Northwest Fisheries Science Center

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Document Type: Contract Report
Center: NWFSC
Document ID: 6177
Title: Relative survival of subyearling Chinook salmon that have passed Bonneville Dam via the spillway or Second Powerhouse turbines or bypass system: adult recoveries through 1991
Author/Editor: Lyle G. Gilbreath, Earl M. Dawley, Richard D. Ledgerwood, Paul J. Bentley, Stephen J. Grabowski
Publication Year: 1993
Publisher: National Marine Fisheries Service
Contracting Agency: U.S. Army Corps of Engineers. Portland, Oregon
Contract Number: E96910013
Pages: 26
Date: 1993

We conducted a preliminary analysis of passage-survival differences among various fish-passage routes through Bonneville Dam Second Powerhouse, tailrace, and spillway.  Results will not be final for several years, pending completion of adult recovery data.  Recovery data reported here are preliminary; based on these data, adult contributions to the various fisheries and returns to hatcheries from study fish have been less than for other upriver bright fall chinook salmon released at Bonneville Hatchery.   Because of these low recovery percentages, our ability to identify statistically significant differences among treatments was much less than the planned 4–5% (at this date the range of detectable difference is 11.9–39.8%).

Trends observed in juvenile recovery data suggest bypass system passage did not substantially improve survival over turbine passage.  Our ability to identify statistically significant differences among treatments was about 5% for juvenile recovery data from all years combined.  We speculated that bypass-released fish had decreased survival as a result of:

  1. Increased predation because of point-source release location and water currents directing fish toward the shoreline
  2. Increased predation because of poor predator avoidance due to stress and injuries from passage
  3. Increased indirect mortality because of synergistic effects of stress and injuries incurred from passage through the bypass system combined with the high water temperatures and diseases incurred during migration downstream

We must emphasize that these data represent only summertime conditions encountered by subyearling size fall chinook salmon at Bonneville Dam Second Powerhouse, tailrace, and spillway.  Also, test fish used in this study were transported and released directly from a hatchery.   Naturally migrating fish may not show these same trends because of differences in predator avoidance and migration behavior.