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Corps Report: Pinniped Predation Consumed 4.7 Percent Of Salmonids In 2017 In Bonneville Tailwater
Posted on Friday, March 16, 2018 (PST)

Some 5,384 salmonids were eaten by sea lions in Bonneville Dam’s tailwater in 2017 before they could migrate up over the dam. That’s 4.7 percent of the entire runs of spring chinook and summer/winter steelhead that passed the dam between Jan. 10 and  June 2, 2017, which is the period the U.S. Army Corps of Engineers monitored the number of pinnipeds and how many and what kinds of fish they consumed, according to a recent report.


The number of salmonids taken by sea lions in 2017 is far below the 2016 total of 9,525 or the 2015 total of 10,859.  However, the sizes of the spring chinook and steelhead runs were much smaller in 2017 so the percentage of the run consumed by sea lions remained largely the same.


Breaking the numbers down, some 4.5 percent of spring chinook, a total of 4,951, and 9 percent of summer and winter steelhead (322 fish) were taken by both Steller and California sea lions in 2017, which arrived at the dam last year in increasing numbers.


“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 says.


Of the 5,384 total salmonids devoured by sea lions, Steller sea lions (SSL) ate 3,242 – 2.8 percent of the combined run – and California sea lions (CSL) took 2,142 fish – 1.9 percent of the combined run.


SSLs consumed 2,860 spring chinook or 2.6 percent of the run and CSLs consumed 2,091 or 1.9 percent of the run. The run size as counted at Bonneville Dam during the observation period was 105,583 spring chinook.


SSLs consumed 269 winter and summer steelhead or 7.6 percent of the steelhead run of 3,241 at the dam, and CSLs consumed 53 or 1.5 percent of the run.


In 2016 (run size of spring chinook during the observation period was 186,633), CSLs ate 4.1 percent of the run and SSLs consumed 1.7 percent of the run. The 2015 percentage (run size was 239,326) was 3.3 percent by CSLs and 1 percent by SSLs. In the first year of observations, 2002, CSLs took 1,010 salmonids or 0.4 percent and SSLs were not observed consuming the fish.


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


During the monitoring period an average of 15.4 Steller sea lions and 5.1 California sea lions were observed each day at the dam.


The report, “Evaluation of Pinniped predation on adult salmonids and other fish in the Bonneville Dam tailrace, 2017,” was published March 5 by the Corps’ Portland District Fisheries Field Unit in Cascade Locks, Oregon (


The report’s authors are Kyle S. Tidwell, Bjorn K. van der Leeuw, Lindsay N. Magill, Brett A. Carrothers, and Robert H. Wertheimer.


The number of sea lions and the high percentage of salmonids they are eating is having an impact on fish listed as threatened or endangered under the federal Endangered Species Act.


Prior to the 1972 Marine Mammal Protection Act, according to the report, sea lions and seals were targeted for eradication, but the MMPA reversed that and pinnipeds reached all-time high levels in the following 30 years. However, salmonid stocks declined and many are now listed under the ESA.


“Thus, the flux of predator and prey in the Columbia River has now transitioned to high numbers of protected pinnipeds, and low levels of endangered salmonids,” the report says.


“Analyses of pinniped-salmonid interactions in or near the Columbia River suggest that all life stages of salmonids are at risk of predation by pinnipeds, and that some salmonid runs are at greater risk of depredation and potential extinction than others,” the report continues.


California sea lions were first seen eating salmonids at Bonneville Dam in the late 1980s and Steller sea lions were first seen in 2003. Their presence can turn into a feast as fish slow their migration at the dam while searching for fish ladder entrances, making them vulnerable to predation. In 2017, high water and swift river flows also contributed to a late run of chinook.


Sea lions typically follow spring chinook into the Columbia River, but their journey begins along the coastlines of California and Oregon in rookeries where they breed – Steller sea lions mostly at the Rogue Reef outcroppings along the southern Oregon coast and California sea lions in the Channel Islands off the coast of southern California.


Males lose weight when breeding, so they leave the rookeries after breeding and find their way to a better food supply. While sea lions – all males – have been seen at the mouth of the Columbia River for a couple hundred years, the report says, it’s only been in the last 20 years that they’ve journeyed as far up the river as Bonneville Dam for food.


About 7 percent of the California sea lions that make it to the river’s mouth go upstream to the dam. They are a mixture of ages: juveniles that are 2 – 4 years old; sub-adults that are 5 – 8 years old and adults, older than 8 years.


Some 156 individual pinnipeds were observed at the dam in 2017 (92 California, 63 Stellers and 1 harbor seal). That compares with 203 in 2016 and 264 in 2015. Prior to that, abundance varied from 166 in 2010 to 31 in 2002.


Four CSLs were observed transiting Bonneville Dam in 2017 and two were seen at The Dalles Dam. One animal remained up there year-round. The other three came back downstream. One of those animals was removed by the states.

The first SSL was observed in August 2016 and the number reached a peak of 63 May 5, 2017. No SSLs were present after May 31. SSL numbers were below the 10-year average in March and April, but above the 10-year average in May.


