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658 GREAT BASIN NATURALIST Vol. 46, No.4 the isolated drainages and depauperate faunas of today reflect Cenozoic conditions. They do not. Instead, the fossil record shows that the large regional desert environs of the South- west are "geologically new" (M. Smith 1981) and not typical in the development of life history attributes of the fish fauna. This has led M. Smith (1981) to propose that the eco- logical history of the fishes suggests they should be considered generalists, not special- ist species. In this case Ptychocheilus would have developed the capability to utilize both riverine and lacustrine habitat depending upon the climatic conditions prevailing. During the late Cenozoic, estuarine condi- tions in the lower basin and widespread lacus- trine habitat during pluvial periods would provide eutrophic conditions that could be exploited by a top carnivore like Ptycho- cheilus. These same areas, however, might not have provided the best spawning and nursery conditions because of adverse envi- ronmental (e. g., oxygen, substrate) and bio- logical (e.g., predation) factors. If Ptycho- cheilus could move between preferred spawning and feeding areas, it might have the best of both. G. Smith (1981) proposed that migration would be a major adaptation to dry seasons for intermountain desert fishes like Ptychocheilus, and that emigration of young fish to unoccupied areas might be selected for in genotypes. If movement and/or migration is highly adaptive, this behavior would have evolved with modem Ptychocheilus. Another consideration in the evolution of Ptychocheilus is large adult size. The popular notion of a richer food supply in the recent past is interesting, but is probably not the factor driving the adaptation to large body size. In the intermountain desert G. Smith (1981) noted the tendency for large habitats to produce large fishes, and, in view of the low food ration available, suggested that life his- tory adaptations to the growing season and differential mortality are primary determi- nants. Species experiencing low adult mortal- ity that grow larger and live longer could be expected to produce a large number of off- spring in the desirable wet years and outcom- pete those species that sacrifice size and longevity for early reproduction. Since Ptychocheilus and various salmonids are the only large native predators throughout most of the Colorado River, survival to a moderate Ul a: \OJ I- \OJ :=; o ..J ;;;: MAY JUNE SEPT JULY AUG MONTH Fig. 2. Movement of radiotelemetered Colorado squawfish, Yampa and Green rivers, 1983 and 1984 (after Archer et aI. 1985). Mouth ofYampa River = 0 km. size would insure low adult mortality. Thus, modern Ptychocheilus should display rapid growth and delayed reproduction to favor a large adult size if these attributes have selec- tive advantage. STRATEGIES OF PTYCHOCHEILUS LUCIUS As stated previously, modern Ptycho- cheilus exists today in conditions different from those in which it evolved. An examina- tion of the known life history attributes of P. lucius contrasted with the conditions and po- tential adaptations to late Cenozoic conditions may reveal life strategies in its evolution that would aid in its survival and potential recov- ery. Migration, Movement, and Habitat Selection As predicted by G. Smith (1981), P. lucius makes extensive use of migration in its life :- ~