Northwest Fisheries Science Center

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Document Type: Contract Report
Center: NWFSC
Document ID: 8872
Title: Relative survival of juvenile salmon passing through the spillway of The Dalles Dam, 1997
Author/Editor: Earl M. Dawley, Lyle G. Gilbreath, Edmund P. Nunnallee, Benjamin P. Sandford
Publication Year: 1998
Publisher: National Marine Fisheries Service
Contracting Agency: U.S. Army Corps of Engineers. Portland, Oregon

In 1997, the National Marine Fisheries Service initiated a study at The Dalles Dam to evaluate survival of juvenile Pacific salmon (Oncorhynchus spp.) when 64% of the river flow was passed through the spillway.  The high spill is presumed to provide increased protection for the migrants.  Research has generally found that survival of fish passed through spillways at dams on the Columbia and Snake Rivers is higher than for fish passed through turbines.  However, at The Dalles Dam, two conditions associated with the spillway under high spill levels are unlike other dams:  1) the stilling basin is shorter and the tailrace is shallower, resulting in severe turbulence that may cause physical injury to migrant salmon; and 2) the large proportion of water that passes through the shallow area may substantially increase predation on salmonids by gulls, (Larus spp.) and northern squawfish (Ptychocheilus oregonensis).

Approximately 43,000 yearling coho salmon O. kisutch (April and May) and 53,000 subyearling fall Chinook salmon 0. tshawytscha (June and July) were collected from the juvenile bypass system at Bonneville Dam Second Powerhouse and tagged with passive integrated transponder (PIT) tags.  Half were released upstream from the spillway at The Dalles Dam and half downstream at a site away from turbulence and predation.

After migrating through 74 km of reservoir, a portion of the test fish passed through the Bonneville Dam PIT-tag interrogation equipment located in the juvenile fish bypass systems.  An average of 12.0% of the coho salmon and 14.1 % of the subyearling Chinook salmon released into the tailrace of The Dalles Dam were interrogated at Bonneville Dam.  Relative survival estimates for spillway passage were 87.1% (95% CI 80.4-93.9%) for coho salmon and 92.1% (85.5-98.7%) for subyearling Chinook salmon.  There were no apparent survival trends related to the date of release, the spill bay through which passage occurred, or the volume of water flow through the spillway or individual spillbays.  Survival appeared higher for fish that passed the spillway at night compared with those that passed during the day.  However, recoveries of fish were insufficient for this difference to be statistically significant (P > 0.05).

Results of this study suggest that when 64% of the river flow is passed through The Dalles Dam spillway at high river volumes (spring flows ranged from 379,400 to 526,500 ft3/second and summer flows ranged from 242,200 to 529,100 ft3/second), survival of juvenile salmon passed through the spillway is lower than at other dams (generally considered to be about 98% ).  Based on limited hydroacoustics data at The Dalles Dam, the efficiency of spill for passing fish appears to decrease above 30% spill, and 80% fish passage efficiency may be achievable at spill volumes lower than 64% of the river flow.  Thus, passage survival of juvenile salmonids at spill rates lower than 64% warrants further investigation at The Dalles Dam.