Quantification and characterization of bull trout annually entrained in the major irrigation canal on the St. Mary River, Montana, United States , and identification of operations changes that would reduce that loss

The entrainment of fish in irrigation canals has long been a management concern in western North America. In north‐central Montana, United States, the presumed but unexamined entrainment of the “threatened” bull trout ( Salvelinus confluentus Suckley) in the St. Mary Canal was considered a major impediment to the species' Federal Endangered Species Act recovery. Between April and September, 2002–2006, we systematically netted fish entrained at four canal gates. Among the total 9,981 fish caught during 21,064 net hours, 207 were bull trout mainly 120–200 mm total length (91%) and age 2 (60%) or age 3 (38%). Because catch per unit sampling effort was small and positive values few, the data were pooled among years. Monte Carlo simulation was used to estimate entrainment within months, hours of the day, and individual nets, as well as total annual entrainment. Bull trout entrainment was lowest during daylight and highest during darkness. Seasonally, entrainment was highest in spring, lowest in summer, and of moderate size in early fall. We estimated 140–274 (95% prediction bounds) bull trout were entrained annually under current canal operations; a 90% reduction in that entrainment if there were no April–May operations; and an 80% reduction if operations were limited to daylight hours. That final result suggested bull trout entrainment may be reduced by operation of non‐physical barriers (e.g., artificial illumination of the river) near the canal gates at night. Our approach provided a tool that could be used to reduce bull trout entrainment while potentially meeting water‐delivery needs.