Irrigation Canals as Sink Habitat for Trout and Other Fishes in a Wyoming Drainage

—Irrigation canals can be a major source of mortality for fish in the Rocky Mountain region. Our study looked at how fish were affected by the irrigation canal system in the Smiths Fork, a tributary to the Bear River in western Wyoming. There are two native species of conservation concern in the Smiths Fork drainage: Bonneville cutthroat trout Oncorhynchus clarkii utah and northern leatherside chub Lepidomeda copei. Our objectives were to determine the relative abundance of each species within the canals and the fate of trout (Bonneville cutthroat trout and brown trout Salmo trutta) that enter canals. During the summer of 2003 we sampled 30 sites within the Covey Canal system, which is the largest canal system withdrawing water from the Smiths Fork. Because fish were observed to accumulate at certain spots in the canal system, we developed a sampling scheme that incorporated both random sample sites and sites known to attract fish. We estimated that between 6,300 and 10,400 fish encompassing 10 species were entrained in this canal system. The two most abundant species were speckled dace Rhinichthys osculus (29% of all fish) and mountain sucker Catostomus platyrhynchus (37% of all fish). Bonneville cutthroat trout and northern leatherside chub each comprised 2% of the total entrained fish. We implanted 30 Bonneville cutthroat trout and 13 brown trout with radio transmitters to determine whether entrained trout could leave the canal system when water levels were reduced in late summer. We found that 77% of the transmitter-implanted fish died within the canals, indicating that this system functions as sink habitat for Bonneville cutthroat trout and brown trout. Based on this mortality rate, we estimated that 120 Bonneville cutthroat trout (95% confidence interval, 75–165) and 299 brown trout (280–317) perished in the Covey Canal system during the summer of 2003. Fisheries biologists are increasingly taking a landscape approach to the management of fish populations (Schlosser and Angermeier 1995; Fausch et al. 2002). One reason for considering management issues at larger spatial scales is our increased understanding of the importance of movement in the ecology of many stream fishes (Brown and Mackay 1995; Bunnell et al. 1998). This is especially true for species such as cutthroat trout Oncorhynchus clarkii that use widely separated areas within lotic systems during their life cycle (Jakober et al. 1998; Schrank and Rahel 2004; Colyer et al. 2005). The ability of fish to move among widely separated habitats to fulfill life history requirements can be negatively affected by water development activities. For example, dams and their associated reservoirs can block upstream migration of spawning fish (Schmetterling 2003), impede downstream migrations of juvenile fish (Raymond 1979) or subject juvenile fish to increased predation (Beamesderfer et al. 1996). Canals built for water conveyance and irrigation also disrupt fish migrations when they entrain fish (Clothier 1953; Gale 2005; Post et al. 2006; King and O’Connor 2007). In many cases, water is diverted into irrigation canals only during the agricultural growing season, and thus the canals are dry and unsuitable as fish habitat for part of the year. From a landscape perspective, canals that entrap and kill fish can be considered sink habitats because they are unsuitable for reproduction or longterm survival (Pulliam 1988; Dunning et al. 1992). Although studies have documented the entrainment of fish in irrigation canals (Clothier 1953; Leslie et al. 1990; Reiland 1997; Gale 2005; Post et al. 2006; Carlson and Rahel 2007; King and O’Connor 2007), little is known about the ability of fish to escape from canals and return to streams when the canals go dry. In natural river systems, fish species may use seasonally flooded side channels for reproduction or feeding but then return to main channel habitats as flood waters recede (Kwak 1988). By contrast, irrigation canals often have a headgate structure that may limit the movement of fish back into the main stem. Because irrigation canals are a widespread feature of the landscape in arid regions, it is important to know whether they serve as a seasonally available habitat for fish, or as a sink habitat that causes additional mortality on fish populations. * Corresponding author: [email protected] 1 Present address: Cooperative Institute for Limnology and Ecosystems Research, School of Natural Resources and Environment, University of Michigan, 2205 Commonwealth Boulevard, Ann Arbor, Michigan 48105-2945, USA. Received March 13, 2007; accepted November 29, 2007 Published online May 22, 2008 951 Transactions of the American Fisheries Society 137:951–961, 2008 Copyright by the American Fisheries Society 2008 DOI: 10.1577/T07-058.1 [Article]