Preliminary Report Detailsby Mike O'Bryant
A preliminary report out of the NOAA Fisheries Northwest Fisheries Science Center in Seattle shows survival for juvenile salmon and steelhead through Snake and Columbia River reservoirs and dams this spring is near the 1995-2002 average.
The report that covers the past three years beginning with 2001, a drought year, shows that 2003 in-river survival for yearling chinook salmon and steelhead is similar to survival in 2002 and near average for the years 1995-2002. Survival is far better than during 2001 when Snake River steelhead survival fell to 4.2 percent in travels from Lower Granite Dam to Bonneville Dam.
The survival study, funded by the Bonneville Power Administration, used PIT-tagged hatchery salmon and steelhead to conduct the studies.
"Estimated survival for Snake River yearling chinook salmon in 2003 was near the average of past years through most reaches," the report says.
The mean estimated survival from the tailrace at Lower Granite Dam on the Snake River to the tailrace of Bonneville Dam was 53.2 percent in 2003, while it was slightly higher in 2002 at 57.8 percent and much lower during the drought year of 2001 when survival was estimated at 27.9 percent.
However, steelhead survival was up in all reaches of the Snake and Columbia rivers. Survival in 2003 was 30.9 percent, Lower Granite to Bonneville, and 26.2 percent in 2002. The report points to the reach of river between the Lower Monumental Dam tailrace and the McNary Dam tailrace as the area where steelhead take the greatest hit in the Snake River system. Survival through that reach in 2001 was 29.6 percent, 65.2 percent in 2002 and 70.8 percent in 2003, while survival from Lower Granite to Little Goose Dam was 80.1, 88.2 and 94.7 percent respectively, and from Little Goose to Lower Granite survival was 70.9, 88.2 and 89.8 percent.
Part of the problem for that reach -- Lower Monumental to McNary -- is predation by a Caspian tern population that has taken up residence during the juvenile migration on Crescent Island in the Columbia River just downstream from the confluence of the Snake and Columbia rivers.
"Survival has remained depressed in this reach since 2001, most likely due to avian predation, primarily by Caspian terns," the report says. "Of all the PIT-tagged fish leaving Lower Monumental Dam in 2001, at least 21 percent were later deposited on McNary pool by bird colonies. In 2002, the percentage of PIT tags found on bird colonies was lower (10 percent), but still represented a high avian predation on steelhead in the reach." Predation data is not yet available for 2003.
Jim Ruff of NOAA Fisheries added that the birds also eat about 1 percent to 2 percent of chinook and that steelhead appear to be more vulnerable to predation by the terns because they tend to be a larger target.
Buried in the numbers is a disparity in survival between chinook and steelhead from the upper Columbia River and those that originate in the Snake River. Some 53.2 percent of yearling chinook and 30.9 percent of steelhead survived the passage from Lower Granite Dam to the Bonneville Dam tailrace in 2003. However, survival from fish originating in the mid-Columbia River is much higher: 76.7 percent of yearling chinook that arrived at McNary Dam (data on these fish begin at McNary Dam because there are insufficient detection facilities in the mid-Columbia River) and 69.5 percent of steelhead survived the ride down through Bonneville Dam.
Looking even further into the numbers, survival at dams in the lower Columbia River begins to fall once fish reach John Day Dam. Survival from McNary Dam to John Day Dam, the next dam downstream, was 89.3 percent for yearling chinook and 87.9 percent for steelhead in 2003 for fish that originated in the Snake River. But survival from the John Day Dam tailrace to the Bonneville Dam tailrace falls to 81.8 percent for chinook and 63 percent for steelhead.
For fish originating in the mid-Columbia River, 90.2 percent of chinook and 95.4 percent of steelhead survived the trip from McNary to John Day. In this case, survival drops to 84.8 percent (chinook) and 78.6 percent (steelhead) from John Day to Bonneville, again hitting steelhead the hardest.
This report looks at just in-river survival. Most chinook and steelhead this year, even with spill, were transported. The report estimates that 56 percent of non-tagged spring-summer chinook salmon and 74 percent of non-tagged steelhead that arrived at Lower Granite Dam were transported from one of the collector dams in 2003. The final survival report for spring migrants will be available in November.
Survival of Downstream Migration, National Marine Fisheries Service, 12/21/00
NOAA Fisheries Northwest Region: www.nwr.noaa.gov
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