Untested assumptions: Effectiveness of screen diversions for conservation of fish populations

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diversion, streams, fish screen, screens, fish population, fish mortality, mortality, intake, power plant, restoration

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Diversions from streams are often screened to prevent loss of fish. Becauseconstruction of fish screens competes for scarce dollars with other fish conservation projects, thewidely accepted premise that fish screens protect fish populations merits thorough examination.We reviewed literature on fish screen projects in California's Central Valley, where there are over3,000 diversions. We found few studies that even attempted to evaluate the effectiveness ofscreens in preventing losses of fish, much less declines in fish populations. The limited publishedliterature suggests that this lack of evaluation is typical throughout the western United States,despite millions of dollars spent annually on screens and their maintenance. Nevertheless evensmall diversions can be important sources of fish mortality, given their large number and thecombined volume of water they divert. The impact on fish populations of individual diversions islikely highly variable and depends upon size and location, as demonstrated by evaluations ofcooling water intakes for power plants. Studies are needed to determine which diversions havethe greatest impact on fish populations in order to set priorities for screening, to make the bestuse of limited public funds available for restoration and conservation, and to provide scientificsupport for effective screening policies.





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