Northwest Fisheries Science Center

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Document Type: Contract Report
Center: NWFSC
Document ID: 5424
Title: Descaling and orifice passage efficiency studies at McNary Dam, 1995
Author/Editor: R. Lynn McComas, Benjamin P. Sandford, Douglas B. Dey
Publication Year: 1997
Contracting Agency: U.S. Army Corps of Engineers
Contract Number: E86910060
Pages: 35
Date: 1997

Extended-length submersible bar screens (ESBSs) have been tested at McNary Dam since 1991 as alternatives to standard-length submersible traveling screens (STSs) for guiding downstream migrating juvenile salmonids out of turbine intakes.  During the spring and summer juvenile migration periods of 1995, we evaluated gatewell orifice passage efficiency (OPE) for Chinook salmon and steelhead using an ESBS with a newly designed vertical barrier screen and an inlet flow vane.  An auxiliary study compared juvenile fish descaling associated with two beam extension modifications in gatewells equipped with these new guidance devices.

Two methods were used to compare OPE between north and south orifices for each juvenile migration period.  First, orifice traps provided an absolute measure of the proportion of migrants passing through the test slot during a 22-h period.  Second, a mark/recapture method furnished an estimate of egress from the gatewell by marked Chinook salmon over 22 h.  Mean orifice passage efficiency was over 70% (range 43-100%) for all salmonids using either method.  Estimates of OPE based on mark/recapture methods were significantly higher than numbers from orifice trapping for both yearling and subyearling Chinook.

There was no significant difference in OPE between north and south orifices for either yearling or subyearling Chinook using orifice traps, or for yearling Chinook salmon using the mark/recapture method.

There was no significant difference in descaling for any species between gatewell and orifice traps, or between beam extension types in ESBS test slots.  Descaling in the gatewell of the STS control slot was 11.2% for yearling Chinook salmon and 13.9% for steelhead. These descaling rates were significantly higher than in either of the two ESBS test slots for yearling chinook salmon (7.2 and 8.9%) and steelhead (9.1 and 9.3%).  There were no statistical differences in descaling for subyearling Chinook salmon among the three test slots.