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Decline of the razorback sucker in the Colorado River was noted as early as 1930, and Dill (1944) reported that they were rare and primarily restricted to reservoirs. Dill (1944) also noted an increased abundance of non-native fishes and suggested that they may be negatively impacting native fishes. Moffet (1942) recorded razorback suckers from below Lake Mead but did not report their abundance. Wallis (1951) stated that most native fishes in the region were declining in abundance but suggested that large numbers of razorback suckers had adjusted to the lacustrine conditions in Lake Mead. Razorback suckers in the Colorado River (LCRB) are now limited primarily to man-made lentic habitats. These include Mead, Mohave, and Havasu reservoirs on the Colorado River mainstream and Senator Wash Reservoir, California (Medel-Ulmer 1980, Minckley 1983, Bozek et al. 1984, Loudermilk 1985, Marsh and Papoulias 1989). The largest concentration of razorback suckers is in Lake Mohave, the newest of the mainstream lower Colorado River reservoirs. Razorback suckers in Lake Mohave have been the subject of long-term study (Minckley 1983, Bozek et al. 1984), and probably represent the largest remaining razorback sucker population in the entire Colorado River Basin. A multiple '~ census population estimate in spring 1989 suggested that about 60,000 (range of 15,000 to 120,000) razorback suckers occurred in Lake Mohave (Marsh and Minckley 1989a). The unique morphometry and hydrology of Lake Mohave produce patchily distributed areas where large numbers of razorback 9