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NW Fishletter #294, September 22, 2011

[1] High Flows Play Havoc With Juvenile Hydro Survival

NMFS scientists say juvenile survival of PIT-tagged salmon and steelhead through the hydro system was lower in 2011 than the previous two years, despite one of the highest flow regimes of the last 40 years.

In a memo to assistant regional administrator Bruce Suzumoto, the agency's Science Center in Seattle reported that yearling chinook survival (hatchery and wild combined) from Lower Granite Dam to Bonneville Dam was nearly 52 percent--close to the long-term average, but lower than 2010's estimate of 59 percent and 2009's 55.5 percent.

For juvenile steelhead (hatchery and wild combined), the agency estimate was 57.7 percent, higher than the long-term average of 42.9 percent, but lower than 2010's 60.8 percent and 2009's 67.6 percent.

The memo reported that, except for higher flows in early April, flow volumes and patterns in the Snake River during April and May were most similar to 2009, while later May and June were closer to 2006 and 2008.

Around May 10, flows in the Snake jumped from 90 kcfs to over 200 kcfs and stayed above 150 kcfs until the end of that month.

Average spill at lower Snake dams was on the high side of average, around 30 percent, until around May 15, when spill percentages rose to over 60 percent before the end of the month, levels not seen in recent years.

Sockeye survival between Lower Granite and McNary dam averaged 65.9 percent, but poor detection rates in the lower river kept the agency from estimating survival all the way to Bonneville Dam. Survival in 2010 was 72.3 percent, and 74.9 percent in 2009.

For upper Columbia hatchery spring chinook, the agency estimated survival from McNary to the Bonneville tailrace at 70.1 percent, and 65.1 percent for steelhead from upper Columbia hatcheries. Last year, survival was 73.5 percent for chinook and 62.6 percent for steelhead, lower than 2009's 89.5 percent and 75.6 percent, respectively.

Sockeye from the upper Columbia averaged 51.3 percent survival from Rock Island to Bonneville this year, compared to last year's 48.8 percent and 2009's 81.7 percent.

NMFS said high flows increased debris problems at dams this year, especially at juvenile bypass systems. Screens had to be removed from some turbine units at Bonneville Dam from May 19 until July 12, which meant more juvenile fish passed through turbines, reducing survival. The action reduced PIT-tag detections so much that survival could not be estimated for that period.

"Therefore," said the memo, "any annual survival estimate for chinook or steelhead that includes the John Day to Bonneville reach in 2011 is most likely an overestimate of the actual survival of the population." The researchers estimated that about 40 percent of the chinook run and 45 percent of the steelhead run had not passed John Day Dam when the screens were removed.

Other dam operations affected juvenile survival as well. The powerhouse shutdown at Little Goose between May 24 and June 2 closed the juvenile bypass system and drastically reduced PIT-tag detections during that period. NMFS said it did not include fish detected above the dam during that time frame in its calculations to reduce estimation problems.

The powerhouse shutdown at Little Goose also caused higher levels of spill and led to high dissolved gas levels at Lower Monumental, the next project downstream. "These increased gas levels may have resulted in increased smolt mortality for both yearling chinook salmon and steelhead," said the memo.

The relatively high spill levels this year also meant that fewer fish were barged than usual. Only 35.2 percent of the spring/summer chinook and 40.7 percent of the steelhead got a ride downstream.

NMFS reported when transportation began on May 1 at Lower Granite Dam, nearly 30 percent of the spring chinook run and 36 percent of the steelhead run had already passed the dam. Eighty percent of the chinook had passed by May 14.

Barging also stopped for 10 days in May due to high flows and a lock repair downstream at The Dalles, and fish collection at Little Goose was suspended for the week the powerhouse was being repaired. -Bill Rudolph

The following links were mentioned in this story:

Draft Survival Memo

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NW Fishletter is produced by Energy NewsData.
Publisher: Cyrus Noë, Editor: Bill Rudolph
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