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

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 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.