Fish screen

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A fish screen is a barrier designed to prevent fish from swimming or being drawn into an aqueduct, dam, or other diversion on a river, lake, or other waterway where water is taken for human use. Fish screens are typically installed to protect endangered species of fishes that would otherwise be harmed when passing through hydroelectric generators, entering water municipal water supplies, etc.

1 Design
2 Legal requirements
3 Geographic spread
4 See also

[edit] Design

Fish screens may be positive barriers (devices such as a perforated metal plate that physically prevents fishes from passing) or behavioral barriers (devices that encourage fishes to swim away). Most behavioral barriers are experimental and of unproven effectiveness. Positive barriers are effective and widely used. Besides simply preventing fishes from passing, fish screens are designed to minimized stress and injury that occur when fishes impact the screen or are subjected to changes in water velocity and direction caused by the diversion.

Most fish screens are designed to protect a single species of fish (for example, salmon) and are not necessarily effective at protecting other fish species.

The cost of a fish screen varies from thousands of US dollars for small, low-flow-rate screens to millions of US dollars in the case of very large custom-designed systems that filter a large flow of water. Maintenance costs can be significant, including repairs, removing trash, and adjusting the equipment for changes in stream conditions.

[edit] Legal requirements

In the United States, NOAA's National Marine Fisheries Service mandates positive-barrier fishscreens in most new diversions from waterways where endangered or threatened fish species occur. Some existing unscreened diversions whose construction pre-dates fish-screen mandates are allowed to continue operating by grandfather rule.