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SYNOPSIS <br />INTRODUCTION <br />The Colorado pikeminnow (Ptychocheilus lucius) is the largest native minnow in North America <br />and an endemic species in the Colorado River Basin. Historically, it was once widely distributed in the <br />upper and lower Colorado River basins, but now the population range has been reduced by 80 percent <br />because of interactions with introduced fishes, construction of dams, and other habitat modifications <br />(Carlson and Muth 1989; U.S. Fish and Wildlife Service 1992; Tyus 1991). Although the Colorado <br />pikeminnow no longer exists in lower basin, its populations still continue to thrive in the upper basin. <br />The greatest numbers are found in the Green River subbasin (Tyus and McAda 1984; U.S. Fish and <br />Wildlife Service 1990; Tyus 1991). <br />Colorado pikeminnow migrate during June and July to two spawning sites, Yampa Canyon or <br />Gray/Desolation Canyon, where they spawn over cobble substrates. Newly hatched larvae drift <br />downstream to one of two nursery areas in alluvial reaches of the Green River (McAda et al. 1998; Tyus <br />and Haines 1991), one downstream of the Yampa Canyon spawning site in the middle Green River (RM <br />200-319), and the other downstream of the Gray/Desolation Canyon site in the lower Green River (RM <br />0-120). The numbers of age-0 Colorado pikeminnow in the nursery areas varies greatly from year to year <br />(Bestgen et al. 1998; McAda et al. 1998), but standardized monitoring between 1986 and 1997 showed <br />average catch was three times as great in the lower Green nursery (McAda et al. 1998). Age-0 Colorado <br />pikeminnow occupy low velocity backwater habitats and may have only two or three months to grow and <br />accumulate fat reserves before entering their first winter (Thompson et al. 1991). <br />Overwinter survival during the first year of life is a primary factor determining year-class strength <br />of most temperate zone fishes (Garvey et al. 1998), including Colorado pikeminnow (Haines et al. 1998). <br />Winter is a period of reduced growth, depletion of energy reserves, and heightened mortality risk. Small <br />changes in the rate of growth or mortality of eggs, larvae, and juvenile have been shown to substantially <br />ii