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<br />c::> <br /> <br />C"~ <br />C\J <br />C <br /> <br />22 <br /> <br />A marked decline in populations of razorback suckers can be attributed to <br />construction of dams and reservoirs. introduction of nonnative fishes. and <br />removal of large quantities of water from the Colorado River system. Dams on <br />the mainstem Colorado River and its major tributaries have segmented the river <br />system and drastically altered flows. temperatures. and channel geomorphology. <br />Major changes in species composition have occurred due to the introduction of <br />numerous nonnative fishes. many of which have thrived due to man-induced <br />changes to the natural riverine system. <br /> <br />The current distribution and abundance of the razorback sucker have been <br />significantly reduced throughout the Colorado River system (McAda 1987. McAda <br />and Wydoski 1980. Holden and Stalnaker 1975. Minckley 1983. Marsh and Minckley <br />1989. Tyus 1987). The only substantial population exists in Lake Mohave with <br />an estimated population of 60.000 adult razorback suckers (Minckley et al. <br />1991). However. they do not appear to be successfully recruiting without <br />intensive management whereby adults are collected. spawned. and reared in <br />diked-off coves and/or ponds and released back into the lake. While limited <br />numbers of razorback suckers persist in other locations in the lower Colorado <br />River. they are considered rare or incidental and may be declining. <br /> <br />In the Upper Basin above Glen Canyon Dam. razorback suckers are found in <br />limited numbers in both lentic and lotic environments. The largest population <br />of razorback suckers in the Upper Basin is found in the upper Green River and <br />lower Yampa River (Tyus 1987). .Lanigan and Tyus (1989) estimated that from <br />758 to 1.138 razorback suckers inhabit the upper Green River. Modde et al. <br />(1996) estimated their numbers to be approximately 500 individuals and <br />suggested that the presence of fish smaller than 480 mm during the study <br />period implied some minimal recruitment. While some recruitment may be <br />occurring in the wild, such minimal recruitment to the Green River population <br />of razorback sucker is not sufficient to maintain the existing small <br />population (Modde et al. 1996). In the Colorado River. most razorback suckers <br />occur in the Grand Valley area near Grand Junction. Colorado; howev~r. they <br />are increasingly rare. Osmundson and Kaeding (1991) report the number of <br />razorback sucker captures in the Grand Junction area has declined dramatically <br />since 1974. In 1991 and 1992, 28 adult razorback suckers were collected from <br />isolated ponds adjacent to the Colorado River near Debeque. Colorado (Burdick <br />1992) . <br /> <br />Razorback suckers are in imminent danger of extirpation in the wild. The <br />specific causes of this species' continued decline are somewhat speculative at <br />this time. As Bestgen (1990) pointed out: <br />