Untested assumptions

Diversions from streams are often screened to prevent loss of fish. Because construction of fish screens competes for scarce dollars with other fish conservation projects, the widely 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 over 3,000 diversions. We found few studies that even attempted to evaluate the effectiveness of screens in preventing losses of fish, much less declines in fish populations. The limited published literature 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 even small diversions can be important sources of fish mortality, given their large number and the combined volume of water they divert. The impact on fish populations of individual diversions is likely highly variable and depends upon size and location, as demonstrated by evaluations of cooling water intakes for power plants. Studies are needed to determine which diversions have the greatest impact on fish populations in order to set priorities for screening, to make the best use of limited public funds available for restoration and conservation, and to provide scientific support for effective screening policies.