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INTRODUCTION Demographic parameters that describe birth, movement, and mortality rates, and population abundance are useful to understand dynamics and status of animal populations. Responses of those population parameters to biotic or abiotic drivers are of interest to ecologists attempting to understand the fundamental basis for population change. They are also useful to managers attempting to maintain or enhance abundance of free-ranging animal populations, particularly when the species of interest is rare. Endangered Colorado pikeminnow Ptychocheilus Lucius, is a large, migratory, and predaceous cyprinid endemic to the Colorado River Basin. Once widespread and abundant throughout warm-water reaches of the basin, wild Colorado pikeminnow are presently restricted to the Upper Colorado River Basin in the San Juan, Colorado, and Green River sub-basins. Reasons for decline of Colorado pikeminnow center mainly on negative effects of habitat alteration from river regulation and non-native fishes (Minckley 1973; Carlson and Muth 1989; Tyus 1991). Over 140 main stem and tributary dams and reservoirs and several trans-basin water diversions provide agricultural and municipal water supplies to a rapidly expanding human population and have transformed water in the Colorado River Basin into one of the most tightly controlled supplies in the world (Iorns et al. 1965; Carlson and Muth 1989). Effects of main stem dams on biota have been particularly damaging because they restrict movements of fishes such as Colorado pikeminnow, reduce seasonal variability of discharge, water temperature, and sediment load, and increase daily hydrograph variation (Vanicek and Kramer 1969; Holden 1979; Ward and Stanford 1979; Stanford et al. 1996; Poff et al. 1997). No fewer than 60 non-native fishes have been established in the Colorado River Basin, many of which prey upon or compete with various life stages of native species (Carlson and Muth 1989; Ruppert et al. 1993). The 9