A Basinwide Perspective on Entrainment of Fish in Irrigation Canals

—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 populationlevel 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. Irrigation in the mountain west region of the United States accelerated in the midto late 1800s after development of agricultural systems in the arid climate (Smythe 1969). Irrigation often entailed building extensive canal systems, and it was soon recognized that fish would readily enter these canals and subsequently die when flows were terminated. As early as 1893, screening programs were implemented to protect fishery resources (Clothier 1953). In the absence of such preventative measures, large numbers of fish may be lost to irrigation canals (Gardner 1941; Clothier 1953; Gebhards 1959; Hallock and Van Woert 1959). Early work on reducing fish mortality in irrigation canals focused on anadromous salmonids because of their economic value and migratory patterns (Hallock and Van Woert 1959; Fleming et al. 1987). However, recent work has emphasized that inland stream fishes often move among complementary habitats during their life cycle (Schlosser 1995; Fausch et al. 2002) and thus may be susceptible to entrainment in canals (Clothier 1953; Spindler 1955; Reiland 1997; Roberts 2004; Gale 2005). For example, Schrank and Rahel (2004) found that 23% of 40 cutthroat trout Oncorhynchus clarkii implanted with radio transmitters became entrained and subsequently died in an irrigation canal during a postspawning migration in a Wyoming stream. Currently, there is little information about population-level effects of fish entrainment in irrigation canals (Moyle and Israel 2005). Several recent studies on inland fish species have estimated the numbers of fish entrained in canal systems (Reiland 1997; Roberts 2004; Gale 2005). An important next step is to evaluate the magnitude of these losses at the population level. Basinwide consequences of mortality may not be evident from studies that consider only local mortality levels (Schill and Beland 1995; McMichael et al. 1998). Bonneville cutthroat trout O. clarkii utah undergo extensive seasonal migrations throughout the Bear River system, Wyoming, and its tributaries (Hilderbrand and Kershner 2000; Schrank and Rahel 2004; Colyer et al. 2005). Brown trout Salmo trutta have established a naturalized population that probably also migrates throughout the Bear River system. Consequently, river connectedness and anthropogenic disturbances that disrupt natural streamflows may affect fish populations throughout the system. On one of these tributaries, the Smiths Fork drainage in western Wyoming, we estimated the number of Bonneville cutthroat trout and brown trout entrained in several irrigation canals and we examined this source of mortality within a larger, basinwide population context. Our objectives were to (1) estimate the number of fish larger than 150 mm total length (TL) that were entrained in the major canals of the Smiths Fork drainage, (2) compare the estimated numbers of entrained fish with the total population sizes of each species within the Smiths Fork main stem and the * Corresponding author: [email protected] 1 Present address: Minnesota Department of Natural Resources, Section of Fisheries, 1601 Minnesota Drive, Brainerd, Minnesota 56401, USA. Received April 11, 2006; accepted March 30, 2007 Published online August 13, 2007 1335 Transactions of the American Fisheries Society 136:1335–1343, 2007 Copyright by the American Fisheries Society 2007 DOI: 10.1577/T06-111.1 [Note]