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Ecology and salmon related articles

BPA Releases Draft
Juvenile Study of Spring Migrants

by Bill Rudolph
NW Fishletter, December 20, 2001

BPA released a draft report yesterday that takes a closer look at last spring's fish survival through the hydro system than a NMFS memo reported in September, but found that spill benefits were "not strongly exhibited." However, the agency said that didn't mean such benefits didn't really exist.

The earlier results were focused on Snake River spring chinook, which showed the lowest survival rates since PIT tag studies began in 1993. In its September memo, NMFS believed the low survivals were "a consequence of low flow and lack of spill. Because both conditions occurred simultaneously, determining how much each factor contributed to the decreased survival will be difficult." The agency said Snake spring chinook survived at a 24 percent rate, about half that found in a normal water year, but steelhead were hit a lot harder, showing less than a 3 percent survival rate to the Columbia River estuary.

Over 90 percent of the spring migrants (both chinook and steelhead) were barged from the Snake, so fewer fish were actually migrating inriver this year than on average. According to the latest report, "Only a very small percentage of unmarked Snake River fish were subjected to the poor migratory conditions faced by migrants passing downstream through the hydropower system."

The new report, completed by NMFS' Seattle-based science center and U.W. researcher John Skalski, found that Snake survivals were slightly higher, 27.6 percent for spring chinook and a little over 4 percent for steelhead. The chinook survivals were similar for upper Columbia spring chinook PIT- tagged at hatcheries ( 28 percent for Winthrop fish that must pass eight dams and 33.5 percent for Leavenworth fish that pass six dams) and higher (48.7 percent) for the summer-fall chinook released at Rocky Reach Dam that must pass seven projects.

The new draft report attempts to quantify the effects of spill on survivals in the lower reaches of the Columbia, but recognizes limitations in the analysis--from few PIT tag detections for some stocks to timing blocks for the study that weren't randomized because they were divided into pre-spill, spill and post-spill groups. It also said the results were confounded by another trend observed over the years, with survival increasing gradually through the early part of the season.

For nine different stocks, only two showed evidence that spill could account for increases in survival and they had such small sample sizes in the period after spill stopped, there were large standard errors and "poor power to detect differences.

"But in five of the stocks where survival during the spill period was significantly different from the other periods, it was higher," the draft report says." However, in the other two cases (Snake River spring-summer chinook salmon and steelhead migrating from Lower Monumental Dam to McNary Dam), survival was significantly higher in the pre-spill period than in the spill period."

The report also suggested that low steelhead survivals could have been affected by increased predation by Caspian terns; water clarity, and residualism.

"The fact that spill benefits were not strongly exhibited this year does not mean they do not exist," says the report. "As mentioned above, the experimental design that we happened upon this year was by no means ideal." The scientists said the low flows and limited or no spill in the drought year added 10 to 30 days' migration time from Lower Granite to Bonneville Dam, which left some fish to face higher reservoir temperatures.

They said a "more rigorous experimental design" would be needed to resolve the issues and that the feasibility of such a complex experiment "would require careful consideration."

A "preliminary" analysis of the spring migration completed by the Fish Passage Center said that increased spring survival evident between McNary and John Day dams was "a result of spill."

Bill Rudolph
BPA Releases Draft Juvenile Study of Spring Migrants
NW Fishletter, December 20, 2001

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