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BIOLOGICAL AND HYDROLOGICAL DATABASE Endangered Fish Distribution. Abundance and Habitat Use Colorado squawfish Colorado squawfish are found in the Yampa River between Craig, Colorado, and the confluence of the Green and Yampa rivers (Figure 3, from Tyus and Karp 1989). The upper Yampa River (RM 50.7 - 123.3) is considered a concentration area for overwintering adults (Archer et al. 1986), as evidenced by radio transmitter monitored fish (Tyus et al. 1987) and abundance data (Miller et al. 1982, Wick et al. 1985, data incorporated information from a high flow winter, DEC-FEB mean = 386 cfs, and low winter flow, DEC-FEB mean = 161 cfs). During winter, adult Colorado squawfish use backwaters, runs, and eddies, but are most common in shallow, ice covered shoreline areas (Wick and Hawkins 1989). Local, nonmigratory movements of adJlt Colorado squawfish in nonbreeding seasons may be indicative of home-range behavior among a wide range of flows (Tyus et al. 1987,.Tyus 1991, Wick and Hawkins 1989). In the spring and early summer, adult Colorado squawfish were most often located in backwater habitats or flooded bottomlands in the Yampa River. Use of flooded bottomlands was most prevalent in flood years. Wick et al. (1983) observed that in 1982 (an average flow year), adult Colorado squawfish used flooded shoreline areas in spring, but, moved to backwater habitats as the river elevation dropped. During the late spring many adult Colorado squawfish underwent spawning migrations to a major spawning area located in the lower 25 miles of the Yampa River (Tyus 1986). Following spawning, adult Colorado squawfish. occupied a variety of habitats in mid to late summer, but were most common in eddies, pools, runs, and shoreline backwaters over sand and silt substrates (Tyus and Karp 1989). Radio tracked fish were most often located in deeper shoreline habitats, where movements suggested heavy use of eddy-run interface (Tyus et al. 1987). Early life stages of the Colorado squawfish reside only temporarily in the Yampa River. Larval Colorado squawfish emerge as sac fry from cobble spawning bars in the Yampa Canyon and drift downstream (Tyus et al. 1982b; Haynes et al. 1984) and relocate in shallow backwater habitats in the Green River (Tyus et al. 1982b, 1987, Ongoing research by the Larval Fish Laboratory, Colorado State University). Presumably, young fish are dispersed by river currents from upstream spawning areas to nursery habitats (Tyus and McCada 1984, Tyus 1986). Juvenile (60 - 450 mm) Colorado squawfish are rare in the Yampa River. Therefore, adult fish occupying the Yampa River represent individuals which have been reared downstream and move upstream as adults to occupy useable habitat. 6