SONGS coastal view

Mitigation Projects > Behavioral Barriers > Background

[Behavioral Barriers] Background

diagram of entrained and impinged fish
diagram of intake screen well structure
Diagram showing how fish are entrained and impinged in the SONGS once-through cooling system. In-plant losses of adult fish are greatly reduced by a Fish Return System that guides fish in the screenwell to an elevator and pipe that returns them to the ocean. Diagram showing the intake screen well structures and the location of the fish return chamber.

All cooling water that enters the power plant passes through screens that filter out large material (including fish) that can clog cooling tubes in the plant. Each screen array consists of a series of sections forming a loop called a traveling screen because the loop revolves such that the sections facing into the cooling water move up. Small fish, eggs and larvae pass through the screens and enter into the plant where they are killed by the warm, high velocity water associated with plant operations. Larger fish that are caught against the screens (i.e. impinged) or carried up on the shelves are removed to trash receptacles at the top of the screen structure. Units 2 and 3 at SONGS are each equipped with a Fish Return System (FRS). Each FRS is located at the end of the cooling water intake structure, adjacent to the traveling screen array. The FRS consists of vanes and louvers that direct fish away from the traveling screens to the fish return chamber where an elevator periodically raises them to the surface of the intake screen well and releases them via a sluiceway back to the ocean. Although the FRS diverts a large fraction of the fish taken into the plant back to the ocean, some are impinged against the traveling screens and killed.

One component of mitigation for SONGS mandated by the California Coastal Commission (CCC) requires SCE to implement a device, technique or protocol that diverts additional fish from the traveling screens to the FRS to further reduce fish impingement losses. To comply with this requirement SCE installed mercury vapor lights in Units 2 and 3 and subsequently conducted studies to determine whether the lights would reduce impingement by at least 2 metric tons. SCE in consultation with the CCC also considered other alternatives, including strobe lights, air bubble curtains, pneumatic guns, poppers and electrified nets. Results from these studies varied inconsistently among species for any given technique, and most of the techniques were either potentially damaging to marine life or presented severe installation and maintenance concerns. As a result, the CCC determined that at the present time there are no alternative behavioral barriers that are likely to be effective or feasible at SONGS.

diagram of Fish Return System
Diagram of the Fish Return System in SONGS Units 2 and 3.

Unlike the lights and other deterrents tested, two operating protocols for SONGS, the Fish Chase and the Fish Return System (FRS) have proved to be effective behavioral barriers. SONGS operations include periodic heat treatments that involve re-circulating approximately two-thirds of the normal discharge flow back through the condenser to achieve a temperature of 105°F in the screen well to control the growth of fouling organisms, especially mussels and barnacles. Prior to 1991 all fish residing in the screen well died during heat treatments due to the elevated temperatures. Since then, SONGS operation was changed to include a unique Fish Chase procedure that involves manipulating crossover gates in the screen well during heat treatments to slowly warm the discharge water. These manipulations also create eddy currents that dislodge fish that have aggregated in areas of low flow. The combination of slow heating and current manipulations herds the fish into the Fish Return System, and prevents them from dying in the screen well during the heat treatment. The FRS operates both during the heat treatments and at least twice daily during normal operations.


On October 12, 2000, the CCC concurred with the Executive Director’s Determination that the mitigation requirement to reduce impingement losses could be met by continued implementation of the Fish Chase Procedure.