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2010 Juvenile Salmon Survival To Below Bonneville Dam Above Average, Less Fish Transported
Posted on Friday, October 08, 2010 (PST)

Survival of Snake River yearling chinook salmon and steelhead swimming down through the Columbia-Snake hydro system this year was above average (1993-2010) despite relatively low flow conditions, according to preliminary estimates for the spring migration.


The survival of steelhead passing downstream through eight dams and reservoirs was particularly strong this year, ranking only behind 2009.Steelhead survival from above Lower Granite Dam on the lower Snake to the tailrace below Bonneville Dam on the lower Columbia was 61.7 percent, which was well above the average of 40.4 percent but slightly less than the 67.8 percent survival in 2009.


NOAA Fisheries Service’s Paul Wagner this week previewed for the Technical Management Team preliminary data compiled this year as part of a long-running study headed by the agency’s Northwest Fisheries Science Center. The TMT’s federal, state and tribal members mull potential shifts of day-to-day hydro operational strategies intended to improve the survival of migrating fish, including numerous upriver salmon and steelhead stocks that are listed under the Endangered Species Act.


The survival memo is linked to the TMT’s Oct. 6 agenda:


Study results compiled through the monitoring of PIT-tagged wild and hatchery produced fish are summarized in a Sept. 13 memo from the NWFSC’s John Ferguson to Bruce Suzumoto, NOAA Fisheries assistant regional administrator, Hydopower Divison.


The summary notes river conditions and survival estimates, as well as preliminary estimates of the proportion of Snake River smolts that were collected at juvenile bypass systems at three lower Snake dams and transported downstream through the hydro system aboard barges.


The “summer memo” cautions that the results are preliminary and could be changed following the more detailed analysis to be undertaken for a final report due by year’s end. In the past differences have ranged up to 3 or 4 percent in estimated survival values.


Wagner said that the high survival last year was a bit of a surprise. The next best survival rate dating back to 1997 was 48.4 percent in 1997.


But 2009 was a relatively normal “water year,” which bodes well for river flows and fish migrations. The 61.7 percent survival rate in 2010’s low flow year is an “out of the ballpark” surprise, Wagner said. The final water supply forecast for flows past Lower Granite was 77 percent of normal. And that July 8 forecast was made after a drenching June that set precipitation records in many parts of the Columbia River basin.


Wagner said he had expected survival similar to 2007 (36.9 percent) when similar conditions prevailed during the spring migration – relatively low flows and high levels of spill which is provided to provide enhanced passage for migrating juvenile fish.


The memo gives some of the credit for improved survival in recent years to improvements at the dams.


“The adjustable spillway weir (ASW) installed in 2009 at Little Goose Dam was in its second year of operation. The removable spillway weir (RSW) at Lower Monumental Dam and the temporary spillway weirs (TSW) at John Day Dam were in their third year of operation in 2010. Also the new spillway guidance wall at The Dalles Dam, which was partially complete in 2009, was completed in March of 2010,” the memo said.


The weirs, which provide more surface oriented, “natural,” passage than turbines or bypass systems, and the guidance wall, which steers young fish toward the deeper, faster area of the river where they are less vulnerable to predators, were all installed with the goal of improving survival. The more surface oriented steelhead have shown a particular affinity for the new weirs.


The memo also says the “Estimated percentages of yearling Chinook salmon and steelhead transported from Snake River dams were among the lowest seen from 1995-2009.


“High spill percentages, in combination with surface bypass collection at each of the collector dams on the Snake River, resulted in low proportions of fish entering juvenile bypass systems,” the memo says.


Because flows were relatively low, planned spill took up a larger percentage of the river, particularly early in the migration season. At Lower Granite, as an example, the spilling of 20,000 cubic feet per second is required in springtime.


“If the flow is only 40 kcfs, and it was at times, it’s 50 percent spill,” Wagner said.


“For yearling Chinook in 2010, estimated percentages entering the bypass systems at Lower Granite Dam, Little Goose Dam, and Lower Monumental Dam were 26 percent, 26 percent, and 8 percent for wild, and 16 percent, 12 percent, and 2 percent for hatchery, respectively. The transport percentage was lower for hatchery than for wild Chinook salmon because of the difference in percentages entering bypass systems.


“For steelhead in 2010, the estimated percentages entering the bypass systems at Lower Granite, Little Goose, and Lower Monumental were 23 percent, 22 percent, and 6 percent for wild, and 20 percent, 23 percent, and 6 percent for hatchery, respectively. These bypass percentages are among the lowest estimated from 1995-2009 for both yearling Chinook and



The memo notes that the fish transportation operations began a week earlier this year at Lower Granite than in 2009, as well as four days earlier at Little Goose Dam and two days earlier at Lower Monumental Dam.


“Despite the earlier transportation start dates and relatively later run passage timing in 2010 than in 2009, the higher spill percentages and consequent lower collection rates resulted in lower percentages of fish transported,” the memo says.


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