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<br />observed or collected in recent years. <br />The apparent lack of young individuals in the bonytail chub and <br />razorback sucker populations suggests that recruitment is limited <br />(Gustafson 1975a,b; Minckley 1973; Minckley 1983). Jonez and Sumner <br />(1954) noted that predators were numerous in the vicinity of razorback <br />sucker and bonytail chub spawning areas. Predation on early life stages <br />of both species is believed to be the principal factor limiting <br />recruitment in these reservoir populations (Minckley 1973; Minckley <br />1983). <br />The U. S. Fish and Wildlife Service initiated this investigation in <br />order to further evaluate the population status and reproductive success <br />of razorback suckers and bonytail chubs in Lake Mohave. The principal <br />objectives were to determine (i) the distributions and abundances of <br />both species, (ii) whether successful spawning occurs for either <br />species, and (iii) the primary factors affecting spawning success. <br />1.2 History of the Lake Mohave Area <br />The construction of Hoover Dam in 1935 stabilized flows and <br />drastically reduced, suspended sediment loads in the Colorado River below <br />tfie dam (Dill 1944). The river was transformed from a seasonally warm, <br />silt laden, turbulent stream to a cold, crystal-clear regulated river <br />(Jonez and Sumner 1954). The swift currents degraded the river bottom <br />and exposed large, stable substrates to 70 km below the dam by 1947 <br />(Jonez and Sumner 1954). This allowed for the extensive colonization of <br />Cladophora sp. in shoal and riffle areas (Moffett 1942). Mayflies <br />(Callibaetis sp.) were abundant in the river while midge larvae, <br />caddisflies, dragonflies,,damselflies, and microcrustaceans (Daphnia sp.