Overall consumption of salmon by California sea lions has continued to rise during the past three years, but federal and state officials still feel they are making progress toward reducing predation by removing marine mammals from the area immediately below the lower Columbia River’s Bonneville Dam.
Studies show that the 40 sea lions removed from the area during those years were among the most voracious of the pinnipeds known to prey on salmon, steelhead and other fish species at Bonneville, according to the October research report, “Evaluation of Pinniped Predation on Adult Salmonids and Other Fish in the Bonneville Dam Tailrace, 2008-2010.” The report was produced by U.S. Army Corps of Engineers researchers Robert J. Stansell, Karrie M. Gibbons, and William T. Nagy.
Data collected by the researchers show that while those sea lions represented only 9.5 percent (40 of 420) of the sea lions identified over the years, they accounted for 36.5 percent (3,388 of 9,275) of all the salmonid catch events attributed to specific individuals. The removed animals had a history of staying longer at the dam and eating more fish per capita than other animals seen at the dam.
“This indicates that the removal program has indeed targeted those animals most likely to stay for a long time and consume many salmonids,” the report says. “Consumption estimates and presence metrics for 2008, 2009, and 2010 undoubtedly would have been higher if these select sea lions had not been removed.”
The Corps research was launched in 2002 to evaluate the effect of the California sea lions growing presence below the dam on salmon stocks headed upriver in springtime on their spawning run. The fish include stocks that are protected under the Endangered Species Act. Bonneville is the first hydro project the fish encounter upon their return from the Pacific Ocean.
Historically only a few California sea lions were observed at Bonneville, which is located 146 river miles from the Pacific. But as spring chinook salmon returns swelled early this century, so did the number of marine mammals camped out below the dam’s fish ladders. As many as 104 different California sea lions (2003) have been seen at the dam. The total was 82 in 2008, 54 in 2009 and 89 this past spring.
The number of individual sea lions observed at Bonneville Dam has increased from an average of 83.0 per year between 2002 and 2007 to 123.7 per year for the last three years, according to the Corps report. That is primarily due to an increase in the presence of Steller sea lions, which averaged 5.0 per year before 2008 and 46.7 from 2008 to 2010. Overall number of California sea lions at Bonneville each spring averaged 76.2 per year before 2008 and 75.0 during the past three years.
The adjusted salmonid consumption rate based on observations extrapolated to include totals for unobserved days were 4,294 fish in 2008, 4,037 in 2009 and 5,095 in 2010.
The overall number of salmon passing Bonneville increased from 2008 to 2009 and again from 2009 to 2010 so the actual percentage of the run taken by California sea lions shrank each year. The 2010 run, 267,184 passing Bonneville, was the second largest since 2002. The 2001 upriver spring chinook salmon run, as measured at the mouth of the Columbia, was the largest on record at 439,885.
The report is among the materials being considered by the Pinniped-Fisheries Interaction Task Force as it evaluates the effectiveness of a sea lion removal-deterrence program begun in 2008 by the states of Oregon and Washington, with the help of Columbia River Inter-Tribal Fish Commission member tribes, federal agencies and other entities. The states were granted lethal removal authority in March 2008 by NOAA Fisheries Service under Section 120 of the Marine Mammal Protection Act.
The 18-member task force, formed in 2007 to help evaluate the states’ application for lethal take authority, includes representatives from academic, scientific and conservation communities, tribes and federal and state agencies.
The task force was reconvened Oct. 25-26 in Portland, and will meet again Nov. 9-10. When it completes its deliberations, the task force will submit recommendations to NOAA Fisheries on how the program, initially approved for a five-year run, should proceed. The federal agency will then decide whether to continue the program unchanged for the next two years, whether to modify the removal authority and/or terms of its letter of authorization or to determine that the permitted lethal removals have been effective and disband the task force.
The number of California sea lions captured during the March 1-May 31 period over the past three years includes 37 trapped below Bonneville and three at Astoria, Ore., near the river mouth. Ten were accepted by zoos or aquariums, 22 were euthanized and five died accidentally. The approval allows the removal of up to 85 animals yearly though NOAA Fisheries predicted that the states would probably be able to capture and remove 30 of the big marine mammals at best in any given year.
“This is likely the cause of the decline in CSL mean daily presence and maximum numbers seen on any given day, as most of the removed individuals had returned many years and remained at Bonneville Dam for long periods of time,” the Corps report says.
Trapping next year would likely target about 35 California sea lions of the 78 animals that are now eligible for removal. That’s because 37 of the eligible animals have not been seen at the dam in two or more years and six others were not seen last year. Researchers know from past experience that sea lions that do not return in consecutive years are unlikely to return at all.
NOAA Fisheries approval letter says that the states can lethally remove “individually identifiable predatory California sea lions that are having a significant negative impact on ESA-listed salmonids.” To make the removal list a California sea lions must have been:
-- observed eating salmonids in the "observation area" below Bonneville Dam between Jan 1 and May 31 of any year; and
-- observed in the observation area below Bonneville Dam on a total of any five days (consecutive days, days within a single season, or days over multiple years); and
-- sighted in the observation area below Bonneville Dam after they have been subjected to active non-lethal deterrence.
The report noted that, despite the removal of some of the most successful predators in 2008 and 2009 the overall number of California sea lions visiting the dam grew in 2010.
“We expected the results from the 2010 season to show a steep decline in CSL numbers, which should have also resulted in reduced salmonid predation by CSL. However, this was not the case, as many new CSL ventured up to Bonneville Dam this year, if only briefly,” the report says. “It may be that removing 11 to 15 animals each year is not enough to prevent substantial recruitment of new individuals and increased predation, and that it would take more additional measures (e.g. the removal of about 30 individuals) each year to see and document a significant reduction in CSL numbers and salmonid predation.”
One sea lion this past year may have moved to No. 1 on the most wanted for removal list. The report notes that a pinniped branded as C287 took the most fish in one day at Bonneville Dam since observations began in 2002.
“He was seen to take 12 Chinook on April 12, 2010,” the report says. “If we use an average Chinook weight of 6.6 kg per fish (Brown, et al., 2010) this equates to about 85.8 kg in one day consumed. This is almost triple the maximum observed daily consumption by weight of that reported in Kastelein et al. (2000) from captive male CSL over 10 years old in the Netherlands. C287 was first observed at Bonneville Dam on March 22 this year, his sixth year observed at Bonneville Dam.”
“This is not to say every CSL consumes this many fish, but it does give us an indication of how unusual a situation pinniped predation at Bonneville Dam has become when compared to natural or captive consumption studies, and what some CSL are capable of consuming.”
The Corps report and other materials related to the task force evaluation of the removal program can be found at: