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North American Journal of Fisheries Management 22:917–928, 2002 � Copyright by the American Fisheries Society 2002 Changes in Habitat Availability and Habitat Use and Movements by Two Trout Species in Response to Declining Discharge in a Regulated River d

By Matthew R. Dare, Wayne, A. Hubert and Kenneth G. Gerow


Abstract.—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 m 3 /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

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