[4] Juvenile Steelhead Survival Soars In Hydro System

NOAA Fisheries scientists say preliminary analyses show that inriver migrating juvenile steelhead in 2009 beat average survival for every reach in the hydro system over the past six years.

According to their Sept. 14 survival memo, about 80 percent of the PIT-tagged hatchery and wild steelhead made it from Lower Granite to McNary Dam, better results than any year since 1995.

From the McNary Dam tailrace to Bonneville Dam's tailrace, survival was even higher, more than 86 percent.

From the head of Lower Granite Reservoir to Bonneville, steelhead averaged about 70-percent survival; yearling chinook averaged 53-percent survival. In 2008, steelhead averaged 48-percent survival, yearling chinook 46 percent.

More good news came from the upper Columbia, where PIT-tagged hatchery chinook averaged 84-percent survival from McNary to Bonneville, the highest in the 2002-2008 dataset. Steelhead survival was about 73 percent.

According to the memo, overall steelhead survival was unusually high from Lower Granite to Little Goose dams, and from John Day to Bonneville dams. With operations similar to 2008, the big difference was a temporary spillway weir in place for the first time this year at Little Goose. Also, a new spillway guidance wall at The Dalles Dam was partly completed by this spring, which may have helped to boost survival.

With steelhead survival the highest yet estimated between Lower Granite and Little Goose, the memo said the new weir may have had something to do with it. Acoustic-tag results will be available later, to "inform this conjecture," says the memo.

Timing of the steelhead run was much earlier than the past two years. By the end of April, 44 percent of the run had passed Lower Granite. In 2007 and 2008, only 18 percent of the run had passed by then.

With transportation starting later this year, many more steelhead migrated inriver than the past two years. NMFS scientists say the larger numbers of inriver fish may actually reduce overall predation.

Avian predation by gulls below John Day Dam was also reported down compared to 2008 as well.

Steelhead survival may have also improved with cooler temperatures during most of the migration. When water temperatures stay down, fewer steelhead revert to parr and residualize, stopping their migrations and sticking around the reservoirs for the rest of their lives.

Yearling chinook survival was about average, with nearly 79 percent survival from Lower Granite to McNary. From McNary to Bonneville, it averaged about 71 percent, with about 56 percent overall survival from Lower Granite to Bonneville Dam. The spring chinook showed no meaningful difference in survival between Granite and Goose from the added weir.

The springers actually showed similar survival from the head of Lower Granite Reservoir to Bonneville (53 percent) in 2005, before the court-ordered spill program began. Spring flows were considerable lower in 2005 than 2009. -B. R.

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