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<br />Copeia, 2000(2), pp. 402-412
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<br />
<br />Temperature Effects of Hypolimnial-Release Dams on Early Life Stages
<br />of Colorado River Basin Big-River Fishes
<br />
<br />ROBERT W. CLARKSON AND MICHAEL R. CHILDS
<br />
<br />Declines of native big-river fishes of the Colorado River Basin, southwestern Unit.
<br />ed States, have been attributed in part to emplacement of hypolimnial-release dams.
<br />Lowered spring-summer tailwater temperatures inhibit spawning and embryonic de-
<br />velopment, depress swimming performance and growth, and reduce survival of early
<br />life stages. We examined effects of three temperatures (10 C, 14 C, 20 C) on aspects
<br />of growth, development, and physiology of larval and early juvenile life stages of
<br />Xyrauchen texanus (Razorback Sucker), Catostomus latipirmis (Flannelmouth Sucker),
<br />Gila cypluz (Humpback Chub), and Plychocheilus lucius (Colorado Squawfish) in the
<br />laboratory. Lengths, weight~, and specific growth rates of all species were signifi-
<br />cantly lower at 10 C and 14 C than at 20 C, and time to transformation from larval
<br />to juvenile life stage was delayed at colder temperatures. Transfer of fishes from
<br />20 C to 10 C (simulating transport from warm tributaries to cold tailwaters) caused
<br />loss of equilibrium in youngest life stages. AIl these effects increase exposure of
<br />larvae to existing sources of mortality. Conservation of the big-river species in hy-
<br />polimnial-release tailwaters will require thermal modification of dam discharges to
<br />lessen detrimental effects of cold water temperatures. Because such action may also
<br />benefit nonindigenous biota, recovery of the native fauna may ultimately depend
<br />upon implementation of comprehensive (and likely expensive) control measures
<br />against nonnative species.
<br />
<br />,.
<br />
<br />MAlNSTEM and major tributary rivers of
<br />the Colorado River basin, western United
<br />States, have been altered by more than a dozen
<br />major dams. These dams have contributed to
<br />declines and endangerment of indigenous,
<br />"big-river" fishes (those that range throughout
<br />the system in larger streams; Minckley et aI.,
<br />1986) by blocking migration routes (Holden,
<br />1979), and altering temperature, discharge, and
<br />sediment-transport patterns (Holden, 1979).
<br />The latter changes disrupted physical processes
<br />that created and maintained conditions neces-
<br />sary for reproduction and survival of the big-
<br />river ichthyofauna. In addition, lentic habitats
<br />created upstream from dams and the down-
<br />stream hydrographic changes facilitated estab-
<br />lishment of nonindigenous aquatic organisms
<br />that negatively impacted native fishes (Moyle et
<br />aI., 1986; Minckley, 1991).
<br />Hypolimnial-release dams in the basin espe-
<br />cially affected big-river fishes through depres-
<br />sion of spring-summer tailwater temperatures.
<br />Low temperatures (< 15 C) inhibit gonadal
<br />maturation, spawning (Holden and Stalnaker,
<br />1975; Minckley, 1991) and embryonic develop-
<br />ment (Hamman, 1982; Marsh, 1985) of these
<br />species. In studies of Ptychocheilus lucius (Colo-
<br />rado Squawfish), low temperatures depressed
<br />growth and swimming performance of juveniles
<br />(Black and Bulkley, 1985; Bestgen, 1996; Childs
<br />and Clarkson, 1996) and caused behavioral
<br />
<br />changes and mortality in larvae due to cold
<br />shock (Berry, 1988). A host of other negative
<br />life-history impacts may accrue to Colorado Riv-
<br />er basin warmwater native fishes caused by rear-
<br />ing in cold temperatures, including increased
<br />early life mortality and decreased survival to sex-
<br />ual maturity (Kaeding and Osmundson, 1988),
<br />reduced condition, lipid stores, and body size
<br />that result in elevated overwinter mortality for
<br />young-of-year fishes (Thompson et al., 1991)
<br />and lowered fecundity (McAda and Wydoski,
<br />1983) .
<br />Changes to the native big-river ichthyofauna
<br />downstream from hypolimnial-release dams are
<br />well documented and appear in large part a re-
<br />sponse to depressed spring-summer water tem-
<br />peratures. For example, the 1962 closure and
<br />subsequent operation of Flaming Gorge Dam
<br />on Green River, Utah, lowered spring-summer
<br />tailwater temperatures to near 6 C from a pre-
<br />vious range of 7-21 C (Vanicek et aI., 1970).
<br />Several native species including Gila robusta
<br />(Roundtail Chub), Rhinichthys osculus (Speckled
<br />Dace), and federally endangered P. lucius, G. cy-
<br />Pha (Humpback Chub), G. elegans (Bonytail),
<br />and possibly Xyrauchen texanus (Razorback Suck-
<br />er) ostensibly disappeared from uppermost por-
<br />tions of the 104-km tailwater reach above Yampa
<br />River following dam closure (Vanicek et aI.,
<br />1970; for discussion of Gila spp. in this reach,
<br />see Holden, 1991), a result likely hastened by a
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<br />@ 2000 by the American Society of Ichthyologists and Herpetologists
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