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INTRODUCTION <br />Background <br />Warm-water reaches of the Colorado River upstream of Westwater Canyon <br />historically supported a native fish community that included four species that are today <br />classified as endangered: Colorado pikeminnow Ptychocheilus lucius, razorback sucker <br />Xyrauchen texanus, humpback chub Gila cypha and bonytail Gila elegans. Bonytail are <br />extirpated from this reach (Kaeding et al. 1986) and wild razorback sucker have become <br />exceedingly rare (Bestgen 1990). One small self-sustaining humpback chub population <br />occurs at Black Rocks, a 2-km reach just upstream of the Utah-Colorado state line <br />(Kaeding et al. 1990, McAda 2002). Colorado pikeminnow occur in small numbers <br />throughout the area upstream to Palisade, Colorado, where further upstream movement <br />is blocked by the Price-Stubb diversion dam (Osmundson and Burnham 1998). <br />Declines of these species are generally attributed to physical habitat alteration, <br />flow regime modification, contaminants, and competitive and predatory effects of <br />introduced fishes (Holden 1974, Hawkins and Nesler 1991). Low-velocity backwater <br />and flooded bottomland habitat are believed critical to the early life phases of both <br />Colorado pikeminnow (Tyus and Haines 1991) and razorback sucker (Wydoski and <br />Wick 1998), although apparently not for humpback chub (Chart and Lentsch 1999). <br />Early-life habitat preferences or requirements for bonytail are unknown. <br />Non-native fishes numerically dominate fish assemblages within upper Colorado <br />River backwaters and their interaction with young of both Colorado pikeminnow and <br />razorback sucker may be important impediments to recruitment for both species <br />(Trammell et al. 2002). Although wild razorback sucker are almost extirpated, a <br />supplemental stocking program begun in 1996 (Burdick 2002) will only be successful if <br />natural reproduction and recruitment result in a self-sustaining population (USFWS <br />2002). Additionally, reintroduction stocking of bonytail began in the upper Colorado <br />River in 2001 (W. Elmblad, Colorado Division of Wildlife, personal communication). <br />Survival rates of stocked bonytail and their progeny may also be affected by predation <br />from non-native fish.