A fish screen is designed to prevent fish from swimming or being drawn into an aqueduct, cooling water intake, dam or other diversion on a river, lake or waterway where water is taken for human use. They are intended to supply debris-free water without harming aquatic life. Fish screens are typically installed to protect endangered species of fishes that would otherwise be harmed or killed when passing through industrial facilities such as steam electric power plants, hydroelectric generators, petroleum refineries, chemical plants, farm irrigation water and municipal drinking water treatment plants. However, many fish are killed or injured on screens or elsewhere in the intake structures.
Fish screens may be positive barriers (devices such as a perforated metal plate that physically prevents fish from passing) or behavioral barriers (devices that encourage fish to swim away). Most behavioral barriers are experimental and of unproven effectiveness.:p. 4-19
Positive barriers are often effective at keeping aquatic organisms from entering a cooling system, but may also kill them by impinging them on the screens. These barrier types are widely used and include:
- Modified traveling screens
- Fish handling and return systems
- Horizontal, flat-plate screens with bypass water return systems
- Cylindrical wedgewire screens
- Fine-mesh screens
- Fish net barriers.
Besides simply preventing fish from passing, fish screens are designed to minimize stress and injury that occur when fish impact the screen or are subjected to changes in water velocity and direction caused by the diversion.:pp.4-3ff
The U.S. Environmental Protection Agency (EPA) has evaluated other barrier technologies and identified some as potentially effective, although not widely demonstrated (as of 2004):
- Aquatic microfiltration barriers
- Angled and modular inclined screens
- Velocity caps.:pp.4-15ff
Some fish screens are designed to protect a single species of fish (for example, salmon) and are not necessarily effective at protecting other fish species. Some screens are capable of protecting more than one species or type of life. Additionally, some screens may effectively protect juvenile and adult fish, but not fish eggs and larvae.
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. Some manufacturers are able to produce a screen that has no moving parts, requires no electricity, and has very little need for maintenance.
Impacts on aquatic life
Many power plants and other industries in the U.S. continue to use screens that impinge fish. For example, at the Bay Shore Power Plant in Ohio, 46 million fish were killed over an 18-month period. Organisms that pass through the screens are killed or stressed (depending on the species) as they become entrained in the cooling system. 208 million fish eggs and over 2 billion small and larval fish were entrained at the Bay Shore plant over the same 18 months. EPA estimates that billions of fish and other organisms are killed each year in cooling water intakes.
In the United States, the National Marine Fisheries Service, a division of NOAA, 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.
The U.S. Clean Water Act requires EPA to issue regulations on industrial cooling water intake structures. The agency issued final regulations for new facilities in 2001 (amended 2003). Other EPA regulations for existing facilities were challenged in litigation and EPA published new proposed regulations in April 2011.
- U.S. Environmental Protection Agency (EPA), Washington, DC. (2004). "Efficacy of Cooling Water Intake Structure Technologies." Chapter 4 of Technical Development Document for the Final Section 316(b) Phase II Existing Facilities Rule. Doc. No. EPA-821-R-04-007.
- Schmitz, Christy (2009-08-19). "One Fish, Two Fish, 46 Million Lake Erie Dead Fish". Toledo City Paper (Toledo, OH: Adams Street Publishing Co.).
- EPA (2002). "National Pollutant Discharge Elimination System—Proposed Regulations to Establish Requirements for Cooling Water Intake Structures at Phase II Existing Facilities; Proposed Rule." 67 F.R. 17136. 2002-04-09.
- "Introduction to the Farmers Screen". Farmers Conservation Alliance. Retrieved 9 December 2013.
- Henry, Tom (2010-03-30). "FirstEnergy to install devices to divert fish". Toledo Blade.
- EPA (March 2011). "Proposed Regulations to Establish Requirements for Cooling Water Intake Structures at Existing Facilities." Fact sheet. Document no. EPA-820-F-11-002.
- Clean Water Act, Section 316(b), 33 U.S.C. § 1316.
- EPA. "Phase I—New Facilities." Cooling Water Intake Structures. Final rule: 2001-12-18, 66 F.R. 65255. Amended: 2003-06-19, 68 F.R. 36749.
- EPA. "National Pollutant Discharge Elimination System—Cooling Water Intake Structures at Existing Facilities and Phase I Facilities." Proposed rule. Federal Register, 76 F.R. 22174. 2011-04-20.
- Cooling Water Intake Structure Technology Fact Sheets - U.S. EPA
-  - Time lapse video of a small modular fish screen being installed in Idaho.