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45 LIFE HISTORY AND ECOLOGY OF THE COLORADO SQUAWFISH (PTYCHOCHEILUS LUCIUS) IN THE UPPER COLORADO RIVER BASIN. Karl Seethaler, Utah State University. The Colorado squawfish (Ptychocheilus lucius) was once abundant throughout the Colorado River system; it is now an endangered species found in small numbers only in limited por- tions of the upper basin. The major cause of this decline is attributed to man-made alterations of the river environment. The Endangered Species Act of 1973 was enacted in response to increased public concern for vanishing native wildlife, although the rationale for the preservation of species is not yet universally understood. Federal and state agencies charged with the management of endangered species and their environments found that they needed to gather considerable information on such endangered species as the Colorado squawfish. This work constitutes a broad synthesis of current knowledge about the Colorado squawfish: distribution; abundance, habitat requirements, systematics, reproduction, early life development, age and growth, food habits, movement, maturity, diseases and parasites, causes of decline, and the phenomenon of its endangered status. The main objective was to synthesize this knowledge into a single volume to aid in effective management decisions. An exhaustive chapter on historical and present distribution of the Colorado squawfish documents every known sighting of this species in all rivers and tributaries in the Colorado River system since 1825. A wide-ranging piscivore, this species frequents all habitat types in the river except the cold headwaters, though it was most often found in eddies, backwaters, and deep holes. Field observations were made primarily in Dinosaur National Monument and adjacent areas of the upper Green and Yampa rivers, and at Grand Junction on the Colorado River main stem. Trammel nets, seines, and occasionally electrofishing were used to capture fish. The Colorado squawfish, one of four species of the genus Ptychocheilus, evolved with the Colorado River system. Attesting to its reputation as a food and sport fish, it has locally acquired such names as "salmon", "whitefish", and "pike". In fact, the species is the largest cyprinid in North America, a family which arrived in this continent about the time of the Miocene epoch. Although captured from two widely separated geographical localities in the Colorado and Green Rivers, the species apparently consists of a single population. Analysis of electrophoretic and meristic data failed to show any intraspecific differences between fish captured from these two locations. Positive identification of larval and juvenile fish often presents a problem to collectors. Because there is a need for