Northwest Fisheries Science Center

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Document Type: Contract Report
Center: NWFSC
Document ID: 6572
Title: Effects of bypass system passage at Bonneville Dam Second Powerhouse on downstream migrant salmon and steelhead: direct capture assessment, 1990-1992
Author/Editor: Earl M. Dawley, Lyle G. Gilbreath, Richard D. Ledgerwood, Paul J. Bentley, Benjamin P. Sandford
Publication Year: 1998
Publisher: National Marine Fisheries Service
Contracting Agency: U.S. Army Corps of Engineers. Portland, Oregon
Contract Number: DACW57-85-H-0001
Pages: 92
Date: 12/01/1998

From 1987 to 1992, passage survival tests at Bonneville Dam Second Powerhouse indicated substantially poorer survival for coded-wire-tagged subyearling Chinook salmon that used bypass systems vs. those that used other passage routes.  In 1990, we began research designed to identify conditions and physical features of the bypass system that debilitated or injured juvenile salmonids during passage at the Second Powerhouse.  Marked fish were released into the bypass system and then captured at the terminus of the discharge conduit using a trap-net. Fish were immediately evaluated for stress and injury.

Results from aggregate tests conducted in 1990 and 1991 showed few consistent adverse passage effects except that descaling of river-run coho and yearling Chinook salmon was high (8.5-28.6%).  Although less definitive, injury and mortality were greater than expected for both hatchery and river-run subyearling Chinook salmon.  There were no obvious detrimental impacts to releases of hatchery steelhead, yearling Chinook salmon, or coho salmon, with only minor scale losses and low percentages of injury, mortality, and delayed mortality.

In late 1991, tests were conducted at low tailwater elevation to assess worst-case effects; i.e., greatest water velocity and greatest shear forces at the pipe terminus.  Results of those tests showed markedly increased descaling and mortality.  Through further testing in 1992, it became apparent that descaling and impingement were occurring in conjunction with capture of test fish, and that impacts from bypass passage could not be separated from impacts occurring during trap-net recovery of test fish.  However, differences in descaling and mortality between release groups provided relative comparisons of stress and fatigue related to various flow conditions and sections of the bypass encountered by fish during individual tests.  

We concluded that injury and mortality of test fish was not occurring during passage through the bypass system at the Second Powerhouse.  However, passage through the bypass system likely caused stress and fatigue in juvenile migrants and could contribute to diminished predator avoidance.  Additionally, northern pikeminnow predation is particularly intense at the outlet of the bypass system.  Therefore, we speculate that

  1. Point source release from the bypass discharge conduit allows for increased predation.
  2. Migration through the low-velocity tailrace basin results in increased predation.
  3. The location of the bypass outlet on the north side of the tailrace, in conjunction with the southward bend in the river, tends to direct juvenile migrants shoreward toward rip-rap areas that are prime habitat for northern pikeminnow.