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Juvenile Chinook Survival Second Highest in Recent Years by Bill Rudolph NW Fishletter, October 19, 2012

Federal scientists from the NOAA Fisheries Science Center in Seattle have released their preliminary results from this year's salmon migration through the Columbia/Snake hydro corridor, noting that nearly 60 percent of juvenile spring chinook survived from the Snake River trap (near Lewiston, Idaho) to below Bonneville Dam.

The Oct. 12 memo said this year's migration showed the second highest survival of PIT-tagged hatchery and wild fish since 1999. Only 2006 was better, when survival from the trap to the Bonneville tailrace was just over 61 percent.

Juvenile survival was significantly higher that last year's 49 percent, despite 2011's higher spring flows and 4th all-time best water year since 1960. The memo also noted that compared to long-term averages, survival in 2012 was 5.5 percent higher between Lower Granite and McNary dams and 10.3 percent higher between McNary and Bonneville dams. The 2012 water supply for the Columbia Basin ended up 5th highest since 1960.

Juvenile steelhead survival was estimated at nearly 60 percent, third highest since 1999, and close to last year's 59 percent. Only 2008 and 2009 were better.

"The pattern through time of flow volume in the Snake River in 2012 was unique among the last eight migration seasons," said the memo, "in that the highest peak in flow occurred in late April rather than in May, and in fact, the flow throughout May was no higher than in April."

The overall average of daily average flow volume in 2012 was higher than the long-term average, the memo reported, "but no other year's flow pattern had those characteristics." Snake flows stayed around 100 kcfs for the first three weeks in April and peaked quickly at 178 kcfs on April 27, dropping back in the 100-kcfs range by the end of the following week.

Mean spill volume at lower Snake dams was close to the 2006-2011 average for the first few weeks in April, then jumped when flows spiked. Spill in the first three weeks in April 2012 as a percentage of flow was below average compared to recent years, but hit nearly 50 percent when flows rose later in the month. In May, spill declined to near-to-below average.

Upper Columbia hatchery spring chinook exhibited an 84-percent survival rate from the McNary Dam tailrace to below Bonneville Dam, nearly 20 percent higher than last year.

Upper Columbia hatchery steelhead showed 100-percent survival from McNary to Bonneville Dam, compared to last year's 65 percent. But survival from release to McNary, was way down, only about 28 percent , compared to last year's 44 percent. The difference could be caused by higher predation from birds colonizing Potholes Reservoir this year, but the memo did not mention anything about it.

Snake River sockeye showed a 47-percent survival rate from Lower Granite to Bonneville this year, compared to more than 54 percent in 2010, a much lower flow year. Survival estimates to Bonneville in 2011 were not available because of low PIT-tag detections caused by debris and pulled screens during high flows last year.

Upper Columbia sockeye showed much higher survival than last year, with a preliminary estimate of more than 79 percent making it from Rock Island Dam to Bonneville. In 2011, the survival rate was barely above 50 percent.

The memo also reported that during May and June, changes to turbine operations at Bonneville Dam to reduce descaling of juvenile sockeye "drastically reduced" PIT-tag detection probabilities and made for "very low precision" of survival estimates, and impossible for some weekly groups.

Another result of the odd flow pattern was the lowest percentage level of fish barged from Snake dams in the past 20 years. Preliminary estimates found only 23 percent of wild spring chinook being transported this spring, along with about 25 percent of hatchery spring chinook. For wild steelhead, it was estimated that about 28 percent were barged, along with 27 percent of hatchery-origin steelies.

The memo said fish collection began on May 2 at Lower Granite, but the large spike in flows in late April meant that most juvenile spring chinook (67 percent) and steelhead (59 percent) had passed the project before barging even began. High spill levels, along with surface bypass collectors kept numbers of fish entering bypass systems low compared to other passage routes. But during May, about 60 percent of the smolts that passed Lower Granite ended up being barged from there or a collector dam downstream.