Evaluation of angled bar racks and louvers for guiding silver phase American eels
Biology, Management and Protection of Catadromous Eels
American Fisheries Society
The ability of downstream migrant silver phase American eels Anguilla rostrata to guide along various configurations of angled bar racks (25- and 50-mm clear spacing) and louvers (50-mm clear spacing) was evaluated in a laboratory flume. Guidance tests were conducted with bar racks and louvers angled at 45 and 15 degrees to the approach flow at velocities of 0.3 m/s to 0.9 m/s. A full-depth bypass was used for all tests. Guidance efficiency was calculated by dividing the number of fish recovered from the bypass by the total number recovered downstream (bypass and entrainment combined). Mean guidance efficiency (MGE) with the 45-degree, 25-mm bar rack ranged from a low of 56.8% at 0.6 m/s to a high of 65.9% at 0.9 m/s. MGE of the 45-clegree, 50-mm bar rack decreased from 72.7% at 0.3 m/s to 54.5% at 0.9 m/s, while MGE of the 45-clegree louver ranged from a low of 34.9% at 0.3 m/s to a high of 61.9% at 0.6 m/s. Guidance efficiency was considerably higher for tests with the 15-degree structures, exceeding 88% at all velocities during tests with a solid bottom overlay placed over the lower 30 cm of each structure. During tests without the overlay in place and at a velocity of 0.6 m/s, guidance efficiency of both bar racks and louvers decreased to 83.3 and 60.7%, respectively. The estimated guidance efficiencies indicate that angled bar racks and louvers have potential for diverting American eels away from hydro intakes, particularly if a shallow angle is employed (e.g., 15 degrees to the approach flow). However, we believe our estimates of guidance efficiency are higher than would be experienced at an actual intake due to the full depth bypass, the limited depth of the flume, and the short length of each rack configuration that we evaluated.