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The best example in westem North America of extensive introgression resulting <br />from introduction of non-native~fishes involves introduced rainbow trout (Oncorhynchus <br />mykiss) and endemic cutthroat trout (O. clarki) (Leary et al., 1984; AllendorF and Leary, <br />1988; Leary et al., 1995). Over a period of approximately 60 years, hatchery-reared <br />rainbow trout have repeatedly been introduced by state and federal resource agencies <br />into the range of the native cutthroat trout. This has resulted in numerous hybridization <br />events. However, similar events can also occur over much shorter time frames. Echelle <br />and Connor (1989) documented large-scale introgressive hybridization between the <br />exotic pupfish Cyprinodon variegatus and the endemic C. pecosensis over 430 km of <br />Pecos River (NM) in a scant four-year period following introduction. The latter study <br />demonstrates that the process of genetic swamping cannot be underestimated by <br />resource managers simply because it is slower when compared to the ecological effects <br />of predation. Invasions and resulting hybridization can still occur relatively rapidly and <br />over extended distances. <br />Hybridization in native frshes-The problem of introgressive hybridization is not <br />restricted solely to interactions between exotic and endemic species. Hybridization also <br />occurs amongst closely related endemic fishes, and as such, has been along-standing <br />problem with regard to native fish management in western North America. For example, <br />past hybridization has been identified in sympatric species of Gila from the Colorado <br />River basin (DeMarais et al., 1992; Dowling and DeMarais, 1993). However, the classic <br />example of endemic fish hybridization is that between Flannelmouth Sucker <br />(Catostomus latipinnis) and Razorback Sucker (Xyrauchen texanus). One such hybrid <br />was even named by Jordan (1891) as a new species, X. umcompahgre. The <br />distributions and occurrences of these hybrids, in conjunction with the parental forms, <br />has been a topic of interest for approximately 40 years. <br />Banks (1964:74), in apre-Flaming Gorge Dam study, noted that Razorback <br />Suckers (i.e., "humpback" suckers) and their hybrids appeared to select the Green <br />River rather than the Yampa River, and attributed this to differences in run-off, <br />temperature, and turbidity differences between the two rivers. However, Vanicek (1967; <br />s <br />