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17 species may eliminate another through genetic swamping (Moyle 1976). The resulting hybrids may also out-compete the native parent species or dilute wild genetic stocks with continued hybridization. Introduced white sucker hybridize with native tlannelmouth or bluehead sucker (Wick et al. 1981, 1985, 1986; Valdez et al. 1982a and b), and tlannelmouth sucker hybridize ",ith razorback sucker (Hubbs and Miller 1953; Tyus and Karp 1990), but there are no known wild hybrids between introduced fishes and the four endangered species. Disease or parasite introductions New diseases or parasites can be carried and introduced by nonnative species. Native fish may also become more susceptible to natural disease or parasites when stressed by interactions with nonnatives. Flagg (1981) conducted a survey of disease agents in native and introduced species in the Colorado River Basin. One objective was to determine if disease agents associated with introduced species adversely affected endangered species. Pathogens were identified from seven native and six introduced species. These included the four endangered species, green sunfish, smallmouth and largemouth bass, channel catfish, black bullhead, and red shiner. Disease agents associated with introduced species did not appear to adversely impact endemic species. Native fishes harbored a distinctly different pathogen group than did the introduced fishes. Flagg (1981) suggested that disease was probably not a factor in the decrease in endangered fish populations. Osmundson (1987) reported the first occurrence of Asian tapeworm in the upper Colorado River basin and determined that it came from Colorado squawfish raised at Dexter National Fish Hatchery. Osmundson and Kaeding (1989b) identified the negative impact parasites like Asian tapeworm can have on Colorado squawfish and the importance of screening forage fish for parasites prior to their introduction into grow-out ponds. Incidental take by angling Osmundson and Kaeding (1989a) noted the significant threat posed by high angler activity for channel catfish within the 15-mile (24 Ian) reach of the Colorado River. Vulnerability of Colorado squawfish increased with decreased tlows which concentrated Colorado squawfish and increased fisherman access. Vanicek (1967) noted the ease of hook and line take of Colorado squawfish, and the heavy exploitation of this species by local fishermen. Wick et al. (1985) reported capturing a Colorado squawfish on the Yampa River by angling with cut sucker meat, a bait often used when fishing for channel catfish. Review of tagging records (Wick et a1. 1985) indicated that Colorado squawfish were often collected by researchers using angling gear in Colorado, on the Yampa, Gunnison., and Colorado rivers. In some years up to 10% of all Colorado squawfish Carlin tagged have been recaptured by fishermen (Tyus and Karp 1989). t ~I I i ~ , i ~