7713

• UCREFRP Catalog Number: 7713
• Author: U.S. Fish and Wildlife Service, C. R. F. R. T.
• Title: Colorado Squawfish Recovery Plan.
• USFW Year: 1991.
• USFW - Doc Type: \
• Copyright Material: NO

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The Colorado River has changed dramatically since the turn of the century.
More than 20 dams have been constructed on the mainstem and tributaries since
1913. Declines of native fishes directly downstream from reservoirs are
clearly related to colder water temperatures (Vanicek et al. 1970). Other,
more subtle factors include changes in stream nutrients, altered seasonal and
daily discharge patterns, and lowered turbidity. Nutrients that ence occurred
in the rivers now are retained in the phytoplankton and zoop?ankton
populations of reservoirs. Water from the hypolimnetic layer of deep
reservoirs carries far less dissolved materials and fine particulates to
fertilize downstream river reaches. :sediments are trapped by reservoirs so
that downstream channel bottoms trans~Form from sand to armored cobble and
boulder. Channelization below dams has reduced the number and size of
backwaters and sloughs that are sought after by Colorado squawfish and other
native fishes for nursery and resting areas. The natural cycle of flood and
drought is replaced by stable discharges and water levels; seasonal
fluctuations are replaced by variable demands for irrigation water or
hydroelectric power. This combinatioin of factors effectively eliminated
Colorado squawfish and most other native species in 105 km (65.6 miles) of the
Green River below Flaming Gorge Dam ('Vanicek and Kramer 1969; Vanicek et al.
1970), caused vast biological modification in essentially the entire 389-km
(243-mile) reach of the Colorado River mainstem in Marble and Grand canyons
below Glen Canyon Dam (Carothers and IMinckley 1981), and resulted in the
exclusion of most warm-water fishes, both native and introduced, from long
reaches of the Colorado below Davis Dam (Minckley 1979).
Specific streamflows and water temperatures are particularly important for
young Colorado squawfish. Representative shallow, ephemeral backwater and
shoreline habitats in the Green River have been seined from 1979-88 to
determine the growth and relative abundance of larval Colorado squawfish (Tyus
and Haines 1991). The lowest relative larval growth and fish abundance were
observed in 1983 and 1984, and were correlated with abnormally high summer
flows from Flaming Gorge Dam that inundated nursery habitat (Tyus and Haines
1991). Streamflow modifications below major Federal reservoirs are currently
being evaluated by the Bureau of Reclamation and the Service to determine the
relationship between flows and survival of young Colorado squawfish.
Kaeding and Osmundson (1988) provided data indicating a relationship between
slow growth of Colorado squawfish in the Colorado River above Lake Powell and
the limited availability of warm water temperatures to support growth. They
suggested that slow growth decreases reproductive potential by lengthening the
time it takes an individual to reach sexual maturity. Also, this long growth
period may increase the susceptibility of young Colorado squawfish to
mortality. During recent times, mortality rates have probably increased due
to habitat changes and the competition by nonnative fish species.
Higher spring flows may be beneficial to Colorado squawfish and detrimental to
introduced fishes. Wick et al. (1983) suggested a relationship between spring
flows and fish abundance in the Yampa River. Catch rates of young squawfish
in the Colorado River are lower in years of low spring flow while numbers of
introduced minnows greatly increase (Osmundson and Kaeding 1989; McAda-and
Kaeding 1989). Higher spring flows also may provide terrestrial food for
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