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and stocks in the upper basin. Hatchery stocks had lower, but adequate m~DNA diversity (0.71-0.91; Dowling et al. 1996). Recent monitoring studies of razorback suckers taken as larva from Lake Mojave and repatriated there as older fish indicate that the repatriates represent the same high genetic diversity as the wild stocks (P. Marsh, pers. comm. 1998). Concern also has been raised about possible genetic introgression in brood stocks proposed for use in reintroduction efforts (Minckley et al. 1991). Allozyme studies of Lake Mojave brood fish and their progeny from Dexter National Fish Hatchery indicated that the degree of hybrid introgression between the razorback sucker and other suckers was no higher than that reported for other catostomid species, i.e., introgression was rare (Buth et al. 1987). Reasons for Decline Decline of the razorback sucker has been associated with major changes in its riverine ecosystem. The native fish fauna of the Colorado River basin evolved in a river system characterized by a diverse mix of riverine, floodplain, and lacustrine habitats (Maddux et al. 1993), and an extreme seasonal variation in flow and turbidity (Carlson and Muth 1989). The geographical isolation of the basin led to a high degree of endemism in the fish fauna, especially within the big river fish community (Miller 1961, Minckley et al. 1986). Several of the big river fishes, including the razorback sucker, are now threatened with extinction (Minckley et al. 1991). Decline of the razorback sucker has been so extensive that it now occupies only a small fraction of its historic range. Continuing decline is expected for the near future because there is virtually no recruitment to wild populations, despite successful spawning and dispersal of larval razorback suckers in some locations. The decline in abundance of the big river fishes has occurred at the same time that major changes occurred in their physical, chemical, and biological environment. Physical changes were primarily a consequence of the construction and operation of the many dams and diversions in the Colorado River basin since 1905. These structures deplete water, alter flow regimes, change water quality, and fragment habitat. Chemical changes are primarily contaminants, which mainly have increased in reservoirs, and increases in the concentration of selenium in impounded areas and in irrigation return flows. At the same time the physical and chemical attribute of the riverine environment was being altered by human actions, the nature and composition of the fish community was altered dramatically by the introduction of nonnative species, many of which did well in the changed environments. As a result, native fish species were confronted with competitors and predators with which they had no evolutionary "experience" (Molles 1980, Johnson et al. 1993). The complexity of the new system, both physically and biologically suggests that recovery of the razorback sucker may necessitate a new viewpoint that considers management of the riverine ecosystem as a whole. 16