Fish screen efficiency and effects of screened and unscreened irrigation canals on the downstream movement of westslope cutthroat trout juveniles in Skalkaho Creek, Montana

Abstract

Fish screens were installed in three of seven irrigation canals that divert water from Skalkaho Creek, a tributary of the Bitterroot River, in 2003 to prevent the loss of fluvial-adfluvial westslope cutthroat trout to irrigation canal entrainment. A study conducted in 2003 and 2004 established that fish screens were effective at reducing the loss of adult and age-0 westslope cutthroat trout. The efficiency of fish screens at preventing the loss of age 1-4 juveniles, the effect of screening on age-0 westslope cutthroat trout downstream movements, and the magnitude of entrainment at unscreened canals remained unclear after this initial study. The goal of my study was to address these information gaps. Fish screens prevented the loss of about 82% of entrained juvenile westslope cutthroat trout and bull trout. About 69% of entrained juveniles exited the screened canals through the headgates, 14% were bypassed, 12% remained in the canals, and the fate of 5% remained unknown. Entrained fish took about 7 days to exit the screened canals. Fish screen efficiency, the route used by fish to exit, and the amount of time taken by fish to exit varied by canal.

Only about 1% of all age-0 westslope cutthroat trout that moved downstream into the irrigation affected reach of Skalkaho Creek migrated beyond the Ward-Hughes diversion dam in 2005 and 2006. Downstream movement of age-0 westslope cutthroat trout beyond this point may have been selected against through predation by introduced brown trout, dewatering, increased water temperature, years of entrainment into unscreened canals, or other unknown causes. About 67 to 70% of downstream-migrating westslope cutthroat trout and bull trout that encountered the three largest unscreened irrigation canals were entrained as they attempted to emigrate to the Bitterroot River in 2006. Although fish screens were relatively efficient at returning entrained fish to Skalkaho Creek, additional entrainment-preventing measures (such as siphons) at unscreened canals and water management strategies that prevent dewatering are needed to enhance the fluvial-adfluvial population of westslope cutthroat trout in Skalkaho Creek. Fish screens, siphons, and creative water management strategies could be valuable tools in the recovery of fluvial-adluvial fish populations in heavily irrigated watersheds.