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1.0 INTRODUCTION At least 40 nonnative fish species occur in the Upper Colorado River basin (UCRB) (Nesler ~ 2003). Along with habitat modification, negative interactions of nonnative fishes are considered a major cause of the decline of native species in the UCRB (Behnke and Benson 1983; Stanford and Ward 1986; Carlson and Muth 1989; Bestgen 1990; Minckley 1991; Hawkins and Nesler 1991; Carlson and Muth ~ 1993, Lentsch et al. 1996). Nonnative fishes that inhabit the UCRB include piscivorous gamefish species (e.g. northern pike Esox lucius, channel catfish Ictalurus punctatus, largemouth bass Micropterus salmoides, smallmouth bass Micropterus dolomieu), and smaller cyprinid species (e.g., red shiner ~ Cyprinella lutrensis, sand shiner Notropis stramineus, and fathead minnow Pimephales promelas). All nonnative species may negatively impact native populations through competition, predation, or both mechanisms (Hawkins and Nesler 1991, Lentsch et al. 1996, Tyus and Saunders 1996, Tyus and ~ Saunders 2000). It is likely that the nonnative cyprinid (NNC) species impact early life stages of native species due to their prevalence in habitats used by larval and young-of-year (YOY) native fishes (Tyus et al. 1982, , McAda et al. 1994a, and Trammell et al. 1999a, and 1999c). Where distributions overlap, predation may be a significant cause of mortality in larval fishes (Pepin 1988, Bestgen 1997, Bestgen et al. 1998). Ruppert et al. (1993) established that red shiner preyed on fish larvae in the Yampa and Green rivers in Colorado. The susceptibility of early life stages of native fishes to predation and competition has thus been a concern to researchers and was the main impetus for nonnative cyprinid removal projects in the UCRB (Beyers et al. 1994, Muth and Snyder 1995). The Upper Colorado River Endangered Fish Recovery Program (Program) has identified control of nonnative species as a necessary component of the recovery plan and has been conducting nonnative fish control experiments since 1993 (Pat Nelson, personal communication). Management primarily involves some form of active removal, a method suggested by Minckley (1991) as a means to enhance ~ native fish survival in the UCRB. Lentsch et al. (1996) examined life histories of all nonnative species within the UCRB and proposed possible means for their control. The suggested control methodology for nonnative cyprinids included mechanical removal by seining in low-velocity habitats, blocking access of ~ -1-