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INTRODUCTION General The razorback sucker (Xyrauchen texanus) is one of several native fishes that were formerly common and widespread throughout warm-water reaches of the Colorado River Basin, primarily in the mainstem and large tributaries from Wyoming to Mexico. Razorback sucker numbers have declined appreciably since human alteration of the aquatic environment began in the basin near the turn of the 20' century. They currently exist naturally only as a few disjunct, aging populations or scattered individuals (Minckley et al. 1991). Most investigators (e.g., Tyus et al. 1987; Osmundson and Kaeding 1989; Bestgen 1990) consider the razorback sucker to be much more rare than the endangered Colorado pikeminnow (Ptychocheilus lucius). The decline of the razorback sucker is attributed to changes in physical and biotic factors leading to very low recruitment because of high mortality during the larval and juvenile life stages. Although the causes are not known, recruitment failure may be due to deterioration of water quality and/or predation on eggs, larvae, and juvenile razorback sucker by nonnative fish (Marsh and Langhorst 1988; Minckley et al. 1991; Pacey and Marsh 1998). Reduction of high spring flows has altered the natural flooding cycle, and some investigators (McAda 1977; Tyus and Karp 1989; Osmundson and Kaeding 1991) attribute lack of recruitment, in part, to the reduced availability of inundated floodplains which historically provided spawning and feeding for adults and nursery habitat for young. Because of its precarious status, the U. S. Fish and Wildlife Service (FWS) listed the razorback sucker as an endangered species (effective 22 November 1991; Federal Register, Vol. 56, No. 2105, 23 October 1991) under authority of the Endangered Species Act of 1973 (FWS 1973). The largest extant population occurs in Lake Mohave, Arizona, but no natural recruitment to this population has been documented in recent decades and the adult population is declining (Marsh 1994) despite recent successful augmentation efforts (Mueller 1995; Marsh 1997). The largest riverine population occurs in low gradient reaches of the Green River Basin (middle Green River, Utah), with the center of distribution between the mouth of the Duchesne River to the mouth of the Yampa River (Tyus 1987; Tyus and Karp 1990; Modde et al. 1996) (Figure 1). However, abundance estimates for wild adult razorback sucker in the middle Green River, Utah, have declined substantially from 300 to 600 adults in the late-1980s and early-1990's (Modde et al. 1996) to only about 80 to 180 individuals in the late-1990s (Bestgen et al. 2002). Recruitment failure was speculated as the reason for this recent decline.