The good news for salmon advocates was that California sea lions’ consumption of fish below the Columbia River’s Bonneville Dam was down in the spring of 2011 after steady increases each year since 2004.
But the bad news may be that Steller sea lions were snapping up more spring chinook salmon and steelhead than ever before, according to the “2011 Field Report: Evaluation of Pinniped Predation on Adult Salmonids and Other Fish in the Bonneville Tailrace, 2011” prepared by U.S. Army Corps of Engineers researchers Robert Stansell, Karrie M. Gibbons, William T. Nagy, and Bjorn K. van der Leeuw.
The report can be found at:
The Corps since 2002 has been observing the behavior of predatory California sea lions, as well as other pinnipeds, in an attempt to estimate the big marine mammals’ impact on steelhead and spring chinook salmon stocks that are listed under the Endangered Species Act.
Until recent years Stellers in the vicinity seemed to focus almost exclusively on white sturgeon that have tended, at least in recent years, to huddle together in relatively large numbers below the dam. And the California sea lions stuck mostly to chinook salmon.
The CSLs in 2011 did again take the majority of the salmon and steelhead that were observed taken below the dam, but their share is slipping.
“The CSL were the primary predator of salmonids, accounting for 70.9 percent (n=1,550) of the 2,186 observed catches in 2011,” according to the report released this week. “This percentage is lower than was seen in previous years, as observed salmonid catch by SSL increased from 0.3 percent (n=12) in 2007, 3.8 percent (n=162) in 2008, 10.1 percent d (n=300) in 2009 and 16.2 percent (n=634) in 2010 to 29.1 percent (n=636) in 2011.”
The estimates of salmon take includes observed daytime catch that is interpolated for hours and days not observed and adjusted to factor in captured fish that observers could not identify.
Observers were stationed at each of the three major tailrace areas of Bonneville Dam -- Powerhouse 1 near the Oregon shore, Powerhouse 2 on the Washington shore and in the spillway mid-river spillway. They used binoculars to observe and record pinniped presence, identify and record fish catches, and identify individual California and Steller sea lions when possible.
Observers completed over 3,315 hours of observations between Jan. 7 and May 31, 2011.
“During that period, observers saw pinnipeds catch and consume 4,489 fish of several species. Adult salmonids were the primary prey item, comprising 48.7 percent (n=2,186) of observed catches. White sturgeon and American shad (Alosa sapidissima) were the second and third most commonly identified prey types, comprising 30.1 percent (n=1,353) and 2.1 percent (n=93) of total observed catch respectively.
“Observers were unable to identify 18.6 percent (n=833) of the fish caught and consumed by pinnipeds during this period. This was higher than any previous year and was primarily due to Steller sea lions taking prey in the far downstream range of the viewing areas,” the report says.
“While surface observations are a useful tool for assessing sea lion diet at Bonneville Dam, pinnipeds can consume smaller prey underwater unseen by observers, so all consumption estimates and associated impacts outlined in this report should be considered minimum estimates,” according to the report.
The report notes that “the last few years, SSL were often observed swallowing steelhead whole, suggesting that they could consume steelhead and chinook salmon jacks entirely below the surface. All consumption estimates provided are minimum estimates, but it should be noted that SSL predation may be underestimated more than CSL predation by the current surface observation methods.” Stellers are the larger of the two species with some males topping out at as much as 2,500 pounds. The largest California sea lion males don’t get much over 1,000 pounds, according to species descriptions posted by the National Marine Fisheries Service.
The researchers’ adjusted salmon consumption estimate for 2011 includes 2,689 taken by CSLs and 1,282 taken by SSLs. The total salmonid (salmon and steelhead) take estimate amounted to 1.8 percent of the total number of spawners reaching the dam, which is located 146 river miles from the Pacific Ocean. That’s the lowest percentage of take estimated since at least 2004.
“The estimated percent of the run taken has declined each year since a high of 4.2 percent in 2007, reflecting an increase in the run size each year since 2007,” until a slight dropoff this year.
The estimates of the actual number of salmonids taken by salmonids has risen each year, until 2011. The peak estimate was 6,321 in 2010 (2.4 percent of the run that year; the estimate this year is 3,970).
“In 2011, the expanded white sturgeon consumption estimate for our study area was 2,178, continuing the upward trend of predation on sturgeon in the Bonneville Dam tailrace,” according to the report. “When unidentified catch was divided proportionally according to daily catch distributions and added to the expanded sturgeon consumption estimate, the adjusted consumption estimate was 3,003.”
During the first year (2005) that Steller sea lion consumption of sturgeon was tallied only one was observed taken. But the number of Stellers camped out below the dam in late winter and spring, and their consumption, has increased every year since.
“White sturgeon were the most commonly observed prey for SSL. Stellers made 99.8 percent (n=1,350) of the 1,353 observed sturgeon catches in 2011.”
The sea lion population that zeroes in on salmon, white sturgeon, shad, lamprey and other fish below the dam has become dominated by the Stellers.
“The estimated number of individual pinnipeds observed at Bonneville Dam in 2011 was 144, lower than last year but the second highest since observations began in 2002,” the report says. “SSL numbers continued to rise in 2011 to 89 individuals. The 32 SSL observed on one day in 2011 was not as high as the 53 SSL seen last year.” In the first year of the study there were no SSLs observed at the dam, but their number started to grow slowing with each passing year. The number at the dam nearly tripled from 2009 (26) to 2010 (75).
“CSL numbers dropped in 2011 to 54 after jumping up to 89 in 2010,” the report says. The CSL total matched 2009’s, which was the lowest number since the first year of the study.
“Over the past two years, unusually large numbers of CSL have moved north of California after the summer breeding season. In 2009 this was likely the result of a significant warm water event related to El Nino that caused many CSL to move northward in search of cooler waters and abundant prey.
In 2009 and 2010, increasing numbers of young, sub-adult sea lions have been observed at many locations in Oregon and Washington (Robin Brown, ODFW, Steve Jeffries, WDFW, pers. comm.),” the report says. “The increase in CSL at Bonneville Dam in 2010, many of which were not seen at the dam before, could be the result of this large group of young males exploring new areas, such as the Columbia River, to prey on fish. Many of these animals had not been seen at the dam before.”
The report suggests that the 2011 numbers may reflect the removal of 40 California sea lions, most of which were trapped below the dam, during 2008-2010. The states of Idaho, Oregon and Washington in the spring of 2008 was granted the authority to lethally remove individually identifiable California sea lions that are known to prey on listed salmon. The authority was granted by NOAA Fisheries via an exemption to Marine Mammal Protection Act protections. That avenue is not available to control Steller sea lions, which themselves are ESA protected.
NOAA’s decision was declared illegal by a federal appellate panel in November 2010.
The report noted the “… large drop in both the CSL salmonid predation and CSL abundance for 2011 to levels not seen since 2003. These results show the full impact of the three years of the CSL removal program conducted 2008 through 2010, as the full impact of those animals removed in 2010 could not be fully realized until the results of the 2011 season were in.
“It does appear to indicate that the removal program was gradually reducing the abundance and predation on salmonids caused by CSL. However, the unusual event of the influx of large numbers of new CSL males showing up at Bonneville Dam tailrace in 2010, coupled with the virtual halting of removal actions in 2011, have and will make further analysis of this program more difficult.
“The increasing presence and salmon predation by SSL at Bonneville Dam could also continue to complicate the issue, if current trends persist.”