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138 The Southwestern Naturalist vol. 30, no. 1
<br />closure temperatures ranged from 0.6 to 22.2°C compared with 0.0 to 22.01C
<br />following closure (Vanicek, 1967). "Big river" fishes persist in the upper
<br />Colorado basin only in areas other than those impacted by mainstream
<br />reservoirs or downstream reaches modified by hypolimnetic water releases
<br />(Vanicek, 1967; Joseph et a]., 1977; Seethaler, 1978).
<br />Tailwater temperatures below Hoover Dam from 1941-80 fluctuated
<br />between 9.0 and 15.0°C (USGS, 1980b), and during a more recent period of
<br />record (1977-78) between 11 and 12.5°C (Paulson et al., 1980). This discharge
<br />affects not only the 30-km riverlike reach below the dam, but also strongly
<br />influences water temperatures in the receiving reservoir, Lake Mohave.
<br />Bonytail chub and razorback sucker still occur in Lake Mohave (humpback
<br />chub historically did not occur downstream from Catclaw Cave, now
<br />flooded by the lake), but there has been no documented successful recruit-
<br />ment by either species since perhaps the middle 1950's (Minckley, 1983; orig.
<br />data). Reproductive failure in Lake Mohave may be due in part to cool
<br />water temperatures, but other factors are probably also important, since fer-
<br />tile ova and fry of at least razorback sucker have been collected recently.
<br />As one moves downstream below Lake Mohave on the Colorado River
<br />potential thermal constraints on successful reproduction by fishes of concern
<br />are alleviated. During 1980 the temperature range below Davis Dam (which
<br />impounds Lake Mohave) was 10.0 to 19.0°C, and below Parker Dam (Lake
<br />Havasu), 8.5 to 26.0°C (USGS, 1980b). The greater range and higher maxi-
<br />mum below Parker Dam are due to epilimnetic penstocks in that structure
<br />(Minckley, 1979). Here, factors other than temperature clearly contribute to
<br />reproductive or recruitment failure of native fishes.
<br />Temperature effects upon reproduction by these species below dams thus
<br />have undoubtedly played a role in their population declines. Nonetheless
<br />other factors must be examined to explain extirpation from habitats which
<br />apparently are thermally suitable for life cycle completion. In particular the
<br />role of predation or other interference competition by introduced fishes
<br />should be investigated.
<br />This research was supported by funds from the U.S. Fish and Wildlife Service, Office of Endan-
<br />gered Species, Albuquerque, NM., James E. Johnson of that organization deserves special thanks.
<br />Mark Pisano assisted ably in conduct of this study. Personnel at Dexter National Fish Hatchery,
<br />Buddy Lee Jensen, Troy Winham, and Sharon Coates were helpful with local logistics. Theophi-
<br />lus D. Inslee of Dexter deserves special appreciation for generous contributions of time and effort,
<br />provision of helpful insights and suggestions, and many hours of discussion on hatchery methods
<br />in general and endangered species in particular. His unique expertise and ability were critical to
<br />successful completion of the project. Sara Frischknecht and Pat Chase typed and edited, and C.
<br />Hobbs, N. B. Grimm, W, L. Minckley, and an anonymous reviewer provided constructive reviews
<br />of the manuscript.
<br />LITERATURE CITED
<br />BiAxTER, J. H. S. 1969. Development: eggs and larvae. Pp, 178-252, in Fish Physiology Vol.
<br />III, W. S. Hoar and D. J. Randall, eds. Academic Press, New York, New York and Lon-
<br />don, Engl.
<br />BRETT, J. R. 1956. Some principals in the thermal requirements of fishes. Quart, Rev. Biol.
<br />31:75-87.
<br />BULRLEY, R. V., C. R. BERRY, R. PIMENTEL AND T. BLACK. 1981. Tolerance and preferences of
<br />Colorado River endangered fishes to selected habitat parameters. Final Compl. Rept.,
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