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DRAFT -PART I INTRODUCTION The Colorado squawfish (Ptvchocheilus lucius) is the largest of four living spec~es of the genus Ptvchocheilus. Although. the specific name lucius means "pike like," the Colorado squawfish is taxonomically placed in the-large and diverse minnow family Cyprinidae. It is the largest cyprinid in North America (Mi l er 1961), a voracious predator, and the top native carnivore of the Colo~ado River system. Maximum weights exceeding 36 kg (79 lbs) and. lengths of n arly 1.8 m (71 in.) have been recorded;. however, specimens weighing more than? kg (15 lbs) have been rare in recent times (Minckley 1973; Behnke and Bens~n 1980). Its substantial size and migratory habit resulted in use of the comm n names "white salmon," "Colorado salmon," or simply "salmon" in early literature (Minckley 1973; Behnke and Benson 1980). . The evolutionary history of Ptvchocheilus lucius has been marked by scores of osculations between pluvial- and arid-dominated habitats caused by climatic fluc',tuations during the Miocene, Pliocene, and early Pleistocene epochs (G. Smith 1981). P. lucius was adapted to swift water by the mid-Pliocene (Uyeno and Miller 1965), but fossil evidence indicates that it may have used lake's as well as rivers (G.-Smith 1975, 1981; M. Smith 1981). The species may have developed the capability to adapt to lakes or rivers, depending on previailing climatic conditions (Tyus 1986). Large size,- great mobility, and spawning migrations would be adaptations to drier seasons when suitable spawning habitats are limited or far-removed from other adult habitats (G. (Smith 1981). These adaptive life strategies that formerly benefited the fish'.. (Tyus and McAda-1984; Tyus 1986) may now be contributing to its decline. The other three living members of the genus Ptvchocheilus include the Sacramento squawfish (P. rg andis) of the Sacramento-San~Joaquin, Pajaro, Salinas, and Russian rivers in California; the Northern squawfish (P. oreg_onensis) of the Columbia River Basin in Nevada, Oregon, Washington, Idaho, and Montana, north to the Nass River, British Columbia; and the Umpqua squ wfish (P. umpQUae) in the Umpqua and Suislaw rivers in Oregon (Lee et al. 1980 . Unlike the Colorado squawfish, these three species remain common in their native waters. The'~Colorado squawfish was listed as endangered by the U.S. Fish and Wildlife Service (Service) in the Endangered Species List published in the Federal Re ~ster (Vol. 32[43]:40001) on March 11, 1967. Full protection under the Endangered Species Act of 1913, as amended, occurred upon its listing in the Fed ral Re4ister (Vol. 39[3]:1175) on January 4, 1974. The~~IStates of California, Arizona, New Mexico, Utah, and Colorado each have laws protecting the Colorado squawfish within State waters.