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1 PART I INTRODUCTION Histor The Colorado squawfish (Ptvchocheilus lucius) is the largest of four living species 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 (Miller 1961), a voracious predator, and the top native carnivore of the Colorado River system. Maximum weights exceeding 36 kg (79 lbs) and lengths of nearly 1.8 m (71 in.) have been recorded; however, specimens weighing more than 7 kg (15 lbs) have been rare in recent times (Minckley 1973; Behnke and Benson 1980). Its substantial size and migratory habit resulted in use of the " in early l " l " " " mon sa or simp y Colorado salmon, common names "white salmon, literature (Minckley 1973; Behnke and Benson 1980). The evolutionary history of Ptvchocheilus lucius has been marked by scores of oscillations between pluvial- and arid-dominated habitats caused by climatic ' fluctuations during the Miocene, Pliocene, and early Pleistocene epochs lucius was adapted to swift water by the mid-Pliocene Smith 1981) P (G . . . (Uyeno and Miller 1965), but fossil evidence indicates that it may have used lakes as well as rivers (G. Smith 1975, 1981; M. Smith 1981). The species may have developed the capability to survive in either lakes or rivers, depending 1 on prevailing 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. grandis) of the Sacramento-San Joaquin, Pajaro, Salinas, and Russian Rivers in California; the Northern squawfish (P. oregonensis) of the Columbia River Basin in Nevada, Oregon, Washington, Idaho, and Montana, north to the Nass River, British Columbia; and the Umpqua squawfish (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 Register (Vol. 32[43]:40001) on March 11, 1967. Full protection under the Endangered Species Act of 1973, as amended, occurred upon its listing in the ' Federal Register (Vol. 39[3]:1175) on January 4, 1974. The States of California, Arizona, New Mexico, Utah, and Colorado each have laws protecting the Colorado squawfish within State waters.. ii