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Predators’ Toll Below Bonneville Dips; Stellers Take Lions' Share For First Time In Study History
Posted on Friday, October 12, 2012 (PST)

The 2012 observation season – predominately from Jan. 1 through May 31 – marked “the first time adult salmonids were not the most preyed upon species by number” in the area immediately below the lower Columbia River’s Bonneville Dam, according to a research report released this week.

“This is due to the increased presence of Steller sea lions earlier in the season, and the reduced presence and predation of California sea lions in 2012,” according to the U.S. Army Corps of Engineers’ “2012 Field Report: Evaluation of Pinniped Predation on Adult Salmonids and Other Fish in the Bonneville Dam Tailrace, 2012.” The annual report is created by researchers Robert J. Stansell, Karrie M. Gibbons, William T. Nagy, and Bjorn K. van der Leeuw.

The Corps has used surface observations since 2002 to evaluate the seasonal presence, abundance, and predation activities of pinnipeds, including California sea lions, Steller sea lions and Pacific harbor seals in the area immediately below Bonneville Dam, which is located at about Columbia River mile 146.

The monitoring was prompted because of concerns about the impact from pinniped predation on Columbia/Snake river spring chinook salmon and steelhead trout that are listed under the federal Endangered Species Act. At the turn of the century fish managers noted that more and more California sea lions, in particular, where making their way upriver to feed on spawning salmon milling below the dam in search of a passage route – the hydro project’s fish ladders.

This year, observers completed more than 3,404 hours of daytime observations at the dam and saw pinnipeds catch and consume 2,962 fish of several species. White sturgeon were the primary prey item, comprising 45.3 percent (1,342) of observed, identifiable catches. Adult salmonids and Pacific lamprey were the second and third most commonly identified prey types, comprising 41.4 percent (1,017) and 0.9 percent (40) of total observed catch respectively.

The predator equation has shifted. Through most of the earlier years of the ongoing study, California sea lions represented, numerically, the largest sea lion species population at the dam. They preyed largely on salmon and steelhead. The larger Steller sea lions’ favorite prey had been, almost exclusively, white sturgeon. The sturgeon are not ESA listed, though state managers have expressed concerns in recent years about negative population trends.

In a twist of legal fate, California sea lions are not ESA listed but Stellers are. Both of the sea lions are protected from harm, however, by the Marine Mammal Protection Act.

The states of Idaho, Oregon and Washington has sought and received, through MMPA provisions, federal permission to trap and remove California sea lions that are having a negative impact on listed salmon and steelhead in the Columbia. But that authority, first granted in March 2008, has been challenged in federal court with some success. Removals, totaling 50 animals in all, took place during the springs of 2008-10 and 2012. Another challenge of lethal removal authority is now pending in Portland’s U.S. District Court.

The estimated number of individual pinnipeds observed at Bonneville Dam in 2012 was 112, lower than the last two years, but the fourth highest since observations began in 2002, according to the 2012 final report.

Steller sea lion numbers dropped a bit in 2012 to 73 individuals (down from 89 the previous year. But annual Steller sightings at the dam have generally trended upward since the first three animals of that species were spotted during the 2003 season.

California sea lion numbers dropped for the third straight year, from 89 in 2010 to 54 in 2011 to 39 this year (not counting one seen upstream of Bonneville since April 2011).

In 2012, adjusted (to estimate catch during unobserved time frames and allocated unidentified prey species to predator species) salmonid consumption in the area immediately below the dam amounted to 2,360 fish (or 1.4 percent of the run). Of that total 0.8 percent was credited to the Stellers.

The estimated number of adult salmonids consumed increased each year since 2005 until a decline in 2011 and 2012, which has the lowest predation estimate since 2002, the first year of the study.

The estimated percent of the run consumed has declined each year since a high of 4.2 percent in 2007, reflecting an increase in the run size each year since 2007, at least until drop-offs in 2011 and 2012.

Steller sea lions were the primary predator of salmonids in 2012 for the first time over the course of the study, accounting for 53.3 percent of the 1,216 observed catches.

California sea lions accounted for 46.7 percent of the salmonid catches below the dam this year. This percentage is lower than what was seen in any previous year, and observed salmonid catch by Stellers increased from 0.3 percent of the run in 2007, to 3.8 percent in 2008, to 10.1 percent in 2009, to 16.2 percent in 2010, to 29.1 percent in 2011 and then 53.3 percent in 2012, the report says.

“In 2012, the expanded white sturgeon consumption estimate for our study area was 2,227, continuing the upward trend of predation on sturgeon in the Bonneville Dam tailrace,” the report says. “When unidentified catch was divided proportionally according to daily catch distributions and added to the expanded sturgeon consumption estimate, the adjusted consumption estimate was 2,498. That count is the second highest since the study began tallying white sturgeon predation events in 2005. The record total was 3,003 from Jan. 1 through May 31, 2011.

“White sturgeon were the most commonly observed prey for SSL [Stellar sea lions]. SSL made 100 percent of the observed sturgeon catches in 2012. Predation on sturgeon dropped off dramatically after the first week of April when spring chinook salmon began to show up and became the preferred prey of both SSL and CSL [California sea lions] by mid-April,” according to the report.

“SSL were observed consuming an estimated 627 sturgeon in the Bonneville Dam tailrace between October 1 and December 31, 2011. Adjusting for unidentified prey, the estimated total additional sturgeon consumed in the fall/winter was 828,” the report says. “More sturgeon predation occurs well below the Bonneville Dam tailrace area, but no systematical observation program has been conducted.

“When possible, observers estimated the lengths of sturgeon caught by pinnipeds in one foot increments,” the report says. The estimated lengths of sturgeon caught between 2006 and 2012 ranged from less than 2 feet long to more than 7 feet long, but 82.2 percent of the sturgeon captured by the pinnipeds were four feet long or shorter.

“There continued to be a large drop in both the CSL salmonid predation and CSL abundance in 2012 that began in 2011 to levels not seen since 2003,” according to the researchers. “These results show the impact of the three years of the CSL removal program conducted 2008 through 2010. It appears 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, has and makes further analysis of this program more difficult.”

The report can be found at:

http://www.nwd-wc.usace.army.mil/tmt/documents/fish/2012/sea_lion_hazing2012.html

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