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3 <br />INTRODUCTION <br />Razorback sucker, Xyrauchen texanus, is a large catostomid fish en- <br />demic to the Colorado River system of southwestern North America. Once <br />abundant, the species has declined in distribution and numbers through- <br />out its native range in mainstream rivers from Wyoming to Sonora, Mexico <br />(11inckley 1983, McAda and Wydoski 1980). Although scattered individuals <br />are occasionally taken (Miller et al. 1982, Ulmer 1985) the only <br />substantial remaining population is in Lake Mohave, a Colorado River <br />reservoir in Arizona-Nevada. That population is comprised of large, old <br />individuals with no evidence of successful recruitment since the 1950s <br />(Bozek et al. 1984, Minckley 1983, McCarthy 1986). Yet, razorback <br />suckers in Lake Mohave spawn successfully and produce viable embryos; <br />free-swimming larvae become abundant in parts of the lake each winter, <br />but disappear by late spring (Langhorst 1985, 1986; Langhorst et al. <br />1985). Cause(s) of presumed larval mortality are unknown, but may be <br />due to one or a combination of factors such as predation, resource <br />limitation, or physical stresses. The present study was initiated in <br />attempt to elucidate ecology and fate of larval razorback sucker in Lake <br />Mohave. Major objectives were to assess: (1) impacts of predation; (2) <br />relationships between larval food habits and food resource availability; <br />(3) dispersal of larvae from spawning areas; and (4) larval survival in <br />protected (predator-free) habitats. <br />Numerous individuals and agencies participated in this work through <br />contribution of manpower and equipment. U.S. Fish and Wildlife Service, <br />Office of Endangered Species, Albuquerque, NM, provided funds to a con- <br />current study of razorback sucker in Lake Mohave. Jim Brooks and Tom <br />Liles (Arizona Game and Fish Department), Tom Burke (Bureau of Reclama-