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specialized forms (Smith 1981). Thus, it is not surprising that habitat <br />studies have shown that the endangered fishes make wide use of -every available <br />habitat, and their habitat use changes as the river environment changes. As <br />stated by Stanford (1994): <br />I conclude that throughout their life cycles these fishes are highly <br />adapted to variations in flow velocity, depth, turbidity, and <br />foodweb structure and function associated with the spatially and <br />temporally dynamic biophysical interaction. They simply move around <br />as flow varies, constantly seeking the best energy return on energy <br />invested in foraging. In the case of squawfish, large size <br />apparently provides for considerable movement, which allows them to <br />efficiently use a highly variable environment. <br />It is generally accepted that the natural, variable river regime produced <br />floodplain environments to which the native fishes were well adapted. <br />Changes in the natural riverine environment have reduced the diversity <br />and abundance of the native Colorado River fish fauna. Several fishes, <br />including the Las Vegas dace (Rhinicthyes deaconi), Pahranagat spinedace <br />(Lepidomeda altivelis), and Monkey Springs pupfish (Cyprinodon sp.) are <br />recently-extinct. Of the remaining native freshwater species listed by <br />Carlson and Muth (1989), about 40% (19) are federally listed or proposed for <br />listing as threatened or endangered species. Native fishes restricted to the <br />mainstream ecosystem have fared worse and all are considered to be in danger. <br />The distribution and abundance of four (70%) of the mainstream fishes, i.e., <br />razorback sucker Xyrauchen texanus (Figure 2), Colorado squawfish <br />Ptychocheilus ]ucius (Figure 3), humpback chub Gila cypha (Figure 4), and <br />bonytail chubs Gila elegans (Figure 5) have been drastically reduced, and <br />these species are listed as endangered species. <br />3 <br />