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~()b ~ j j . .~ i ~ i J '/- /1~ Iq86b ~ ~ 0'7303 ......- LIFE STRATEGIES IN THE EVOLUTION OFTHE COLORADO SQUAWFISH (PTYCHOCHEILUS LUCIUS) Harold M. Tyus1 ABSTRACT.-The Colorado squawfish, a large predaceous cyprinid, is a generalist species adapted to the large seasonal water fluctuations, low food base, and changing riverine subsystems of the Colorado River. Extant at least as early as the Miocene epoch, Ptychocheilu8 has survived by incorporating life strategies to deal with changing climates varying from arid to pluvial. Migration and long-term movement patterns appear to have evolved as tactics to perpetuate a grand reproductive strategy for exploiting the changing habitats and general environmental conditions of the late Cenozoic era. Accordingly, high mobility of a large fish would aid in selection of optimum spawning, nursery, and adult habitats in the dynamic lacustrine/riverine system that existed at that time. A spatial separation oflifestages thus produced would aid in the reduction of intraspecific competition. Large size, long life, and late spawning of Ptychocheilu8 indicate that mortality of young must be disproportionately high compared to that of the adult form. Growth to a large size should reduce. predation by other fishes and, once attained, would facilitate long distance movement for reproduction, feeding, and other purposes. Such a strategy, formerly highly adaptive, may now be implicated in the decline of this species in controlled riverine systems. The genus Ptychocheilus includes the largest cyprinids in North America. Repre- sented by four species today, the largest of these, the Colorado squawfish (Ptychocheilus lucius Girard) formerly grew to a size of about 1.8 m and 45 kg (Miller 1961). Endemic to the Colorado River Basin, this fish, once dis- tributed throughout the basin, has declined since the 1930s and is today restricted to the upper Colorado River Basin, where it is classi- fied as endangered by the U.S. Fish and Wildlife Service (1973, 1974). The loss of the Colorado squawfish from parts of the Colo- rado River is apparently related to major wa- ter developments that have ostensibly re- duced P. lucius to about 25% of its former range (Tyus 1984). Although many workers have postulated man-induced changes in riv- erine conditions as primary factors in the re- duction of the range and abundance of this species (Miller 1961, Holden and Wick 1982, Ono et al. 1983), a lack of basic knowledge about its life history, especially in locations where the fish has been lost (Minckley, 1973), has made these implications impossible to prove. Recent research in the Green River Basin (Fig. 1) by the U.S. Fish and Wildlife Service (Tyus and McAda 1984) resulted in the first discovery of a spawning grounds of this species in 1981 and identified migrations and movements as important factors in the 'u.S. Fi,h and Wildlife Service, 1680 We,t Highway 40, Vernal, Utah 84078. reproductive strategy of this species. These findings have been substantiated by the work of Haynes et al. (1984), Wick et al. (1983), Tyus (1985), and others. With the present knowledge of the life his- tory requirements of P. lucius, it is now possi- ble to relate its apparent life strategies with its evolution and adaptations to conditions in the Colorado River Basin. In so doing I have drawn heavily from the works of G. R. Smith (1981) and M. L. Smith (1981), who presented the background on late Cenozoic climates and adaptations of the southwestern fish fauna, particularly P. lucius, upon which this work is based. CLIMATE AND ADAPTATION OF PTYCHOCHEILUS The cyprinid fishes apparently arrived in the New World from Asia in the Miocene epoch, and fossil Ptychocheilus species similar to modern Ptychocheilus lucius have been re- ported from the middle Pliocene in the Colo- rado River system of northern Arizona (Miller 1961). Ptychocheilus had widespread distri- bution in the Pliocene, as evidenced by fossils in Lake Idaho (Smith 1975), the Great Basin (G. Smith 1981), and Arizona (Miller 1961). Furthermore, the similarity between the Pliocene fossils and modern forms suggests that the adaptation to swift water habitat had 656