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INTRODUCTION Background The Colorado pikeminnow (Ptychocheilus lucius) is a large piscivorous cyprinid indigenous to the Colorado River Basin, reaching lengths of 1.8 m and weights of 36 kg (Miller 1961). Once common throughout the basin, extensive water development, land use practices, pollutants, and non- nuti ve fishes led to extirpation of the species from the lower basin by the early 1970s, and a reduction in distribution and abundance throughout the upper basin (Tyus 1991). The Colorado pikeminnow was included in the first List of Endangered Species issued by the Office of Endangered Species on II March 1967 (32 FR4001), and it received protection under the Endangered Species Actofl973, as amended (39 FR 1175). A Colorado Squawfish (Pikeminnow) Recovery Plan was approved on 16 March 1978 and revised 6 August 1991 (U.S. Fish and Wildlife Service 1978). Critical habitat was designated for the Colorado pikeminnow, along with three other mainstem species, on 21 March 1994 (50 FR 13374). The largest numbers of Colorado pikeminnow remain in the Green River sub-basin of the Upper Colorado River Basin, including the Yampa River and Green River (Haines et al. 1985, Tyus 1986, 1990, 1991). Adults make extensive migrations and home to specific spawning sites in canyon-bound reaches (Tyus 1990). The majority of spawning in the Green River sub-basin has heen associated with the lower Yampa River in Yampa Canyon and the Green River in lower Desolation Canyon (Tyus 1985, 1986). Spawning typically occurs July through August when water temperature reaches 18-220C (Vanicek and Kramer 1969; Holden and Stalnaker 1975; Seethaler 1(78). Females are accompanied by males and broadcast adhesive eggs over cobble/gravel bars; egg incubation time is usually 3-4 days (Hamman 1981). Newly hatched larvae are transported by river currents 3-15 days after hatching for varying distances downstream to nursery backwaters in alluvial reaches (Nesler et al. 1988; Tyus and Haines 1991). Age-O Colorado pikeminnow are abundant in hackwaters in late summer and fall (Holden and Stalnaker 1975; Valdez and Williams 1993; Tyus and Haines 1991). The Interagency Standardized Monitoring Program (ISMP, McAda et al. 1997) was implemented in the Upper Colorado River Basin in 1986, with annual fall sampling of age-O Colorado pikeminnow in backwaters. This ISMP was designed to monitor year to year variation and long-term patterns in age-O cohort strength, but single sampling precludes assessment of cohort survival. Additional sampling was incorporated into the ISMP from spring 1988 to spring 1996 to determine overwinter survival indexof age-O Colorado pikeminnow by comparing fall densities with spring densities of the same cohort. Analyses oflSMP data have been performed to assess overwinter survival of age-O Colorado pikeminnow for the Colorado River (McAda and Ryel1999) and for the Green River (Haines et al. 1998). These studies have been unable to quantitatively assess survival and to conclusively demonstrate cause-and-effect relationships between various environmental parameters and survival. Nevertheless, these studies have demonstrated that there are complex relationships among various parameters that affect survival. Indices of overwinter survival were highest in years of moderate to high river flow, and relative densities in spring were often related to relative densities in fall. There