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• <br />provide the required n~atzr quantity and hydrologic regime necessary for recoven• of the fishes. <br />The physical habitat includes aeeas of the Colorado River system that are inhabited or potentiallp~ <br />habitable for use in spanning and feeding, as a nursery, or serve as corridors between these areas. <br />In addition, oxbows, bacllvaters. and other areas in the 100-yeaz floodplain, when inundated, <br />provide access to spanning, nursery, feeding, and rearing habitats. <br />BIOLOGICAL BACKGROUND <br />Colorado Pikeminnow <br />The Colorado pikeminnow evolved as the main predator in the Colorado River system, The diet <br />of Colorado pikeminnow longer than 3 or 4 inches consists almost entirely of other fishes <br />(Vanicek and Kramer ] 969). The Colorado pikeminnow is the lazgest cyprinid fish (minnow <br />family) native to North America and, during predevelopment times, may have grown as lazge as <br />6 feet in length and weighed neazly 100 pounds (Behnke and Benson 1983). These large fish <br />may have been 25-50 years of age. <br />Based on eazly fish collection records, azchaeological fords, and other observations, the Colorado <br />pikeminnow was once found throughout warmwater reaches of the entire Colorado River Basin, <br />• including reaches of the upper Colorado River and its major tributaries, the Green River and its <br />major tributaries, and the Gila River system in Arizona (Seethaler 1978). Colorado pikeminnow <br />were appazently never found in colder, headwater aeeas. Seethaler (1978) indicates that the <br />species was abundant in suitable habitat throughout the entire Colorado River Basin prior to the <br />1850's. Historically, Colorado pikemirinow have been collected in the upper Colorado River as <br />faz upstream as Pazachute Creek, Colorado (Kidd 1977). <br />A mazked decline in Colorado pikeminnow populations can be closely correlated with the <br />construction of dams and reservoirs between the I930's and the 1960's,introduction ofnonnative <br />fishes, and removal of water from the Colorado River system. Behnke and Benson (1983) <br />summarized the decline of the natural ecosystem. They pointed out that dams, impotmdments, <br />and water use practices aze probably the major reasons for drastically modified natural river <br />flows and channel chazacteristics in the Colorado River Basin. Dams on the main stem have <br />essentially segmented the river system, blocking Colorado pikeminnow spawning migrations and <br />drastically changing river characteristics, especially flows and temperatures. In addition, major <br />changes in species composition have occurred due to the introduction of nonnative fishes, many <br />of which have thrived as a result of changes in the natural riverine system (i.e., flow and <br />temperature regimes). The decline of endemic Colorado River fishes seems to be at least <br />partially related to competition or other behavioral interactions with nonnative species, which <br />have perhaps been exacerbated by alterations in the natural fluvial environment. <br />• The Colorado pikeminnow currently occupies about 1,030 river miles in the Colorado River <br />system (25 percent of its original range) and is presently found only in the Upper Basin above <br />Glen Canyon Dam, It inhabits about 350 miles of the main stem Green River from its mouth to <br />