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<br />40 <br />boratory preferences, these fishes would avoid cold tailwaters of a <br />reservoir. This avoidance probably explains at least in part the di- <br />sappearance of native species from the Green River below Flaming .Gorge <br />Reservoir. Since-the closure of Flaming Gorge 'Dam, native Colorado <br />fishes have not been found from the dam downstream to the confluence <br />of the warmer Yampa River (Holden 1978; Mc Ada and Wydoski 1980). The <br />decrease from historical summer water temperatures throughout much of <br />the Colorado drainage appears to have effectively reduced the habitat <br />that meets the thermal requirements of fish endemic to the drainage. <br />Brett (1960) described the thermal requirements of fish and recommend- <br />ed not optimal temperatures but temperatures that permit survival at a <br />level which allows for continuity of a species. Neill _(1979) suggest- <br />ed that a successfully thermoregulating fish moves through its envi- <br />ronment in such a way as to maximize time spent at temperatures favor- <br />able for the joint conduct of its life processes. Holden and Stalnak- <br />er (1975b) noted that ,Colorado squawfish::abundance in-..the Yampa "River,. <br />-:rose sharply when river temperatures stabilized at '20-21 C. Squawfish <br />apparently moved into the Yampa from the.:-Green River as :waters warmed <br />to 16-21` C in July~and early August. The distribution and abundance <br />of Colorado squawfish'appears~ to 'be affected by :temperature, :and <br />squawfish °appear to select `areas of preferred temperature when those <br />_. <br />temperatures exist in the wild. <br />I <br />