Effects of fall-to-winter changes in habitat and frazil ice on the movements and habitat use of juvenile rainbow trout in a Wyoming tailwater

Overwinter declines in the abundance of small rainbow trout Oncorhynchus my kiss have been observed in a section of the Big Horn River that lies downst ream from Boysen Reservoir, where reservoir releases prevent surface ice fo rmation. To provide insight into the possible causes of these declines in a bundance, radiotelemetry was used to determine movement and microhabitat us e of juvenile (20-25 cm total length) rainbow trout during the fall and win ter of 1995-1996. Throughout the fall and winter, both stocked (hatchery) a nd naturally spawned (wild) fish were generally found in main-channel pools with cover that reduced current velocities to less than 2 cm/s near the bo ttom and with nearby (<2 m) water velocities that were greater than 15 cm/s . These locations provided refuges from the current, with adjacent flowing water that could deliver drifting aquatic invertebrates. The fish were gene rally associated with cover that was formed by aquatic vegetation early in the fall, but they shifted to cobble and boulder cover (in deeper water) as the aquatic vegetation decomposed and as winter progressed. Episodes of fr azil ice in January and early February were associated with movements of wi ld fish in the upstream portion of the study area-from normal activity area s to refuges at the bottom of deep pools or under shelf ice in shallow wate r near shore. Frazil-ice episodes often initiated long-term movements among fish. Our results suggest that changing habitat features from fall to wint er and frazil-ice episodes can cause juvenile rainbow trout to move and to modify their habitat use, depending on their location in a tailwater.