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<br />U02'f 79 <br /> <br />Native big river fishes have disappeared from about three-fourths of their original <br />habitat while introduced fishes have become more widespread and abundant. Even <br />where physical habitat has been altered relatively little, nonnative fish abundance has <br />increased, and the abundance of native fishes has been reduced. Although it is <br />obvious that suitable physical habitat is a requirement for the native fishes, the <br />suitability of the physical habitat is no longer the only issue. Most suitable physical <br />habitat now is occupied by introduced species, including many of which are <br />predaceous, highly competitive, and harmful to the native fish fauna (Minckley 1982, <br />Tyus et al. 1982, Carlson and Muth 1989). Because nonnative fishes have displaced <br />native fishes even from habitats whose physical attributes should be ideal for the <br />natives, there is a clear implication that natural physical habitat conditions are a <br />necessary, but not a sufficient condition for recovery of the endangered species. <br /> <br />Introduction of Nonnative Species <br /> <br />As important as the physical changes have been in endangering native fishes, <br />the most significant threat to the existence of the native fishes is probably not physical <br />or chemical, but biological. At least 67 nonnative species have been introduced <br />actively or passively into the Colorado River system during the last 100 years (Minckley <br />1982, Tyus et al. 1982, Carlson and Muth 1989, Minckley and Deacon 1981, Maddux et <br />al. 1993). The creation of the US Fish Commission in 1872 is cited as the beginning of <br />large stocking initiatives in the Colorado River basin (Miller 1961). The original <br />recommendations for stocking were apparently based on the assumption that it would <br />benefit the relatively depauperate Colorado River fauna (e.g., Jordan 1891), At least <br />36 fish species, mostly game fishes from the eastern US, were introduced from 1930 to <br />1950 (Miller 1961). By 1980, more than 50 nonnative species had been actively <br />introduced into rivers and reservoirs of the Colorado River basin (Minckley 1982, Tyus <br />et al. 1982, Carlson and Muth 1989). The desire to expand or enhance sport fishing <br />opportunities was the reason for most intentional introductions; other reasons include <br />forage for game species, biological control of unwanted pests, and for aesthetic or <br />ornamental purposes. <br /> <br />The states of Colorado and Utah have curtailed stocking of nonnative fishes <br />directly into waters of the Colorado River basin and now stock only coldwater <br />salmonids such as brown and rainbow trout. The state of Wyoming continues to stock <br />nonnative fishes above Flaming Gorge Reservoir and had been stocking channel <br />catfish in the Yampa basin as recently as the late 1980s. Even though much of the <br />direct stocking has ceased, previous stocking efforts have left a potent legacy that <br />continue to effect native Colorado River species. Well-established populations of <br />warm- and coolwater fishes are recruiting individuals in riverine habitat. For example, <br />smallmouth bass in the Uinta River and northern pike in the Yampa River yield a steady <br />supply of predators to the UCRB. Escapement of predaceous sunfishes, pikes and <br /> <br />7 <br />