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22 A marked decline in populations of razorback suckers can be attributed to construction of dams and reservoirs, introduction of nonnative fishes, and removal of large quantities of water from the Colorado River system. Dams on the mainstem Colorado River and its major tributaries have segmented the river system and drastically altered flows, temperatures, and channel geomorphology. Major changes in species composition have occurred due to the introduction of numerous nonnative fishes, many of which have thrived due to man-induced changes to the natural riverine system. The current distribution and abundance of the razorback sucker have been significantly reduced throughout the Colorado River system (McAda 1987, McAda and Wydoski 1980, Holden and Stalnaker 1975, Minckley 1983, Marsh and Minckley 1989, Tyus 1987). The only substantial population exists in Lake Mohave with an estimated population of 60,000 adult razorback suckers (Minckley et al. 1991). However, they do not appear to be successfully recruiting without intensive management whereby adults are collected, spawned, and reared in diked-off coves and/or ponds and released back into the lake. While limited numbers of razorback suckers persist in other locations in the lower Colorado River, they are considered rare or incidental and may be declining. In the Upper Basin above Glen Canyon Dam, razorback suckers are found in limited numbers in both lentic and lotic environments. The largest population of razorback suckers in the Upper Basin is found in the upper Green River and lower Yampa River (Tyus 1987). Lanigan and Tyus (1989) estimated that from 758 to 1,138 razorback suckers inhabit the upper Green River. Modde et al. (1996) estimated their numbers to be approximately 500 individuals and suggested that the presence of fish smaller than 480 mm during the study period implied some minimal recruitment. While some recruitment may be occurring in the wild, such minimal recruitment to the Green River population of razorback sucker is not sufficient to maintain the existing .small population (Modde et al. 1996). In the Colorado River, most razorback suckers occur in the Grand Valley area near Grand Junction, Colorado; however, they are increasingly rare. Osmundson and Kaeding (1991) report the number of razorback sucker captures in the Grand Junction area has declined dramatically since 1974. In 1991 and 1992. 28 adult razorback suckers were collected from isolated ponds adjacent to the Colorado River near Debeque, Colorado (Burdick 1992). Razorback suckers are in imminent danger of extirpation in the wild. The specific causes of this species' continued decline are somewhat speculative at this time. As Bestgen (1990) pointed out: