An assessment of losses of native fish to irrigation diversions on selected tributaries of the Bitterroot River, Montana
Withdrawals of surface water for irrigation and stock water leave the Bitterroot River and its tributaries chronically dewatered during the irrigation season. These water withdrawals affect local trout populations by entraining migratory trout into irrigation diversion canals at multiple life stages, and through the loss and degradation of available habitat for aquatic species. Irrigation losses may be responsible in part for the low abundances and restricted distributions of migratory native westslope cutthroat trout Onchorhyncus clarkii lewisi and bull trout Salvelinus confluentus in this system. My objectives were to quantify entrainment of fish into irrigation diversions on Lost Horse and Tin Cup Creeks, two tributaries of the Bitterroot River used by migratory adult westslope cutthroat trout for spawning, and to identify characteristics of these diversions that correlate with entrainment. I sampled fish species by snorkeling, electrofishing, fry trapping, and reconnaissance at 60 sites in 2005 and 54 sites in 2006. Annual entrainment estimates for age-0 salmonids were 18,061 and 8,972 in 2005 and 2006 in Lost Horse Creek diversions. Concurrent entrainment estimates for Tin Cup Creek were 2,995 and 2,312. Annual entrainment estimates for juvenile and adult trout (>40 mm TL) were 7,947 and 7,877 in 2005 and 2006 in Lost Horse Creek diversions.Concurrent entrainment estimates for Tin Cup Creek were 2,554 and 1,691. The species composition of trout entrained in Lost Horse Creek was dominated by brook trout, whereas on Tin Cup Creek, the species composition of entrained trout was dominated by westslope cutthroat trout. Entrainment of juvenile and adult trout was significantly associated with discharge, upstream gradient, discharge ratio, length of irrigation season, temperature, diversion dam height, and angle with downstream thalweg. A model containing discharge and upstream gradient was the best model selected using the Akaike Information Criterion for small sample sizes (AICc). Several diversions on Lost Horse and Tin Cup Creeks are responsible for the majority of entrainment occurring on these streams, and should be considered for potential screening projects. In addition, a model containing discharge and upstream gradient may help managers identify candidate diversions that could benefit from fish screens to reduce native trout losses.