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<br />88
<br />
<br />TYUS AND HAINES
<br />
<br />He also found that smaller fish (mean, 33 mm TL;
<br />range, 26-39 mm) that were low in body lipids
<br />experienced a significant mortality rate if starved
<br />for 3 months. Winter conditions in the Green Riv-
<br />er can be expected to be warmer in some habitats
<br />than those simulated, at least during warm years
<br />or intervals of warmer weather, and most age-O
<br />fish we collected (83%, N = 18) in November and
<br />December had food in their guts (R. Muth, Col-
<br />orado State University, personal communica-
<br />tion). Young fish move freely between backwaters
<br />and other main-channel habitats, presumably
<br />seeking preferred temperatures and perhaps other
<br />environmental variables, and we consider over-
<br />winter mortality of age-O Colorado squawfish in
<br />the Green River to be nonsignificant. Although
<br />Thompson (1989) demonstrated that smaller fish
<br />(26-39 mm TL) may not have energy reserves
<br />sufficient to survive 210 d at 3-40C without food,
<br />25% of the young we caught in the spring were in
<br />the same size range as the small fish he studied
<br />(range, 22-39 mm; N = 314). The survival ofthese
<br />fish suggested that winter conditions in the Green
<br />River were more suited to their survival than con-
<br />ditions in the laboratory study conducted by
<br />Thompson (1989).
<br />The Colorado squawfish has been subjected to
<br />altered habitat conditions in the Green River, but
<br />demonstrates a capability to recruit sizeable
<br />standing crops of age-O fish in years of favorable
<br />(historical) water flows. Changes in water man-
<br />agement to provide flow regimes more optimal for
<br />Colorado squawfish (Tyus and Karp 1989) could
<br />overcome recruitment problems we noted in some
<br />years and provide a better basis for management.
<br />The Green River main stem should be managed
<br />as a Colorado squawfish nursery area by provision
<br />of a proper flow regime to optimize backwater
<br />habitats, i.e., summer flows more like historical
<br />conditions. This would provide a better environ-
<br />ment in which to further evaluate other factors
<br />that have also been associated with the decline of
<br />this species.
<br />
<br />Acknowledgments
<br />
<br />This study was funded in part by the Bureau of
<br />Reclamation and Fish and Wildlife Service, U.S.
<br />Department of Interior. B. D. Burdick, C. W.
<br />McAda, R. L. Jones, and S. L. Lanigan were prin-
<br />cipals among the numerous biologists and tech-
<br />nicians that assisted in collecting field data during
<br />the years 1979-1989. C. A. Karp, E. J. Wick, and
<br />R. C. Beamesderfer provided helpful review com-
<br />ments on an earlier draft of the manuscript. D.
<br />
<br />Moses prepared graphics and provided technical
<br />support.
<br />
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