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Preliminary flow-training trials conducted at Utah State University indicate that the red muscle cells, which are essential for sustained swimming in current, increased both in size and number when bonytail (Gila elegans) were intermittently subjected to flow (1 hour of flow/3 hours of no flow) and exercised in circular tanks (Crowl et al. 1995). In the riverine environment, fish not physically trained or exercised to flow may be at a disadvantage because they remain in low velocity areas and may not physically maintain themselves in high velocity for short periods where densities of preferred food might be higher (Personal communication, Todd Crowl). Pond-reared razorback sucker stocked in the Upper Colorado and Gunnison rivers may have not been capable of switching to available food items in the river soon enough because they were not able to move to areas of abundant food resources and, consequently, slowly starved. Therefore, critical for survival might be the ability for stocked razorback sucker to learn to acclimate, move to, and maintain themselves in flowing water habitats to utilize abundant food items. Physical conditioning to flow may be important and subsequently critical for fish to utilize available food items in the environment that they are stocked, and their ultimate ability to survive. Modde and Meyer (1991) believed that the high mortality of captive-reared bonytail stocked in the Upper Green River, Utah, in 1988 and 1989 (Chart and Cranney 1990) was the result of the failure of individuals to adapt to natural conditions rather than the inability of the species to survive in a natural environment. Chart and Cranney (1990) strongly advised against release of any hatchery-reared adult bonytail into a riverine environment, without a lengthy period of conditioning (e.g., flow conditioning). Crowl et al . (1995) hypothesized that bonytai 1 should be raised in flow for as long as possible prior to stocking and simulating a natural environment would assist bonytail to adapt both physiologically and behaviorally to any reintroduction site. The Colorado Division of Wildlife recommends stocking fingerling (3 to 4- inch), hatchery-reared rainbow trout rather than younger (2-inch and smaller) or older, larger trout because post-stocking survival is greater (Nehring 1990). Older salmonid piscivores reared in static water may require more time to adapt both physiologically and behaviorally to lotic water conditions than younger fish. Learning and physical conditioning, similar to imprinting, may occur only early in life, and once past a certain age, cannot be "triggered" or initiated. Older, captive-reared fish, although they may have adequate fat reserves, may take too long to learn to convert to natural feed or simply may not utilize natural feed following stocking. However, several factors such as time of year stocked, type of habitat stocked, acclimation period, predators, handling and transport stress, and genetics (wild versus domestic; Nehring 1990) also influence post-stocking survival. Habitat Use Upper Colorado River Fifty-seven habitat observations were obtained from 12 different fish. Fish used seven different habitat types: main channel, backwaters, eddy/pools, 18