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Item An assessment of losses of native fish to irrigation diversions on selected tributaries of the Bitterroot River, Montana(Montana State University - Bozeman, College of Letters & Science, 2007) Bahn, Leslie; Chairperson, Graduate Committee: Alexander V. ZaleWithdrawals 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.