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Tagging Study Aims To Measure Extent Of Pinniped Salmon Predation From River Mouth To Bonneville Dam
Posted on Friday, December 17, 2010 (PST)

Northwest Fisheries Science Center researchers ventured into new territory this past spring with a first step toward evaluating just what sort of an impact predatory sea lions and seals might be having on salmon in the 146-mile stretch of the Columbia River from its mouth to Bonneville Dam.

 

The pilot study, “A Study to Evaluate Sampling and Tagging Methods for Estimating Survival of Adult Chinook Salmon through the estuary and lower Columbia River,” in its first year aimed to learn how to best catch, handle and implant fish with PIT and acoustic tags for tracking, and obtain preliminary estimates of how well the fish survive after their release back into the river.

 

The study employed commercial fishermen, who used so-called “tangle nets” in the estuary near Astoria, Ore., to corral returning spawners. The tangle nets do not ensnarl and smother the chinook like larger meshed gill-nets.

 

The spring chinook’s physical condition was then quickly assessed and only the most robust-looking fish tagged before their return to the Columbia. The process took about a minute and a half on average per fish, according to the NWFSC’s Michelle Rub. Various methods of handling were investigated, all assuring the salmon were not long out of water.

 

The researchers would like to repeat the pilot study investigations in 2011, provided funding is found for the work. It was funded by NOAA Fisheries this year.

 

Then, once the scientists identify the proper methodologies to be used, a full-fledged three-year study would be launched to determine what the spawner losses are in the lower river, something that’s never been evaluated. Hopefully that research will be conducted in collaboration with marine mammal scientists to help evaluate what portion of lower river losses can be attributed to sea lions and seals, Rub said.

 

Assessing the overall level of lower river mortality is, in a scientific sense, much easier than assessing the lion’s share.

 

“To make that leap, that it’s sea lion predation, that’s more complicated,” said Rub, who previewed the 2010 research data Tuesday for the Northwest Power and Conservation Council’s Fish and Wildlife Committee.

 

The researchers from mid-April through the first week in June captured and outfitted 233 spring chinook salmon spawners with PIT-tags and another 100 with PIT and acoustic tags. The fish were released on 12 different days in groups ranging from three to 58 fish. The fishing was better early in the study period.

 

The releases included 174 PIT-tag-only fish that were later identified through genetic analysis as having originated from tributaries above Bonneville in the mid- and upper Columbia and the Snake River basin. 

 

Of the PIT-tagged fish 74 percent were detected passing up and over Bonneville’s fish ladders, having swum 234 kilometers upriver and passed through the gauntlet of sea lions that assembles each spring below the dam to feast on salmon and sturgeon, primarily. Of the 26 percent that didn’t make it, researchers estimated that 13 percent died because of the stresses resulting from their capture. Rub said the tangle net mortality estimates are probably on the high side, given the selective tagging and release of only unscarred, healthy looking fish. An estimated 7 percent of the missing fish disappeared into commercial fishing nets and sport boats.

 

The average travel time for the PIT-tagged fish was 16 days from the lower estuary. Travel time ranged from six to 35 days.

 

That left an estimated 10 percent “unexplained” mortality in the lower river. The researchers “highly suspect that the pinnipeds are eating the fish,” Rub said, but that has yet to be scientifically proven.

 

Rub said ideally the research would be expanded next year across time. The study got off to a late start because of funding issues and really only tapped into the back end of the spring chinook run, which has hit its peak in early May, as calculated at Bonneville Dam. An earlier start would also allow more data collection when more pinnipeds are in the river. May is also the time when many of the sea lions begin leaving the river to return to breeding grounds elsewhere, and the number of spring chinook is dwindling.

 

There are plenty of mouths to feed in the Columbia, particularly in springtime. Since passage of the Marine Mammal Protection Act in 1972, the Columbia River pinniped population has increased steadily. As many as 7,000 pinnipeds now reside in the lower Columbia River for all or part of the year, including 3,000 California and Steller sea lions and 4,000 harbor seals.

 

The vast majority, more than 90 percent, of those animals spend their time in the lowest part of the river.

 

“Those animals are not very well studied at all” in terms of impacts on salmon runs, Rub said.

 

Efforts have been ongoing since 2002 to document predation taking place each spring in the immediate vicinity of Bonneville Dam. The U.S. Army Corps of Engineers’ study has shown that “expanded” salmon consumption predation on salmon at the dam alone has ranged from 0.4 to 4.2 percent in any given year since 2002. The observed catch is expanded to adjust for missed observation time during daylight hours (mid-day break and weekends).

 

Rub said that researchers have used the best information available – the Corps consumption data and bio-energetics modeling estimating pinnipeds’ likely food needs during their time in the river -- to estimate how much the marine mammal population as a whole would potentially consume in a year. The effort produced an estimate that from 10 to 24 percent of the adult spring/summer chinook salmon return in 2008 (22,500-57,000 fish) could have been consumed by marine mammals.

 

One of the theories underpinning the study is that unaccounted for lower river spring chinook losses could be throwing off other calculations. As an example, smolt-to-adult return rate assessments for upriver stocks are often based on returns to Bonneville Dam.

 

As such, any ‘natural mortality’ (e.g. any mortality not due to fishing) that might have occurred in the estuary or lower river gets attributed to the ocean phase of the salmonid life history, Rub said.

 

That could:

 

-- mask important stressors that adults may encounter in the estuary and lower river;

-- underestimate the true benefits of conservation measures implemented at earlier life history stages, and

-- affect predictions of run size.

 

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