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CURRENT DISTRIBUTION AND ABUNDANCE Upper Basin razorback sucker distribution has been reduced to about 750 miles (Holden and Stalnaker 1975a; Ecology Consultants, Inc. 1978; McAda and Wydoski 1980). In the Upper Basin, the fish persists in the lower Yampa and Green rivers, mainstream Colorado River, and lower San Juan River (Tyus et al. 1982a; Minckley et al. 1991; Platania et al. 1991), but there is little recruitment in these remnant populations. The largest extant riverine population occurs in the upper Green River Basin, but consisted of only about 1,000 fish in 1988 (Lanigan and Tyus 1989). In the absence of conservation efforts, these wild populations will be lost as old fish die and are not replaced. The largest concentration of razorback suckers in the Upper Basin occurs in the upper Green River from the mouth of the Duchesne River upstream to the Yampa River (Tyus 1987; Lanigan and Tyus 1989; Tyus and Karp 1991). Lanigan and Tyus (1989) estimated that 758 to 1,138 razorback suckers still inhabit the upper Green River. In the upper Colorado River subbasin, most razorback suckers occur in the Grand Valley area where the number of adult captures has declined appreciably since 1974 (Valdez et al. 1982b; Osmundson and Kaeding 1989). Recent studies on the San Juan River found razorback suckers in the San Juan arm of Lake Powell near Piute Farms, and one adult male near Bluff, Utah (Platania 1990). In the Lower Basin, the decline of razorback suckers shortly after impoundment of bake Mead in 1935 was noted by several researchers (Dill 1944; Miller 1946,• Wallis 1951; Jonez and Sumner 1954). Now, a substantial population exists only in Lake Mohave estimated at approximately 60,000 adult razorback suckers (Minckley et al. 1991). Small numbers of razorback suckers sporadically occur in Lake Mead and the Grand Canyon and below Lake Mohave in the mainstem and associated impoundments and canals (Marsh and Minckley 1989). Successful spawning has been documented in Lake Mohave and numerous larvae have been collected (Bozek et al. 1984; Marsh and Langhorst 1988). However, juveniles have been extremely rare in collections since the early 1950's (Minckley et al. 1991). No significant recruitment to any populations has been documented in recent years (Tyus 1987; McCarthy and Minckley 1987; Osmundson and Kaeding 1989; Tyus and Karp 1991). The formerly large Lower Basin populations have been extirpated from most riverine environments, and recruitment is virtually nonexistent in the remnant stocks (Minckley et al. 1991). STATUS The razorback sucker has been negatively affected by perturbations to its environment. These perturbations have resulted in genetic isolation, lack of recruitment, and an adult population nearing its maximum life expectancy. As these older fish die, wild populations of razorback sucker will disappear if habitat conditions favoring recruitment do not improve. 11