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Introduction Declines in populations of razorback sucker, X-auchen texarn?s (Abbott), in the Colorado River basin of western North America have been the subject of several recent publications Qt--Ada and Wydoski 1980; Minckley 1983; Tyus 1987; Lanigan and Tyus in press). Lake Mohave, Arizona-Nevada, a mainstream Colorado River reservoir, supports the largest known population of this unique, endemic species. Lake Mohave fish are old (>30 years; McCarthy and Minckley 1987), and there is no evidence of recruitment into the adult papulation. Annual reproduction nonetheless occurs, and substantial numbers of larvae are produced, only to disappear when near 12 mm long (Marsh and Ianghorst 1988). In marine systems, insufficient amounts or qualities of foods are often identified as major causes of mortality of larval fishes, either directly through starvation or indirectly because of predation on sl or weakened larvae (Hjort 1914, 1926; O'Connell and Raymond 1970; Theilacker•1986). Kashuba and Matthews (1984) similarly demonstrated that poor year classes of freshwater shads (Doroscma spp.) in Lake Texoma, Texas-Oklahana, were nutritionally mediated. Cellular atrophy and deterioration of midgut and digestive organs were reported for larval shad during a period when zocplankton density was considered to be low at -100 organisms/L. There is evidence that threadfin shad (Doroscma petenense) experienced similar year-class failure in Lake Mead, Arizona Nevada, tinder conditions of low nutrient input (Baker and Paulson 1983a). Reservoirs on the lower Colorado