The first CSL was observed in October 2016, reaching a peak of 28 May 4 and then declining to 1 by June 2. Similarly, CSL numbers were below the 10-year average in March and April, but above average in May. This is the first year since 2012 that there were more SSLs present at the dam than CSLs.


While staking out their positions at the dam to feast, sea lions consume a variety of fish, not just salmonids. Some 24 white sturgeon were consumed, including 20 by SSLs and 4 by CSLs. The sizes of the sturgeon taken ranged from 2 to 5 feet. However, the number of sturgeon taken by sea lions has dropped considerably in recent years: consumption in 2016 was 90, 2015 was 44 and 2014 was 146, but even those years are far below the years 2007 – 2012. The highest loss was in 2010 when sea lions ate an estimated 3,003 sturgeon. Predation by sea lions on white sturgeon has declined by an average of 67 percent per year since 2011, the report says, while salmon predation has increased.


Some 191 Pacific lamprey were taken by sea lions in 2017. That’s 1.7 percent of the total catch of all fish by sea lions during the year, with SSLs taking 46 and CSLs consuming 145. The 2016 tally for lamprey was higher at 232, 4.8 percent of the total take of all fish by sea lions.


The Corps, along with the states of Oregon and Washington, have used a variety of deterrents or hazing to dissuade pinnipeds from becoming habituated to Bonneville Dam, but that has had a very limited impact on the pinniped population at the dam.


Boat-based hazing by the Columbia River Inter-Tribal Fish Commission began March 7 and ended May 19, a total of 29 days of hazing. A two-week hiatus in boat-based hazing occurred April 9 - 22 due to a tragic boating accident April 7 involving the CRITFC research vessel where fisheries technician Greg George lost his life, the report says.


Dam-based hazing by the U.S. Department of Agriculture began March 7 and ended May 31.


“Dam-based hazing is seemingly most effective when conjoined with boat-based hazing to effectively haze animals away from the fish ladder entrances and ‘drive’ hazed animals downstream, therein, providing a period of time when no pinnipeds are at the fish ladder entrances. Observations indicate that this period of time is commonly less than 20 minutes,” the report says.


Since 2008, Oregon Department of Fish & Wildlife and Washington Department of Fish & Wildlife have staffed a branding and removal program for California sea lions. They branded 18 CSLs and 12 SSLs and removed 24 CSLs from early April to late May 2017. They were authorized by NOAA Fisheries to remove as many as 92 CSLs. Some 57 CSLs were removed in 2016.


One of those CSLs removed by the states in 2017 consumed 23 fish in 22 days, a low compared to previous years. The highest consumption was in 2010 when a single CSL ate 198 fish.


“The removal of 24 predatory CSLs this year undoubtedly allowed more fish to pass, but the rate of removal has been hypothesized to be too low to significantly impact the rate of recruitment to the BON CSL population,” the report says.


The report concludes with this warning:


“As the winter steelhead passing Ballard Locks are now functionally extinct, in part, due to pinniped predation, the UWR winter steelhead are reportedly facing the same challenges due in part to pinniped predation. Managers need to take action and develop plans to ensure the continued existence of endangered and protected species. This should entail a review of the current CSL management plan, and the development of an equitable SSL management plan. For it is now through adaptive management that an ecological balance of interspecific competition can be found to ensure the continued survival of ESA-listed Pacific salmon. Without such management, anthropogenic and pinniped predatory pressures may synergistically function to extirpate runs of a vital prey resource.”


Also see:


--CBB, January 19, 2018, “West Coast California Sea Lion Population Has Rebounded; Meets Marine Mammal Protection Act Goal,”


--CBB, December 1,, 2017, “Recovery Of West Coast Marine Mammals Dramatically Increasing Consumption Of Chinook Salmon,”


--CBB, December 1, 2017, “NOAA Invites Comments On Lethal Removal Of Sea Lions At Willamette Falls; Threat To Listed Steelhead,”


--CBB, September 22, 2017, “Biologists Tell Council That Sea Lion Predation Puts Willamette Winter Steelhead At Extinction Risk,”


-- CBB, Aug. 11, 2017, “ODFW Analysis: With Continued Sea Lion Predation Willamette Winter Steelhead At Risk Of Extinction”


--CBB, June 23, 2017, “Oregon To Seek Permit To Lethally Remove Salmonid-Eating Sea Lions At Willamette Falls,”


--CBB, July 15, 2016, “NOAA Re-Authorizes States To Lethally Remove Salmon-Eating California Sea Lions At Bonneville Dam,”


--CBB, June 17, 2016, “Final 2016 Pinniped Report: Sea Lion Salmon Take Astoria To Bonneville Dam Could Be 20 Percent Of Run,”


--CBB, March 10, 2017, “Corps Report: Sea Lions In Bonneville Dam Tailrace In 2016 Consumed 4.5 Percent Of Spring Chinook,”


CBB, March 3, 2017, “Task Force On Sea Lion-Salmon Predation Mulls Ways To Reduce Pinniped Predation on ESA-Listed Stocks,”


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