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Sea Lion Predation: Most At-Risk Spring Chinook From Clearwater, Salmon, Icicle, Deschutes, Umatilla
Posted on Friday, August 03, 2012 (PST)

Sea Lion predation dynamics change from year-to-year below the lower Columbia River’s Bonneville Dam but a constant, it appears, is that early arriving salmon stocks, including some protected under the Endangered Species Act, take a bigger hit, according to a research article published last week in the American Fisheries Society’s journal, Transactions.

Spring chinook salmon start their run upriver run with a trickle that slowly builds to a peak, then day-by-day diminishes in number until the return is complete. Adult “upriver” spring chinook salmon are fish bound for hatcheries or their natal tributaries upstream of Bonneville in Idaho, Oregon and Washington.

Data shows that early arrivals, because they are relatively small in number, are taken in larger numbers – percentagewise -- than stocks that approach the dam later in season.

“…very low salmonid density appeared to be associated with high predation risk,” according to the research paper produced by researchers from the University of Idaho and the U.S. Army Corps of Engineers.

The California and Steller sea lions have been observed over the course of studies that began in 2001 after, the marine mammals  had uncharacteristically began to travel in bunches up the 146 miles of the Columbia River in spring and winter to huddle, and forage, below Bonneville Dam.

“The recent sea lion aggregations at Bonneville Dam appear to be unprecedented, with no historical observations or archaeological records of otariids found along the Columbia River farther than about 150 km upstream from the Pacific Ocean (Lyman et al. 2002),” the research paper says. “Possible explanations for the novel Bonneville aggregation include sea lion population recovery and range expansion (Carretta et al. 2007) or declines in traditional prey species in the estuary or ocean (e.g., NMFS 2009), or both.

“California sea lions also may have originally responded to large Columbia River salmonid runs in the early 2000s.”

The 2002-2010 data reviewed for the research paper showed that the sea lions began to assemble in midwinter, well ahead of the first arrivals of spring chinook salmon.

“Springtime sea lion abundance steadily increased from 2002 to 2010 and the aggregation formed earlier each winter,” the paper says.

Because of the relative scarcity of salmon during the early part of the runs, sea lions took a higher percentage of the fish. Later, when the spring chinook salmon runs built toward a peak, sheer numbers “swamped” the predation numbers, effectively reducing the percentage of any given stock taken on any given day.

The research paper melds information gathered by the Corps in its pinniped observation study that aims to enumerate sea lions and seals near the dam and document their surface feeding targets species by species, and a UI radiotelemetry study that has since 1996 monitored spring- and summer-run chinook salmon passage through the tailrace and fishways at BonnevilleDam and on their subsequent upstream homing migration.

“We hypothesized that Chinook salmon risk would vary as a function of heritable among-population differences in migration timing (e.g., Quinn et al. 2000; Keefer et al. 2004b; Waples et al. 2004),” the paper says. “More specifically, we expected that salmon populations with relatively early migration timing would be most at risk because the relative predator density (number of predators / number of prey) has been highest early in the spring.”

That hypothesis appears to prove true based on statistical modeling of data gleaned from the two studies.

“The population risk model clearly indicated that Chinook salmon from early migrating populations were most at risk of sea lion predation” below the dam, according to report.

“These included salmon from the Clearwater, Salmon, Icicle, Deschutes, and Umatilla rivers,” according to the paper, “Use of Radiotelemetry and Direct Observations to Evaluate Sea Lion Predation on Adult Pacific Salmonids at Bonneville Dam.”

Lead author is Matthew Keefer of the University of Idaho’s Department of Fish and Wildlife Sciences. Co-authors are Robert Stansell, Sean Tackley, William Nagy and Karrie Gibbons of the Corps and Christopher Peery of the U.S. Fish and Wildlife Service and Christopher Caudill of the University of Idaho.

“Of these, the Salmon and Icicle river salmon populations are listed as threatened under the ESA,” the paper says. Icicle River feeds into the Wenatchee and then the Columbia River in central Washington. The Salmon originates in central Idaho’s high country and flows into the Snake River at the Idaho-northern Oregon border.

The article can be found at:

The latest issue of Transactions includes a variety of research reports focused on Columbia River topics.

-- A paper produced jointly by NOAA Fisheries Service, Idaho Department of Fish and Game and U.S. Geological Survey researchers, “Quantifying Cumulative Entrainment Effects for Chinook Salmon in a Heavily Irrigated Watershed,” looks at the outmigration of juvenile chinook salmon from central Idaho’s Lemhi River basin.

“Under median-streamflow conditions with unscreened diversions, the estimated cumulative effect of the diversions was a loss of 71.1 percent of out-migrating smolts due to entrainment.

“This is a large potential source of mortality, but screening is an effective mitigation strategy, as estimated mortality was reduced to 1.9 percent when all diversions were screened. If resources are limited, targeting the diversions that remove a large amount of water and diversions in locations with high fish encounter rates is most effective,” the paper says.

“Our modeling approach could be used to quantify the entrainment effects of water diversions and set screening priorities for other watersheds.”

-- “Evidence for Parr Growth as a Factor Affecting Parr-to-Smolt Survival,” produced by U.S. Fish and Wildlife and USGS researchers, used “mark-recapture analyses to understand the factors affecting survival of fish estimated between rearing in riverine habitat and dam passage.”

“We suggest that parr growth and reservoir velocity were directly proportional to parr-to-smolt survival because fast growth and downstream movement reduces the time when fish are vulnerable to predators,” the paper says. “The effect of reservoir velocity comports with previous published studies and supports management efforts to increase reservoir velocity.

“Few if any published studies explicitly relate parr growth measured on individual fish to survival estimated for their cohorts in freshwater. This study provides empirical evidence that upholds the long-held belief that any anthropogenic activity that reduces growth of juvenile salmonids during freshwater rearing has the potential to reduce their survival.”

-- “Survival and Growth of Juvenile Pacific Lampreys Tagged with Passive Integrated Transponders (PIT) in Freshwater and Seawater” reports on research intended to better understand the biology Pacific juvenile and adult lamprey and factors contributing to their decline in recent decades in the Columbia River basin.

“We developed a safe and efficient technique for tagging juvenile Pacific lampreys with passive integrated transponder (PIT) tags.”

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