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<br />INTRODUCTION <br />Colorado squawfish (Ptvchocheilus lucius Girard), largest North <br />American minnow, is endemic to the Colorado River basin. It was once <br />abundant and widely distributed (Jordan and Evermann 1896), but now <br />extant populations persist in less than about 20% of its historic range, <br />and it is classified endangered by the U.S. Fish and Wildlife Service <br />(Federal Register, 1967, 32{43}:4001). The Green River in Utah and its <br />tributary, the Yampa River in Colorado, support the largest known <br />concentration of Colorado squawfish (Holden and Wick 1982, Tyus 1990). <br />Abundance, distribution and early life history of age-O Colorado <br />squawfish have been described (Haynes et al. 1984; Tyus et al. 1982, <br />1987; Tyus and Haines, in review) and ephemeral shoreline embayments, or <br />backwaters, identified as primary nursery areas. However, little is <br />known about the influence of environmental parameters on Colorado <br />squawfish abundance. Non-native fish introductions may affect the <br />distribution and abundance of young Colorado squawfish, and a potential <br />exists for negative interactions (Behnke and Bensen 1983, Holden and Wick <br />1982, Joseph et al. 1977, Karp and Tyus 1990, Osmundson 1987). <br />We describe the influence of environmental variables (i.e. backwater <br />area, depth, temperature; peak spring and summer discharge; and <br />abundance of non-native fishes) on the abundance and distribution of <br />age-O Colorado squawfish in backwater nursery habitats. It is our <br />hypothesis that all backwater habitats are not suitable nursery habitat, <br />that presence of some non-native fishes limits habitat use by Colorado <br /> <br />3 <br />