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fish catch (bluehead and flannelmouth suckers, speckled dace, chubs) was <br />positive but not statistically significant (Nesler 1988). The lack of <br />significance in the latter relationship supports the hypothesis that native <br />fishes exhibit greater tolerance to fluctuating flow regimens. <br />Instream Flow Models <br />Flow needs of rare and endangered fishes in the Yampa River, and <br />elsewhere, have been evaluated by use of different techniques, including <br />physical habitat simulation models. Application of these models for <br />predicting optimal flows for endangered fishes in large rivers is <br />controversial Ear several reasons. Poor understanding of limiting factors, <br />stream dynamics, habitat availability and fish habitat needs, has led to <br />improper model application. Early studies were prone to using model outputs <br />without adequate understanding of the biology or distribution of the fishes <br />(e. g., Prewitt et al. 1982). In addition, the PHABSIM model (Bovee 1982) has <br />been used for habitats that violated model assumptions, leading species <br />experts to constrain its applicability (Valdez et al. 1987). <br />The PHABSIM model has been used to predict flow needs for various <br />endangered fishes. Applications of this model in the Yampa River have <br />incorporated depth, velocity, and substrate parameters as descriptors of "fish <br />habitat", and outputs have not agreed with empirical fish data. FWS funded a <br />major effort to obtain habitat utilization curves for all rare and endangered <br />Colorado River fishes, using a massive data set and a team of species experts <br />(Valdez et al. 1987). These utilization curves were applied to hydrologic <br />stations in the Yampa River by FWS (USFWS 1987b). The results indicated that <br />"physical habitat" for adult Colorado squawfish was maximized during the <br />42 <br />