Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 6848
Title: A method for estimating spill effectiveness for passing juvenile salmon and its application at Lower Granite Dam on the Snake River
Author: John W. Wilson, Albert E. Giorgi, Lowell C. Stuehrenberg
Publication Year: 1991
Journal: Canadian Journal of Fisheries and Aquatic Sciences
Volume: 48
Issue: 10
Pages: 1872-1 876

Juvenile salmonids migrating down the Snake and Columbia Rivers in the northwestern United States pass each of the rivers' hydroelectric dams via either the powerhouse or the spillway.  Spill effectiveness, the proportion of juveniles passing through the dam via the spillway, is important to fishery managers seeking to minimize the exposure of fish to turbines.  We outline a method for estimating spill effectiveness and its sampling variance using a multinomial model with categories defined on the basis of information recovered from radio–tagged fish.  We applied this method to data from tagged chinook salmon (Oncorhynchus tshawytscha) juveniles released at Lower Granite Dam in 1985.  For two release conditions, one with 20% and the other with 40% of the total water volume going through the spillway, the maximum likelihood estimates of spill effectiveness were 41 and 61%, respectively.  Increasing the flow of water through spillways may therefore be a useful method of increasing spill passage of juvenile salmonids at Lower Granite Dam.  Similar radio–tag experiments and multinomial models could readily be devised to study spill effectiveness at hydroelectric projects with a wide variety of structural plans. 

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