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<br />410
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<br />COPEIA, 2000, NO.2
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<br />temperatures, sample dates, and ages (except
<br />for the earliest comparisons of some older age
<br />groups). Failure to detect similar length and
<br />weight differences in P. lucius and G. CYPha was
<br />presumably a result of small sample size result-
<br />ing from analysis of only replicate means. We
<br />conclude that lowered water temperature had
<br />the same effect on both catostomids and cypri-
<br />nids.
<br />Our results and those of Berry (1988) suggest
<br />early larval stages of some species (G. CYPha, P.
<br />lucius) enter cold coma for periods of 5-90 min
<br />upon entering cold waters, at least when expe-
<br />riencing the most extreme temperature chang-
<br />es (e.g., from 20 C or higher to 10 C or lower).
<br />Some direct mortality is possible (Berry, 1988).
<br />With smaller temperature differences (e.g.,
<br />from 20 C to 12 C), some cold coma may occur
<br />in larvae, activity levels may be reduced, and
<br />other physiological/behavioral changes may be
<br />evident (see also Berry, 1988). At moderate tem-
<br />perature differences (e.g., from 20 C to 14 C),
<br />we noted few or no short-term effects. Older
<br />larval and postlarval fishes did not exhibit any
<br />apparent physiological/behavioral effects (ex-
<br />cept lethargy) at any temperature change (see
<br />also Berry, 1988). Research on other warmwater
<br />species has also shown that behavioral changes
<br />are lessened with smaller temperature differ-
<br />ences (Speakman and Kenkel, 1972; Griffith,
<br />1978; Burton et aI., 1979) and with older fish
<br />(Pitkow, 1960; Nickum, 1966).
<br />Effects of entering cold coma in the Colorado
<br />River system are potentially severe. Predation
<br />rates may be increased (Coutant et aI., 1974),
<br />and physical damage and death may occur from
<br />abrasion against substrates, entrainment in high
<br />current velocities and turbulence, or from buri-
<br />al if fish settle on the substrate.
<br />Early life stage big-river fishes that enter cold
<br />tailwaters become exposed to essentially perpet-
<br />ual near-winter temperature conditions, with re-
<br />sultant severe growth depression that negatively
<br />acts upon numerous life-history parameters, as
<br />previously noted. Our experiments showed de-
<br />lays in transformation from larva to juvenile at
<br />lower temperatures, in some cases extending
<br />the larval stage through an entire season or
<br />more. Delays in transformation lengthen expo-
<br />sure to existing sources of mortality such as
<br />food scarcity (Papoulias and Minckley, 1990,
<br />1992), hydrological disturbance (Robinson et
<br />aI., 1998), predation (Ruppert et aI., 1993), par-
<br />asitism (Clarkson et aI., 1997), or other factors
<br />(Houde, 1987).
<br />These findings suggest that, short of dam re-
<br />moval, more flexible dam operations (especially
<br />warming of discharges) are needed to conserve
<br />
<br />populations of Colorado River basin big-river
<br />fishes in hypolimnial-release tailwaters. Effects
<br />of tailwater warming via modification of Flam-
<br />ing Gorge Dam resulted in immediate positive
<br />response by native fishes (P. B. Holden and L.
<br />W. Crist, unpubI.). Although tailwater reaches
<br />may not provide all environmental elements re-
<br />quired for successful completion of lifecycles of
<br />some of the big-river species, they should not
<br />be disregarded in recovery efforts, especially
<br />given the deteriorating or tenuous status of
<br />many of these fishes in other river reaches (Lan-
<br />igan and Tyus, 1989; Minckley et aI., 1989; Os-
<br />mundson and Burnham, 1998).
<br />Kaeding and Osmundson (1988), Minckley
<br />(1991), Childs and Clarkson (1996), and others
<br />cautioned that increasing tailwater tempera-
<br />tures to benefit warmwater native fishes may
<br />also advantage nonnative aquatic biota, perhaps
<br />to the detriment of the former. Absent such
<br />modification, however, big-river species will like-
<br />ly continue to decline and disappear from tail-
<br />water reaches, as exemplified by the trend in
<br />Grand Canyon (Minckley, 1991; Weiss, 1993; R.
<br />Valdez and R. J. Ryel, unpubI.). Thermal mod-
<br />ification of tailwaters is the only way to alleviate
<br />the known restriction by cold water tempera-
<br />tures to successful spawning, embryo incuba-
<br />tion, and larval growth of warm water native fish-
<br />es. In concert with provision of more natural
<br />(pre-dam) hydrological release patterns, we be-
<br />lieve expansion of nonnative fish populations
<br />can be minimized to the relative benefit of na-
<br />tives. Should operational changes result in ex-
<br />pansion of nonnative populations to the in-
<br />creased detriment of natives, a return to full hy-
<br />polimnetic discharge temperatures could large-
<br />ly reset the system to its former state, albeit with
<br />continued deterioration of native fish stocks.
<br />Resolution of conflicts between native and
<br />nonnative species interactions likely will require
<br />basinwide implementation of innovative activi-
<br />ties that have not yet been seriously considered.
<br />These should include routine provision of high-
<br />magnitude floods to destabilize nonindigenous
<br />fish populations (Minckley and Meffe, 1987;
<br />Tyus and Karp, 1989), development of new tax-
<br />on-specific ichthyocides, segregation of native
<br />and nonnative (sport) fisheries, and education
<br />of the public about the ecological consequences
<br />of transfaunations and values of native com-
<br />munities. Stopgap measures such as acquisition
<br />of instream flows or increased hatchery produc-
<br />tion and repatriation of native fishes may slow
<br />or stop declines, but recovery of the big-river
<br />native ichthyofauna of the Colorado River Basin
<br />may ultimately depend upon implementation of
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