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<br />also been observed feeding on fishes trapped in isolated pools (Erman and <br />leidy 1975; USFWS, unpublished data). <br /> <br />Age-O Colorado squawfish are most abundant in shoreline backwaters when <br />water temperatures are the same or greater than the main river channel and <br />mark-recapture studies indicated a diel movement of young fish 'between <br />backwaters and the mainstream river in April, October, and November (Tyus <br />1991). Abundance and growth of age-O Colorado squawfish in October was <br />negatively correlated with summer flows and positively correlated with water <br />temperature (Tyus and Haines 1991). <br /> <br />Overwintering mortality of age-O Colorado squawfish was not detected to <br />be significant during the winters of 1985-B6, 1987-88, and 1988-89 (Tyus and <br />Haines 19~1). Low flows in 198B did not reduce growth or standing crops (7.7 <br />fish/100m seined) of young Colorado squawfish. Total length of the fish in <br />October 19BB (41.3 and 45.0 mm, upper and lower Green River respectively) was <br />similar to other years of best growth (Tyus and Haines 1991). Additionally, <br />high spring catches (6.5-28.9 fish/100m2 seined) indicate that overwinter <br />survival may be high in some years. This is supported by laboratory studies of <br />Thompson (19B9) who found that all age-O Colorado squawfish survived simulated <br />winter conditions when fed, and only smaller individuals with low lipid <br />content died when starved 210 days at 3-40C. He found that age-O fish actively <br />foraged in lab and field conditions, and we assume that healthy young would <br />survive winter under normal flow conditions (Tyus and Haines 1991). <br /> <br />late summer and autumn are critical periods for growth and survival of <br />young Colorado squawfish, and flows in the Green River system at this time are <br />historically and predictably low. Tyus et al. (1987) noted that abundance and <br />growth of young Colorado squawfish in the Green River was negatively <br />correlated with late summer and autumn flows. During the late summer and <br />autumn, catch and growth were highest in 1979 and 1980 and lowest in 19B3 and <br />1984 (Tyus et al. 1987). In 19B3 and 1984, unusually high releases from <br />Flaming Gorge Dam in the late summer and autumn inundated backwater nursery <br />areas, and survivorship of young Colorado squawfish was low. These <br />relationships suggest that flows which optimize growth and survival of small <br />Colorado squawfish vary with time of year, and that both reproduction and <br />survival are highest in years whose hydrographs approximated natural flow <br />conditions. This presumably is related to the availability of nursery <br />backwater habitat in autumn (Tyus and Haines 1991). <br /> <br />Aerial photography and videography indicated that more backwaters were <br />avai1ab!e in the upper Green River with a summer flow range of about 31.13 to <br />50.94 m /s than other flows tested (Pucherelli ang Clark 1989). These authors <br />also noted that flows higher than 50.94 - 68.57 m Is reduced numbers of <br />backwaters during their study. 'High spring flows may be needed to maximize <br />backwater formation, and gradually decreasing flows may be required to produce <br />good nursery habitats (Pucherelli and Clark 1989). <br /> <br />A natural flow regime of spring highs and summer lows favors the <br />establishment of ephemeral backwater habitats within the riverine flood zone, <br />and these habitats are used extensively by young Colorado squawfish. However, <br />in some areas of the Green River where flows have been controlled, riparian <br /> <br />20 <br />