Entrainment losses of westslope cutthroat trout into screened and unscreened irrigation canals on Skalkaho Creek, Montana

Abstract

Irrigation canals are known to 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 have evaluated effects on anadromous populations only. Prior to my study, none existed on the benefits of fish screens for non-anadromous salmonids. Large numbers of post-spawn adult and downstream migrant juvenile westslope cutthroat trout (Oncorhynchus clarkii lewisi) are potentially entrained into the seven irrigation canals on Skalkaho Creek, a tributary of the Bitterroot River. I quantified entrainment rates into the canals using telemetry and trapping before (2003) and after (2004) installation of three fish screens. I also examined the efficacy of the screens in returning downstream migrants to the stream. No telemetered adults were entrained in 2003, but most were residents and therefore did not migrate past the canals. In 2004, 79% of the telemetered adult migratory fish were entrained at either screened or unscreened canals, but all adults entrained in screened canals were successfully bypassed back to Skalkaho Creek. Only eight of 117 telemetered age-1 juveniles were entrained, whereas the others were residents and did not migrate. Only one of three age-1 juveniles entrained in 2004 was bypassed. The low number of migratory adult fish and age-1 juveniles I was able to telemeter suggests that the non-migratory, resident life history is now being selected for in this system, but screens should reverse this process. Downstream movement of age-0 westslope cutthroat trout in Skalkaho Creek increased their risk of entrainment. The Highline Canal entrained about 71% of age-0 westslope cutthroat trout moving downstream in 2003. If not for the screen to bypass them in 2004, 38% would have been entrained in the Highline Canal. A total of 6,049 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 to reduce or eliminate entrainment at Skalkaho Creek and may be useful elsewhere to eliminate entrainment of inland salmonids.