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Sea lion predation of salmon increases

Steller sea lion consumption of spawning salmon and steelhead in the waters below the lower Columbia River’s Bonneville Dam continued its upward trend during the late winter-spring of 2013, according to a year-end report prepared by a U.S. Army Corps of Engineers research team.

Consumption of white sturgeon below the Corps-operated hydro project, however, was the lowest in years.

The Corps has since 2002 been from early winter through May marine mammal predation in the area below the dam.

Since the turn of the century a growing number of California sea lions, in particular, had been assembling to feed on spring chinook salmon and steelhead headed upstream to spawn. Five different fish stocks that pass the dam during that season are protected under the Endangered Species Act. Also targeted by sea lions are unlisted fish species such as lamprey and white sturgeon.

California sea lions, males that wander up the Pacific Coast to feed between breeding seasons, have in recent years have been trapped and removed from the below-Bonneville area as a means of reducing their impact on wild, protected salmon. The often lethal removals by the states of Oregon and Washington have been allowed under a Marine Mammal Protection Act exemption.

The Steller sea lions, which also have been protected under the ESA, have been subjected to non-lethal hazing as an attempt to disrupt their predation activities below the dam, but they cannot be harmed or killed.

(See CBB, Oct. 25, 2013, “Steller Sea Lions Delisted: Gives States Option Of Seeking Lethal Removal Below Bonneville Dam” http://www.cbbulletin.com/428830.aspx)

The Corps’ “2013 Field Report: Evaluation of Pinniped Predation on Adult Salmonids and other Fish in the Bonneville Dam Tailrace,” notes that the researchers adjusted salmonid consumption estimate by pinnipeds in the Bonneville Dam tailrace for the period Jan. 1 through May 31, 2013 was 2,928, “which is slightly higher than last year, but still lower than any year since 2003.” Observers positioned along the top of the dam record predation events; the predator species and, when possible, the prey species.

The report can be found at:

http://www.nwd-wc.usace.army.mil/tmt/documents/fish/

“The California sea lion portion of that estimate (1,497) was also slightly higher than last year, although it is the second lowest observed since the first year observations began in 2002,” the report says. “However, the Steller sea lion component (1,431) continues the trend of increasing every year since 2002.

“The daily average abundance of California sea lions in 2013 (3.0 per day) was similar to last year (3.2 per day) both of which are the lowest since 2002,” the Oct. 30 said of the CSL presence at the dam. “However, the daily average abundance of Steller sea lions in 2013 (13.0) continued to be higher than that for California sea lions for the fourth consecutive year.”

“The estimated number of individual pinnipeds observed at Bonneville Dam in 2013 was 136, higher than last year but only the third highest since observations began in 2002,” the report says. “SSL numbers increased a bit in 2013 to 80 individuals. The maximum of 41 SSL observed on one day in 2013 was higher than the past two years but not as high as the 53 SSL seen in 2010.

“CSL numbers increased in 2013 to 56 (excluding four upstream of Bonneville dam). The maximum number of CSL seen on any one day was 21 this year. This year was similar to 2010 in that record numbers of CSL were seen at the east mooring basin in Astoria,” which is located about 133 miles downstream of Bonneville Dam. A high percentage of the CSL visitors seen at Bonneville were younger animals that had not been seen at the dam in prior years.

The high count of different CSLs at the dam was 104 in 2003. The high count of SSLs was 89 in 2011.

In 2013, the expanded adult salmonid consumption estimate for the Bonneville Dam tailrace observation area was 2,714 or 2.2 percent of the adult salmonid run at Bonneville Dam from Jan. 1 through May 31.

Accounting for unidentified fish, the adjusted estimated consumption was 2,928 (or 2.4 percent of the run). A progressive series of tables, broken out for CSL and SSL, showing estimated salmonid consumption (interpolated for hours and days not observed), adjusted salmonid consumption (factoring in unidentified fish caught), and finally adding a nighttime consumption factor after hazing began (in 2006) is presented in the report’s Appendix A. The estimated percent of the run consumed by sea lions had declined from a high of 4.2 percent in 2007 to 1.2 percent 2012.

In 2013, chinook salmon were the most commonly identified prey species, comprising 97.5 percent (n=1,424) of observed adult salmonid catch.

Salmon populations identified with the highest risk included endangered/threatened stocks from the Clearwater and Salmon rivers in Idaho, the Umatilla and Deschutes rivers in Oregon, and the Icicle River in Washington, the report says.

The adjusted white sturgeon consumption estimate (635), exclusively by Steller sea lions in 2013, was only 20 to 50 percent of that observed during the past five years, the report says. The adjusted white sturgeon take total in 2013 was 635 as compared to 2,172 in 2010, 3,003 in 2011 and 2,498 in 2012.

“Whether this was due to a lack of sturgeon abundance or switching prey preference to salmonids is unknown. Predation during October through December 2012 was similar overall to 2011, other than there were a few California sea lions present in 2012 and sturgeon predation was about half that seen in 2011,” the report says.

The report says that the jury is still out as regards the effects of California sea lion removals. A total of 58 of the CSLS have been trapped and removed since 2008. Some have been placed in zoos or aquariums; the majority have been euthanized.

“Although higher than last year, CSL salmonid predation was still lower than any other year since 2003,” according to the report. “Mean daily CSL abundance in 2013 was again lower than any year since 2002 and continues the downward trend that began after 2008, although maximum daily CSL and individual CSL identified increased somewhat since last year.

“These results provide some evidence that the impact of the CSL removal program conducted since 2008 may be at least partially responsible for reducing the CSL abundance and predation on salmonids by CSL at Bonneville Dam.

“However, the unusual event of the influx of large numbers of new CSL males showing up at Bonneville Dam tailrace in 2010 and 2013, coupled with the virtual halting of removal actions in 2011, make further analysis of this program more difficult. It is also likely the reduction in predation on the early Chinook salmon runs is due to the removal of many of the returning CSL that would arrive earlier each year waiting for the arrival of Chinook salmon. The increasing presence and salmon predation by SSL at Bonneville Dam could also continue to complicate the issue if current trends persist,” according to the 2013 final report.

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