

"Ultimate causation of the difference cannot be determined but should be monitored in coming years as removals continue as the potential differences in behavior or motivation to kill spring Chinook Salmon would impact the monitoring program and evaluation of the removal program," the report says.

The report also highlights specific concerns about impacts to steelhead and white sturgeon.

"The number of SSL remain at high levels and new impacts from this species are now being documented. White Sturgeon and winter steelhead are disproportionately impacted by SSL at Bonneville Dam," the report states.

The Corps noted that in 2019, 13 percent of the entire steelhead run was consumed by pinnipeds, and Steller sea lions ate the vast majority of them. Because sampling did not occur during most of the winter steelhead season last year, the drop in steelhead consumption to 8 percent of the run is not a complete picture, the agency noted. The Corps suggested that sea lion predation on steelhead at Bonneville Dam deserves attention due to these previously documented impacts by Steller sea lions and low returns in recent years.

Similarly, the Corps said, trends of white sturgeon consumption by sea lions warrant immediate attention by managers since impacts may be more severe than previously believed, the report says. It notes that while predation on white sturgeon in the spring has dropped, fall consumption of white sturgeon by Steller sea lions now more than makes up for the reduction. In fall 2021, the estimated 1,119 white sturgeon eaten by Steller sea lions was twice the estimate from previous years.

"Why more fish are killed in the fall and winter than the spring is unclear, but the additive mortality of White Sturgeon over time at [Bonneville Dam] may be contributing to the questionable status of the stock," the report says.

The agency also noted that sea lion consumption of chum at Bonneville Dam is likely a subset of the chum eaten closer to Pierce and Ives islands where they spawn, so more attention should be given to predation impacts on chum.

The Corps concluded the lethal removal of California sea lions over the last decade has contributed to a reduced impact on ESA-listed fish, and a similar effort to remove Steller sea lions shows promise as a management tool for further reducing impacts to listed and sensitive stocks.