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adversely affected by cold water of 10°C and 15°C. Young Colorado squawfish are routinely collected in isolated pools in the Green River system (USFWS, unpublished data). These pools form when decreasing flows strand bodies of water from the main channel. Natural fluctuations in river level usually make this a gradual process and allow entrapped fish an escape route. However, abrupt fluctuations in river level, as is characteristic of some regulated systems, could increase mortality of small fishes by cutting off escape routes and thereby increasing potential for competitive interactions and exposure to terrestrial predation. Herons (Ardeidae), raccoons (Procyon lotor), garter snakes (Thamnophis spp.), and other animals have been observed feeding on fishes trapped in isolated pools (Erman and Leidy 1975; USFWS, unpublished data). Age-0 Colorado squawfish are most abundant in shoreline backwaters when water temperatures are the same or greater than the main river channel. Mark- recapture studies indicated a diel movement of young fish between backwaters and the mainstream river in April, October, and November (Tyus, In prep). Abundance and growth of age-0 Colorado squawfish in October was negatively correlated with summer flows and positively correlated with water temperature. Overwintering mortality of age-0 Colorado squawfish was not detected to be significant during the winters of 1985-86, 1987-88, and 1988-89 (Tyus and Haines, in review). In four of six cases, spring catches of young Colorado squawfish were greater than that the previous autumn (Tyus and Haines, in review). Low flows in 1988 did not reduce growth or standing crops (7.7/100m2) of young Colorado squawfish. Total length of the fish in October 1988 (41.3 and 45.0 mm, upper and lower Green River respectively) was similar to other years of best growth (Tyus ~nd Haines, in review). Additionally, high spring catches (6.5-28.9 fish/100m ) indicate that overwinter survival may be high in some years. This is supported by laboratory studies of Thompson (1989) who found that all age-0 Colorado squawfish survived simulated winter conditions when fed, and only smaller individuals with low lipid content died when starved 210 d at 3-4°C. He found that age-0 fish actively foraged in lab and field conditions, and we assume that healthy young would survive winters under normal flow conditions. Results of an interagency monitoring program indicated an increase-in average nu~ber of young Colorado squawfish in primary sampling areas from 20.2 fish/100m in 1986 and 1987 to 54.1 fish/100m in 1988 (U.S. Fish and Wildlife Service 1987b, 1988, 1989). Late summer and autumn are critical periods for growth and survival of young Colorado squawfish, and flows in the Green River system at this time are historically and predictably low. Tyus et al. (1987) noted that abundance and growth of young Colorado squawfish in the Green River was negatively correlated with late summer and autumn flows. During the late summer and autumn, catch and growth were highest in 1979 and 1980 and lowest in 1983 and 1984 (Tyus et al. 1987). In 1983 and 1984, unusually high releases from Flaming Gorge Dam in the late summer and autumn inundated backwater nursery areas, and survivorship of young Colorado squawfish was low. These relationships suggest that flows which optimize growth and survival of small Colorado squawfish vary with time of year, and that both reproduction and survival are highest in years whose hydrographs approximated natural flow conditions. This presumably is related to the availability of nursery backwater habitat in autumn. Aerial photography and videography indicated that more backwaters were available in the upper Green River with a summer flow range of about 31.13 to 19