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!~91 T~ viS l' 1- rlai'\~e,s Transactions of the American Fisheries Society 120:79-89, 1991 7775 Distribution, Habitat Use, and Growth of Age-O Colorado Squawfish in the Green River Basin, Colorado and Utah ~ HAROLD M. Tyus AND G. BRUCE HAINES u.s. Fish and Wildlife Service 1680 West Highway 40, Suite 1210, Vernal, Utah 84078, USA . Abstract. - Age-O Colorado squawfish Ptychocheilus lucius (N = 11,379) were captured as larvae and postlarvae in drift nets and seines in the Green and Yampa rivers from 1979 to 1988. Larvae were captured in drift nets (N = 601) for 2-6 weeks in June and July. Small postlarvae (N = 3,079) were captured by seining shoreline habitats in July and August. In September and October, postlar- vae were most abundant (N = 6,459) in low-gradient reaches of the Green River. Catch data indicated that the postlarvae moved from the Yampa and Green river spawning areas and were concentrated about 150 km downstream by autumn of each year. Spring sampling indicated that young fish (N = 1,240) overwintered in areas occupied the previous autumn. Postlarvae captured in the Green River (N = 5,043) most frequently occupied shoreline embayments (backwaters) that were relatively warm (mean, 17.0oC), deep (mean, 38 cm), large (mean, 826 m2), and turbid. Abundance and size of young Colorado squawfish in the Green River were inversely correlated with high summer and autumn flows, which inundated nursery habitats. Seine catches of young squawfish in 4 years of sampling the upper and lower Green River in autumn and the following spring were not reliable in assessing overwintering mortality ofage-O Colorado squawfish, presum- ably due to differences in capture vulnerability between seasons. Survival of small fish in spring indicated their tolerance of prevailing winter conditions. 'II Colorado squawfish Ptychocheilus lucius, the largest North American minnow, is endemic to the Colorado River basin, where it was once abun- dant and widely distributed (Jordan and Ever- mann 1896). The species was classified as feder- ally endangered by the U.S. Fish and Wildlife Service in 1967 (Federal Register 32[43]:4001) and natural populations presently exist only in the up- per Colorado River basin (upstream of Lee Ferry, Arizona). The Green River in Utah and its largest tributary, and Yampa River in Colorado (Figure I), contain the largest known concentration of Colorado squawfish (Holden and Wick 1982; Tyus, in press). Two major spawning areas have been identified in the Green River basin by radiotracking the mi- grations of Colorado squawfish and capturing ripe adults. One spawning area in the lower Yampa River (i.e., Yampa Canyon) was located in 1981 (Tyus and McAda 1984); a second spawning area in Gray Canyon of the Green River was located in 1983 (Tyus 1985). Successful Colorado squaw- fish reproduction was confirmed at the Yampa Canyon site by collections of newly hatched larvae (Haynes et al. 1984; Nesler et al. 1988). However, the early life history and ecology of larval and postlarval Colorado squawfish are not well known. Events during the first year of life regulate many fish populations (Braum 1978), and a better un- derstanding of such events should aid present re- covery efforts (Rose and Hamill 1988) for this speCies. Construction and operation of Flaming Gorge Dam and Reservoir has significantly altered his- toric flow and temperature regimes in the Green River and this alteration has been implicated in the decline of Colorado squawfish (Seethaler 1978; Holden and Wick 1982). We studied distribution, relative abundance, and growth of age-O Colorado squawfish in order to evaluate effects of present summer and early autumn flow regimes on the fish. We also sampled standing crops of young Colorado squawfish in autumn and spring to eval- uate the effects of winter conditions on survival and growth. Finally, we evaluated summer con- ditions and the sizes (total length) of age-O Colo- rado squawfish in the upper and lower Green Riv- er with the hypothesis that colder water temperatures and fluctuating flows produced by Flaming Gorge Dam should reduce growth of the fish in the upper Green River. Study Area The Green River basin is in eastern Utah, northwestern Colorado and southern Wyoming (Figure I). The primary study area included the lower 552 km of the main stem Green River, downstream of its confluence with the Yampa River, and the lower 30 km of the Yampa River during 1979-1988. However, sampling was also 79