Effectiveness of Fish Screens to Prevent Entrainment of Westslope Cutthroat Trout into Irrigation Canals

Abstract

Irrigation canals entrain anadromous and potamodromous salmonids of all life stages during their annual migrations. Fish screens may reduce or eliminate entrainment, but few studies exist on their benefits and these studies have evaluated effects on anadromous populations only. The benefits of fish screens for nonanadromous salmonids are largely unknown. Postspawn adult and downstream-migrating juvenile westslope cutthroat trout Oncorhynchus clarkii lewisi are potentially entrained into irrigation canals on Skalkaho Creek, Montana. We quantified entrainment rates into the canals using telemetry and driftnetting before (2003) and after (2004) installation of fish screens on three canals. No radio-tagged adults were entrained in 2003, but most adults were residents and therefore did not migrate past the canals. In 2004, 79% of the radio-tagged adult migratory fish were entrained at either screened or unscreened canals, but all those entrained in screened canals were successfully bypassed back to Skalkaho Creek. The uppermost (Highline) canal entrained about 71% of age-0 westslope cutthroat trout moving downstream in 2003. Without the screen, 38% of age-0 fish would have been entrained in the canal during 2004. In total, 6,034 age-0 westslope cutthroat trout were bypassed by all three screens. Most age-0 westslope cutthroat trout entrained at screened canals were successfully bypassed, whereas those entrained at unscreened canals were lost to the population. Fish screens were an effective management tool that reduced or eliminated entrainment at Skalkaho Creek and may be useful elsewhere to curtail entrainment of migratory inland salmonids.