Northwest Fisheries Science Center

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Document Type: Contract Report
Center: NWFSC
Document ID: 1155
Title: Passage behavior and survival for radio-tagged subyearling Chinook salmon at Lower Monumental Dam, 2007
Author/Editor: R. F. Absolon, Eric E. Hockersmith, Gordon A. Axel, Darren A. Ogden, Brian J. Burke, Kinsey E. Frick, Benjamin P. Sandford
Publication Year: 2008
Publisher: National Marine Fisheries Service
Contracting Agency: U.S. Army Corps of Engineers. Walla Walla, Washington
Contract Number: W68SBV92844866
Date: September 2008

This study was initially designed to evaluate passage of subyearling Chinook salmon Oncorhynchus tshawytscha with a removable spillway weir (RSW) at Lower Monumental Dam.  Installation of the RSW in spillbay 8 was scheduled prior to the 2007 juvenile salmonid migration season, but was postponed until spring 2008 due to construction delays.  Although the RSW could not be installed in time for the 2007 juvenile salmonid migration, we proceeded with the study to collect additional baseline information on subyearling Chinook salmon behavior and passage survival.

River-run subyearling Chinook salmon were collected at Lower Monumental Dam from 12 June through 4 July.  Fish were surgically tagged with both a radio and PIT tag, and evaluations were based on detection at the primary telemetry array, 16 km downstream from the dam.

We released 860 radio-tagged fish into the forebay and 833 into the tailrace of Lower Monumental Dam.  Releases were made twice per day during the study period.  Of the 860 fish released into the forebay, 571 were detected and used in the evaluation of relative survival.  The number of fish not detected after release was higher than expected based on previous detection rates at the same time of year.

The fate of non-detected fish is unknown, but likely includes consumption by predators, failure to move downstream to the detection arrays, or failure to move downstream until after the life of the radio tag had expired.  Average total river flow in 2007 was 38.4 kcfs, lower than either 2006 (68.8 kcfs) or the 10-year average (62.8 kcfs) for the study period.  Lower flows may have contributed to lower-than-expected detection rates.

Estimated relative dam survival was 0.762, relative concrete survival was 0.845, relative spillway passage survival was 0.838 overall and 0.903 through spillbay 8.  Pooled relative survival estimates for spillbays 6 and 2 were 0.779 and 0.697, respectively. The pooled relative survival estimate for fish passing through the juvenile bypass system was 0.949.

Passage routes were estimated at 91.4% through the spillway, 6.9% through the juvenile bypass system, and 1.8% through turbines.  Ten fish passed the dam via an unknown route.  Spill efficiency was estimated at 0.914, fish guidance efficiency at 0.796, and fish passage efficiency at 0.982.  Median overall forebay residence time was 3.6 h (range 0.3 141.5 h), and median tailrace egress time was 13.1 min (range 1.3 10,673.6 min).