A Basinwide Perspective on Entrainment of Fish in Irrigation Canals

Abstract

Entrainment in irrigation canals has been considered a major source of mortality for some fish populations. However, the magnitude of fish loss from entrainment is usually not evaluated from a basinwide perspective, thus precluding an assessment of population-level consequences. We incorporated such a perspective in evaluating entrainment losses of Bonneville cutthroat trout Oncorhynchus clarkii utah and brown trout Salmo trutta in a southwestern Wyoming river basin that is used for irrigating agricultural crops. We estimated the number of Bonneville cutthroat trout and brown trout (>150 mm TL) entrained in irrigation canals and then compared these numbers to population estimates within the river main stem and across the entire basin. We also examined previous studies to determine annual mortality rates for assessment of realized losses to irrigation canals. Small percentages of the Bonneville cutthroat trout (1.2-3.3%) and brown trout (0.4-1.2%) populations within the basin were entrained in 2004 and 2005. These values were much lower than the total annual mortality rates for inland riverine cutthroat trout O. clarkii (57.4%) and brown trout (56.4%) in the mountain west region of the United States. Examining the number of entrained fish in context with the basinwide population and total annual mortality allows evaluation of the significance of fish entrainment losses to irrigation canals.