Changes in Habitat Availability and Habitat Use and Movements by Two Trout Species in Response to Declining Discharge in a Regulated River during Winter

—We assessed habitat availability and habitat use by subadult cutthroat trout Oncorhynchus clarki and brown trout Salmo trutta during a four-stage flow manipulation in the Shoshone River, Wyoming, to determine how the physical characteristics of a medium-size river and the habitat use and movement patterns of subadult trout change in response to declining discharge. Discharge was reduced from 20.9 to 5.7 m3/s between December 1998 and February 1999. Changes in riverine physical characteristics included decreased water depths and velocities, increased pool area, and dewatering of riffles. Changes in habitat use differed between cutthroat and brown trout; however, both species were typically observed at locations with greater-than-average water depths and slower-than-average water velocities, especially pools with abundant cover. The observed patterns of habitat use by subadult cutthroat trout and brown trout were similar to those found for salmonids in other variable environments during winter. Selection of stable pool habitat and a tendency by both species to move relatively short distances during winter indicate that short-term (14–21-d) reductions in discharge may have little effect on subadult trout. Though hypolimnetic water releases from upstream reservoirs often dampen fluctuations in water temperature and prevent ice formation in regulated rivers (Ward and Stanford 1979), winter can be a difficult time for salmonids in these systems. Discharge is often reduced to the lowest levels of the year during winter, and decreases in discharge may limit habitat availability and reduce food supplies (Cushman 1985; Baran et al. 1995), with consequences for salmonids that feed throughout winter (Hebdon 1999; Simpkins et al. 2000a) Research on variable discharge has described the effects of increased discharge (Fjellheim et al. 1993; Gido et al. 2000; McKinney et al. 2001) or hydropower-peaking operations (Fraley et al. 1986; Valentin et al. 1996) or has modeled changes in habitat within the context of the instream flow incremental methodology (Baran et al. 1995; Heggenes et al. 1996). Field studies of the effects of * Corresponding author: [email protected] 1 Present address: Biomark, Inc., 149 South Adkins Way, Suite 104, Meridian, Idaho, 83642-8911, USA. 2 The Unit is jointly supported by the University of Wyoming, Wyoming Game and Fish Department, U.S. Geological Survey, and Wildlife Management Institute. Received June 4, 2001; accepted January 6, 2002 decreasing discharge on habitat availability and use by salmonids is lacking. We examined the effects of declining discharge on habitat availability, habitat use, and movements of subadult cutthroat trout Oncorhynchus clarki and brown trout Salmo trutta in the winter of 1998– 1999, based on a controlled flow manipulation in which discharge was reduced by approximately 400% over 3 months. We are unaware of any published research that has directly examined changes in habitat availability and fish habitat use during a controlled decrease in discharge for a system as large as the Shoshone River. Logistical constraints have precluded such research in the past, but we were given the unique opportunity to explore questions pertaining to decreased discharge during winter because of the importance of the Shoshone River fishery in Wyoming. We addressed two questions. First, as the physical characteristics of a river change in response to decreased discharge, how does habitat use by cutthroat trout and brown trout change? Second, do movement patterns change in response to de-