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1358 small and may be extirpated (Platania et al. 1991; Holden 2000). In the Colorado River subbasin, abundance estimates for the 278-river-kilometer (rkm) reach averaged about 600 subadult and adult fish from 1991 to 1994 (Osmundson and Burnham 1998), increasing to about 750 subadult and adult fish in the period 1998-2000 (Osmundson 2002). The abundance of Colorado pikeminnow in the Green River subbasin is unknown but thought to be the largest in existence based on the larger occupied area (about 900 rkm) and relatively high catch rates in basinwide monitoring from 1986 to 2000 (Tyus 1991; McAda 2002). Historically, Colorado pikeminnow 1,800 mm in length and 40 kg in weight have been reported (Jordan and Evermann 1896; Minckley 1973; Tyus 1991; Quartarone 1995), although the largest individual known since intensive sampling began in the late 1970s was 1,240 mm (this study). Large individuals may be 35-50 years old and very slow-growing, based on average growth rates of 10 mm/year or less for recaptured individuals (Hawkins 1992; Osmundson et al. 1997). Sexual maturity is not reached until 7-10 years of age and a length of about 450-550 mm (Vanicek and Kramer 1969; Osmundson et al. 1997; Osmundson and Burnham 1998; Osmundson 2006). Reproduction and recruitment of Colorado pike- minnow in the Green River subbasin is spatially segregated and occurs over a broad scale. Adults migrate up to 745 rkm round-trip to spawning areas in the lower Yampa Canyon of the Yampa River and in Desolation-Gray Canyon in the Green River in late spring (Tyus 1990; Irving and Modde 2000). Colorado pikeminnow typically begin spawning around the summer solstice, when flows decline to near base level and water temperatures range from 160C to 220C (Haynes et al. 1984; Nesler et al. 1988; Tyus 1991; Bestgen et al. 1998). Eggs deposited over geomorph- ically complex riffle habitat hatch within 4-7 d at water temperatures of 18-260C (Hamman 1981; Marsh 1985; Tyus 1990; Harvey et al. 1993; Bestgen and Williams 1994), and 5-9 d later 7-9-mm larvae are transported downstream 40-200 rkm to alluvial river reaches, where they rear in warm, low-velocity shoreline areas (e.g., backwaters) throughout the summer (Nesler et al. 1988; Tyus and Haines 1991; Bestgen 1996; Bestgen et al. 1998). Variation in year-class abundance of young is thought to cause variation in the abundance of adult Colorado pikeminnow (Tyus 1991; Osmundson et al. 1998, 2002), Study Area The Green River subbasin (hereafter, "Green River basin" unless specified otherwise) drains high- elevation mountains in southwestern Wyoming, north- BESTGEN ET AL. eastern Utah, and western Colorado (Figure 1). The study area included warmwater stream reaches desig- nated as critical for recovery of the Colorado pike- minnow (U.S. Fish and Wildlife Service 2002) and consisted of five main areas: the Yampa and White rivers and three reaches of the Green River. We sampled a total of 819 rkm or 88% of the critical habitat, and we believe that nearly all Colorado pikeminnow in the basin were found in our study reaches in spring because of movements to those areas (Bestgen et al. 2005; Kitcheyan and Montagne 2005). The 118-km-long reach of the Yampa River extended from rkm 192 to rkm 74; few Colorado pikeminnow have been documented upstream of that area (Holden and Stalnaker 1975; Finney 2006). The 74-km-long, canyon-bound lower reach of the Yampa River was excluded because few fish occur there outside of the spawning season (Holden and Stalnaker 1975) and logistical constraints prevented sampling. The middle Green River study reach was 143 km long and extended from rkm 539.4 (16.2 rkm downstream of the confluence with the Yampa River) downstream to just above Desolation Canyon at rkm 396 (near the White River confluence) and included the lowermost few kilometers of the Duchesne River. The reach was in an alluvial valley with the exception of Split Mountain Canyon (rkm 528.2-513.8) and the 16.2- km-long Whirlpool Canyon, which were excluded from sampling. The White River study reach was 167.4 km long and extended from just below Taylor Draw Dam downstream to the confluence with the Green River; Colorado pikeminnow are thought to be extirpated upstream of Taylor Draw Dam (Martinez 1986; Martinez et al. 1994). The Desolation-Gray Canyon reach was 189.8 km long and extended from the head of Desolation Canyon at rkm 395.9 downstream to near the lower end of Gray Canyon at rkm 206.1. The lower Green River reach extended 193.2 km through the low-gradient Labyrinth and Stillwater canyons and extended from near the town of Green River, Utah, downstream to the confluence with the Colorado River (rkm 0). Other areas not sampled in the Green River basin included the Price, San Rafael, upper Duchesne, and Little Snake rivers and Lodore Canyon of the upper Green River, Adult Colorado pikeminnow are thought to be relatively rare in those locations because of small stream size or because fish made only temporary seasonal movements to such reaches after abundance estimation sampling occurred (Holden and Stalnaker 1975; McAda et al. 1980; Marsh et al. 1991; Wick et al. 1991; Hawkins et al. 1996; Cavalli 1999; Muth et al. 2000; Kitcheyan and Montagne 2005). River geomorphology in the study area varied; ,