Barrier Net to Reduce Entrainment Losses of Adult Kokanee from Banks Lake, Washington

Abstract

A barrier net 1,341 m long was developed to reduce the entrainment of mature kokanee (Oncorhynchus nerka) into the main irrigation canal intake of Banks Lake in Washington. The dacron net was constructed of 83-mm mesh (stretch measure) and relied on the visual avoidance response exhibited by salmonids. The net was hauled by machine and cleaning was done with high-pressure water jets. The screening efficiency of the barrier was evaluated by numerous methods including sampling the fishes entrained in the irrigation canal with large nets, mark and recapture of adult kokanee in the reservoir, estimates of the number of beach spawners, sonic tracking near the barrier, census of the sport fishery, and mortality of kokanee gilled in the barrier. The annual canal entrainment of kokanee declined from an average of 64% before installation of the net to 10% afterwards, based on 4 years of catch data. An estimated 35,391 adult kokanee, based on mark-and-recapture estimates, were retained in the lake during the fall of 1978 when 96% retention of the population was achieved. Sonic-tracked kokanee were turned back by the barrier and, during October, homed to beach spawning sites. From a creel census, it was estimated that anglers caught 46,427 kokanee in 1978. The catch of kokanee per angler-hour remained stable at 0.216 while the catch of all other species declined from 0.372 to 0.042 from 1972 to 1978. The barrier net enhanced the sport fishery and the spawning population in the reservoir. The number of kokanee gilled in the barrier net was small relative to the population retained, and the net provided an economical means of reducing the entrainment loss of adult kokanee through a spillway.