7047

• UCREFRP Catalog Number: 7047
• Author: Tyus, H. M. and C. A. Karp.
• Title: Habitat Use and Streamflow Needs of Rare and Endangered Fishes in the Green River, Utah
• USFW Year: 1991.
• USFW - Doc Type: Final Report.
• Copyright Material: NO

also 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Leidy 1975; USFWS, unpublished data).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 and
mark-recapture studies indicated a diel movement of young fish between
backwaters and the mainstream river in April, October, and November (Tyus
1991). Abundance and growth of age-0 Colorado squawfish in October was
negatively correlated with summer flows and positively correlated with water
temperature (Tyus and Haines 1991).
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 191). Low flows in 1988 did not reduce growth or standing crops (7.7
fish/100m seined) 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 (zyus and Haines 1991). Additionally,
high spring catches (6.5-28.9 fish/100m seined) 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 days 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 winter under normal flow conditions (Tyus and Haines 1991).
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 (Tyus and Haines 1991).
Aerial photography and videography indicated that more backwaters were
availab~e in the upper Green River with a summer flow range of about 31.13 to
50.94 m /s than other flows tested (Pucherelli an~ Clark 1989). These authors
also noted that flows higher than 50.94 - 68.51 m /s reduced numbers of
backwaters during their study. High spring flows may be needed to maximize
backwater formation, and gradually decreasing flows may be required to produce
good nursery habitats (Pucherelli and Clark 1989).
A natural flow regime of spring highs and summer lows favors the
establishment of ephemeral backwater habitats within the riverine flood zone,
and these habitats are used extensively by young Colorado squawfish. However,
in some areas of the Green River where flows have been controlled, riparian
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