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1 1 1 1 1 1 1 1 1 1 1 1 1 1 Introduction Endangered Colorado squawfish Ptychocheilus lucius, is a piscivorous cyprinid endemic to the Colorado River Basin. Colorado squawfish have been negatively affected by human-induced changes in their physical and biological environment. Physical changes are largely due to regulation of rivers which has reduced magnitudes of discharge, water temperature, and seasonal hydrograph variation, but has increased daily hydrograph variation, and restricted movements of Colorado squawfish (Vanicek and Kramer 1969; Holden 1979; Tyus and Karp 1989; Tyus 1991). Some biological changes can be attributed to indirect effects of modifying the physical environment, but many are directly related to introduction of over 40 non-native fishes into the Basin (Carlson and Muth 1989). Many of these fishes are potential competitors or predators of different life stages of Colorado squawfish. As a result of these physical and biological changes, natural populations of Colorado squawfish have declined and are currently restricted to the San Juan and Colorado River basins upstream of Lake Powell (Platania et al. 1991; Tyus 1991). Within the Colorado River Basin, populations in the Green River system are thought to be relatively large and stable. Recruitment of age-0 Colorado squawfish in fall to these populations has been documented annually since 1979, but is variable (Kesler et al. 1988; Tyus and Haines 1991; Tyus 1991). Establishing causal relationships between putative environmental factors and recruitment patterns has been the goal of the Recovery Implementation Program for Endangered Fish Species in the Upper Colorado River Basin since its inception (Wydoski and Hamill 1991). Well established causal relationships could be used to guide management actions aimed at ensuring 2