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many of which have thrived due to human-induced changes to the natural riverine <br />system. These nonnative fishes prey upon and compete with razorback suckers. <br />Currently, the largest concentration of razorback sucker remaining in the Colorado River <br />Basin is in Lake Mohave on the border of Arizona and California. Estimates of the wild <br />stock in Lake Mohave have fallen precipitously in recent years from 60,000 as late as <br />1991, to 25,000 in 1993 (Marsh 1993, Holden 1994), to about 9,000 in 2000 (USFWS <br />2002b). Until recently, efforts to introduce young razorback sucker into Lake Mohave <br />have failed because of predation by nonnative species (Minckley et al. 1991; Clarkson et <br />al. 1993; Burke 1994). While limited numbers of razorback suckers persist in other <br />locations in the Lower Colorado River, they are considered rare or incidental and may be <br />continuing to decline. <br />In the Upper Colorado River Basin, above Glen Canyon Dam, razorback suckers are <br />found in limited numbers in both lentic (lake-like) and riverine environments. The <br />largest populations of razorback suckers in the Upper Basin are found in the middle <br />Green and lower Yampa Rivers (Tyus 1987). In the Colorado River, most razorback <br />suckers occur in the Grand Valley area near Grand Junction, Colorado; however, they are <br />increasingly rare. Osmundson and Kaeding (1991) reported that the number of razorback <br />sucker captures in the Grand Junction area has declined dramatically since 1974. <br />Between 1984 and 1990, intensive collecting effort captured only 12 individuals in the <br />Grand Valley (Osmundson and Kaeding 1991). The wild population of razorback sucker <br />is considered extirpated from the Gunnison River (Burdick and Bonar 1997). <br />Razorback suckers are in imminent danger of extirpation in the wild. As Bestgen (1990) <br />pointed out: <br />"Reasons for decline of most native fishes in the Colorado River Basin have been <br />attributed to habitat loss due to construction of mainstream dams and subsequent: <br />interruption or alteration of natural flow and physio-chemical regimes, inundation <br />of river reaches by reservoirs, channelization, water quality degradation, <br />introduction of nonnative fish species and resulting competitive interactions or <br />predation, and other man-induced disturbances (Miller 1961, Joseph et al. 1977, <br />Behnke and Benson 1983, Carlson and Muth 1989, Tyus and Karp 1989). These <br />factors are almost certainly not mutually exclusive, therefore it is often difficult to <br />determine exact cause and effect relationships." <br />The virtual absence of any recruitment suggests a combination of biological, physical, <br />and/or chemical factors that may be affecting the survival and recruitment of early life <br />stages of razorback suckers. Within the Upper Basin, recovery efforts endorsed by the <br />Recovery Program include the capture and removal of razorback suckers from all known <br />locations for genetic analyses and development of discrete brood stocks. These measures <br />have been undertaken to develop refugia populations of the razorback sucker from the <br />same genetic parentage as their wild counterparts such that, if these fish are genetically <br />unique by subbasin or individual population, then separate stocks will be available for <br />future augmentation. Such augmentation may be a necessary step to prevent the <br />extinction of razorback suckers in the Upper Basin. <br />17