Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 7129
Title: Impedance bridge fish counter
Author: Kenneth L. Liscom, Charles D. Volz
Publication Year: 1975
Journal: The Progressive Fish-Culturist
Volume: 37
Issue: 1
Pages: 39-42

A SYSTEM IS BEING DEVELOPED to bypass juvenile Pacific salmon (Oncorhynchus spp.) and steelhead trout (Salmo gairdneri) around hydroelectric dams on the Columbia and Snake Rivers.  The fish, which are seaward migrants, are to be passed from turbine intake gatewells through orifices and carried past each dam, either through existing trash sluiceways or by a system of pipes, to the tailrace area. 

As enumeration of fish moving through the bypass is important in managing the fish runs and for evaluating the bypass system, the National Marine Fisheries Service, with the cooperation of the U.S. Army Corps of Engineers, tested an automatic electronic fish counter concurrently with gatewell orifice studies at Ice Harbor Dam on the Snake River in 1965.  The objective was to test the counter for accuracy under conditions applicable to the bypass system. 

The counter used the impedance bridge concept, which detects the difference in resistance between water and fish.  This type of counter requires a minimum of maintenance and gives continuous counts without handling the fish. 

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