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1 <br />1 <br /> <br />f <br />t <br />r <br /> <br /> <br />1 <br />t <br />funding evidence of some genetic isolation between Lake Mohave stocks and stocks in <br />tlhe upper basin. <br />Concern also has been raised about possible genetic introgression in brood stocks <br />proposed for use in reintroduction efforts (Minckley et al. 1991). Allozyme studies of <br />Lake Mohave brood fish and their progeny from Dexter National Fish Hatchery <br />indicated that the degree of hybrid introgression between the razorback sucker and <br />other suckers was no higher than that reported for other catostomid species, i.e., <br />introgression was rare (Both et al. 1987). <br />Reasons for Decline <br />Tlhe native fish fauna of the Colorado River basin evolved in a river system <br />characterized by extreme seasonal variation inflow and turbidity (Carlson and Muth <br />1989). The system contains a diverse mix of riverine, floodplain, and lacustrine <br />habitats (Maddox et ai. 1993). The geographical isolation of the basin ied to a high <br />degree of endemism in the fish fauna, especially for those big river fishes associated <br />wiith the main river channel (Miller 1961, Minckley et al. 1986). Several of the big river <br />fishes, including the razorback sucker, are now threatened with extinction (Minckley et <br />al. 1991). Decline of the razorback sucker has been so extensive that it now occupies <br />only a small fraction of its historic range. Continuing decline is expected for the near <br />25 <br />