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for dispersal from upstream spawning grounds to downstream nursery habitats (Tyus and McAda 1984; Tyus 1986; Paulin et aI., in review). These productive nursery habitats are created during summer by gradually decreasing flows following spring runoff. Postlarval (young-of-the-year) Colorado squawfish are rare in the Yampa River, the farthest upstream record of postlarval Colorado squawfish in the Yampa River is RK24 (Hayues et aI.1984). Juvenile Distribution, abundance, and habitat use of juvenile Colorado squawfish (60--450 nun total length) in the Yampa River are poorly understood because of the rarity of this life history stage (Miller et al. 1982; Wick et a1.1983, 1985). From 1980 to 1988, only 3% (N = 198) of all Colorado squawfish greater than 60 mm collected by Service biologists in the Yampa River were juveniles. The downstream drift of larvae from Yampa River spawning grounds suggests that a long-distance upstream movement by juveniles is needed to repopulate upstream areas (Tyus 1986). Such movement probably occurs during the late juvenile or early adult stage, because only large-sized fish are found in the upper Yampa River. This phenomenon is also supported by data from the Green River showing large concentrations of larger juvenile Colorado sq uawfish (average electroflshing catch, >0.18 fish per hour) in the lower section of the mainstream Green River and greatest concentrations of adults (average electroflshing catch, > 0.6 fish per hour) in upstream sections (Tyus et al. 1987). Humpback Chub Adult Adult humpback chubs ( > 230 mm) were captured in canyon-bound habitat in the lower 89.6 km of the Yampa River (Tyus et al. 1982a; Karp and Tyus 1989; C. Haynes, personal communication) and in the lower 16 km of the Little Snake River (E. Wick, personal communication; Fig. 4). Service biologists captured seven adult humpback chubs in the Yampa River (RK 28.8-75.2) from 1981 to 1985. From 1986 to 1988, expanded efforts between RK 6.4 and 73.6 yielded 88 captures (9 recaptures). Adult humpback chubs were most often collected in eddy habitat (average depth, 2 m), particularly in shoreline eddies created by large boulders and rapids (Karp and Tyus 1989). Adult humpback chub were commonly captured with roundtail chub and the introduced channel catfish (IctalllTllS pllllctatlls; Karp and Tyus 1989). Classification of the adult life history stage was based on the smallest ripe humpback chub captured, a 232-mm male in Whirlpool Canyon, Green River. Spawning Spawning of the humpback chub in Yampa Canyon was documented in 1986 with the capture of two spent females and two ripe males (Tyus et al. 1987). This was confirmed in 1987 with the capture of 2 ripe females, 7 ripe males, and 11 tuberculated fish (sex not determined, 2 recaptures); and in 1988 with the capture of 1 ripe female, 5 ripe males, and 2 tuberculated fish (11987 recapture: Karp and Tyus 1989). Thus, a total of 32 mature humpback chubs (3 recaptures) were captured in spawning condition in Yampa Canyon, RK 19.2-64, in shoreline eddy and run habitat (Fig. 4). Humpback chubs spawn shortly after peak spring flows (Fig. 13; 1986 not included because sampling was initiated following highest spring flows). This relation has also been noted in the Blackrocks area of the upper Coiorado River (Valdez and Clemmer 1982; Archer et al. 1985) and in the Little Colorado River, Arizona (Kaeding and Zimmerman 1983; C. O. Minckley, personal communication). Although specific discrimination of Colorado River Gila is problematical in some areas (Holden and Stalnaker 1970; Valdez and Clemmer 1982), chubs captured in the Yampa River in 1987 and 1988 were ". 1987 '" 17. "'.....160 .5",. ~:: 0 " " ...... ..." ... ", A"" '" 1988 'so -;:300 "'.....260 .5 !I: 200 . 5", ~ 100 " ...... ~, ... ". "" MONTH Fig. 13. Relation between discharge and spawning period for humpback chub. Yampa River. 1987-S8. VeJ1ical bars delineate spawning period. 16