Effects of Entrainment and Bypass at Screened Irrigation Canals on Juvenile Steelhead

Abstract

AbstractExpensive bypass screens have been installed throughout the western United States to prevent the entrainment of valuable anadromous salmonids into irrigation canals. However, few studies have quantified the annual extent and variability of entrainment and bypass success for anadromous salmonids emigrating through screened irrigation diversion networks in a basinwide context. By monitoring the movement of hatchery steelhead Oncorhynchus mykiss surrogates marked with passive integrated transponder tags through three irrigation canals, we estimated canal entrainment, bypass success, and survival for threatened juvenile steelhead emigrating from the Umatilla River in northeast Oregon. Annual entrainment of steelhead was positively related to the days of canal operation and negatively related to mean seasonal river flow. The relationship between flow and entrainment differed by canal; as a result, steelhead entrainment was not always greatest at canals with the largest water diversion rates. Binary models (steelhead entrainment or no entrainment) that best described daily entrainment depicted the influence of annual differences in river flows on canal entrainment and indicated that this relationship can change under different values of thermal experience (sum of daily mean temperatures). Most of the entrained hatchery steelhead (range = 97–100%) were returned to the river by screened bypasses, even when large percentages of the steelhead population were entrained. Based on similarities in movement, hatchery steelhead were good surrogates for the entrainment of naturally reared steelhead. We estimated that the three fish screens in the Umatilla River basin prevented losses of up to 25% of the steelhead population to irrigation canals over 5 years. Interannual variability in fish entrainment should be considered when monitoring irrigation canals, and managers should not necessarily limit entrainment monitoring or screening projects to canals that divert the most water.