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INTRODUCTION <br />The Colorado pikeminnow (Ptychocheilus lucius) is the largest native minnow in North <br />America and an endemic species in the Colorado River basin. Historically, it was once widely <br />distributed in the upper and lower Colorado River basins, but now the population range has been <br />reduced by 80 percent. Construction of mainstem impoundments, which altered historic flows <br />and temperature regimes, and the introduction of nonnative predators are believed to have <br />reduced the Colorado pikeminnow population (Tyus 1991a; Tyus and Haines 1991; Irving and <br />Modde 2000). By the 1970's, the Colorado pikeminnow was extirpated in the lower basin <br />(Minckley 1973), but natural populations of Colorado pikeminnow continue to persist in the <br />northern portion of its historical range. The greatest numbers are found in the Green River <br />subbasin (Tyus and McAda 1984; USFWS 1990; Tyus 1991a). <br />In the Green River, Colorado pikeminnow migrate in June and July to two spawning <br />sites, Yampa Canyon and Gray/Desolation Canyon, where they spawn over cobble substrate <br />(Tyus 1990). Newly hatched larvae drift downstream to one of two nursery areas in alluvial <br />reaches of the Green River (McAda et al. 1998; Tyus and Haines 1991), one downstream of the <br />Yampa Canyon spawning site in the middle Green River (RM 200-319), and the other <br />downstream of the Gray/Desolation Canyon site in the lower Green River (RM 0-120). The <br />numbers of age-0 Colorado pikeminnow in the nursery areas varies greatly from year to year. In <br />the middle Green River between 1986 and 1997 catches ranged from near zero to 1.9 fish per <br />IOm2 seined (average = 0.4), and in the lower Green River for the same time period catches <br />ranged from near zero to 5.6 fish per lOm2 seined (average= 1.4; McAda et al. 1998). Age-0 <br />Colorado pikeminnow occupy low-velocity backwater habitats and may have only two or three <br />A-1