Monitoring Juvenile Rainbow Trout Outmigration Using a Coanda Wedge Wire Screen

We’re using both a tilted wedge wire Coanda screen to monitor outmigrating juvenile rainbow trout in spring-fed tributaries of the Henrys Fork of the Snake River in Idaho. The tilted wedge wire coanda screen is being used on the spillway of the Buffalo River (5.66 m³/s base water flow) Hydroelectric Project. The 5.95 m² screen and supporting framework were designed, constructed, and installed by the Hydroscreens Company at a cost of $11,900. Total water capacity of the trap is about 1.13 m³/s. The trap uses 1 mm wire spacing with a wire tilt of 5 degrees. The shearing action of the tilted wires and the coanda effect help pass the majority of the water through the screen. Fish and debris, along with remaining water, move down the screen and into a bypass pipe to a holding tank. A total of 11,167 fish (84% rainbow trout, 15% brook trout, and 1% whitefish and non-salmonids) has been captured in the trap from June 30 to November 24, 2009 and March 12 to November 19, 2010. Capture efficiency of the trap ranged from 12% to 15% of age-0 rainbow trout during the high flow period of spring 2010. Mortality or injury to fishes from the trap has been < 0.5%. The trap does not require an electrical power source, has no moving parts, and no repairs or structural maintenance of the screen has been required to date.  The screen has required frequent (daily or every other day) cleaning of debris when high aquatic macrophyte loads occur in the summer in the Buffalo River. We recommend the tilted wedge wire Coanda screen for use as a fish trap or as fish exclusion device on spillways and diversions that have similar operating conditions as our installation.