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NW Fishletter #318, June 4, 2013

[5] Bonneville's Turbines Tuned To Reduce Fish Descaling

Since the middle of May, juvenile salmon have been showing up at several dams with excessive descaling after passing through bypass systems. The problem first showed up at McNary Dam, where debris, mainly rafts of tumbleweeds passing downriver from the upper Columbia, were stuck in the tailrace, and impinging on turbine screens.

Nearly 17 percent of subyearling Chinook showed descaling on May 17, and 13 percent of the young sockeye showed descaling on May 9, and again on May 13, along with 10 percent of juvenile spring Chinook sampled. By May 22, sockeye descaling was down in the 4-recent range, and not expected to be much of a problem anymore at the project, said Corps of Engineer's spokesman Doug Baus.

Further downriver, descaled sockeye began showing up in serious numbers at Bonneville Dam's Powerhouse 2 around May 20, when 20 percent of those samples showed signs. Modifications to the gatewells a few years ago, mandated by NOAA Fisheries, seems to have increased water velocities and began causing problems for some juveniles, notably sockeye, who are most prone to descaling compared to other migrating salmon.

A flow-neutral operation was put into place May 15 that increased generation slightly at Bonneville's Powerhouse 1 by operating outside of 1-percent peak efficiency, and reduced by the same amount at Powerhouse 2. Sockeye descaling declined over the next 24 hours from 26 percent to about 9 percent, but rose to the 15-percent range several days later. By May 22, it was down in the 10-percent range. Juvenile sockeye numbers peaked on May 19 and were declining fast by May 23.

At the May 15 Technical Management Team meeting, the Corps of Engineers said it would approve a request by fish managers, including NOAA Fisheries, to suspend barging juvenile fish from McNary Dam this summer. The managers had noted that improvements in dam passage probably reduced any potential benefits from barging juvenile fall Chinook from the dam. -B. R.

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