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17 <br />the mainstem essentially have segmented the river system, blocking Colorado <br />squawfish spawning migrations and drastically changing river characteristics. <br />especially flows and temperatures. In addition, major changes in species <br />composition were caused by introduction of nonnative fishes, many of which <br />have thrived as a result of changes in the natural riverine system (i.e., flow <br />and temperature regimes). The decline of endemic Colorado River fishes seems <br />to be at least partially related to competition or other behavioral <br />interactions with nonnative species, which have perhaps been exacerbated by <br />alterations in the natural fluvial environment. <br />The Colorado squawfish currently occupies about 1,030 river miles in the <br />Colorado River system (25 percent of its original range) and is presently <br />found only in the Upper Basin above Glen Canyon Dam. It inhabits about <br />390 miles of the mainstem Green River from its mouth upstream to the Gates of <br />Ladore (Kevin Bestyen pers.comm.). Colorado squawfish have also been observed <br />in the lower 49 miles of the Duchesne River. Its range also extends 160 miles <br />up the Yampa River and 104 miles up the White River, the two major tributaries <br />of the Green River. In the mainstem Colorado River, it is currently found <br />from Lake Powell extending about 201 miles upstream to Palisade. Colorado <br />(Tyus et al. 1982), and in the lower 60 miles of the Gunnison River, a <br />tributary to the mainstem Colorado River (Burdick 1995). <br />Osmundson and Burnham (1996) estimated the size of the Colorado squawfish sub- <br />adult and adult population in the Colorado River from Palisade, Colorado, <br />downstream to the Green River confluence; Utah, at around 600-650 individuals. <br />The authors further stated that, compared to historical accounts, <br />population abundance today appears low and suggests the long-term trend of <br />this population has been downward." Because of low and fluctuating adult <br />numbers due to the infrequency of strong year classes, the population appears <br />vulnerable to extinction (Osmundson and Burnham 1996). <br />Critical habitat was designated within the 100-year floodplain of the Colorado <br />squawfish's historical range in the following sections of the Upper Basin <br />(59 F.R. 13374): <br />Colorado. Moffat County. The Yampa River and its 100-year floodplain from <br />the State Highway 394 bridge in T. 6 N.. R. 91 W., section 1 <br />(6th Principal Meridian) to the confluence with the Green River in <br />T. 7 N., R. 103 W., section 28 (6th Principal Meridian). <br />