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Large, age-0 Colorado pikeminnow are most abundant in shoreline backwaters, but they also use other habitats, presumably in response to changing water temperature. Larval pikeminnow in the UCR (RM 26.8) used backwaters that were warmer than the river channel (Valdez et al. 1982). Similar findings are reported from mark-recapture studies in the Green River, where young fish were observed making diel movements between backwaters and the main channel (Tyus 1991 b). The movements appear to be associated with temperature because fish occupied warmer backwaters in the day, but moved into the main channel as water temperature decreased in the backwaters at night. The young fish also are sensitive to changes in water level; they move into ~ shallow backwaters as water level increases (Tyus 1991 b) and move out of shallow areas when water recedes, thereby avoiding being trapped in isolated pools (Valdez et al. 1982). Abundance and growth of age-0 fish showed a negative correlation with flows in the river and a positive correlation with water temperature (Tyus and Haines 1991). Thus, the larvae appear to do better when river flow is low and water ~ temperature is high, at least during some times of the year. Juvenile pikeminnow (age 0 and age 1) appear to be very tolerant of winter conditions. In laboratory studies that simulated winter conditions, Thompson (1989) found that most of the larvae survived 210 days of starvation at 3-4°C, and Thompson et al (1991) ~ determined that overwinter survival of age-0 pikeminnow was related to large size and fat content at the onset of winter. Amore recent study also supports the premise that healthy young fish are tolerant of starvation (Bestgen 1996). Healthy larvae would be able to survive winter under normal flow conditions, even with relatively little food (Tyus and Haines 1991). ~ Juveniles. Juvenile Colorado pikeminnow tend to leave the nursery habitat during the next spring runoff, when they are greater than 60 mm long, and gradually move upstream toward adult habitat. The process takes time, and the juveniles (60-200 mm) and subadults (20000 mm) become spread throughout the system (cf. Valdez et al. 1982). Most of the movement probably occurs during the late juvenile or subadult ~ stage, because only large-sized fish are found in the upper Yampa River, and the highest concentration of juveniles is found in the lower Green river (Tyus 1986 and 1990, Tyus et al. 1987). Osmundson et al. (1997) reported that this movement occurs in the UCR when the fish reach a size of about 450 mm. Fifteen of eighteen fishes that entered the Gunnison River via the new Redlands fish passageway were juveniles that ~ came from the lower part of the UCR (F. Pfeifer, personal communication). The search for new habitat is probably associated with a change in diet. The condition of juveniles seems to decline with age if they remain in the lower part of the UCR, whereas condition improves with age in the upper part of the river (Osmundson et al 1997). Sexual maturity can be attained as early as 5-6 years under artificial conditions ~ (Hamman 1981), but probably not until about 8 years in nature (Tyus 1990, Hawkins 1991, Seethaler 1978). Integration of Life History. The life history of the Colorado pikeminnow has been studied in greater detail than that of the other three endangered fishes, and much 17