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>- J Z Q W .-~ U 30 W ~ 20 W ~ 10 W F- /'~•~OWER ---•~ SPAWNING j / ~•--.\ DD\ E ,/ GROWTH ~./_ /• •\•\ - UPPER.;\•~ ./ \ - ~'~ /'/ .~•/• •\• _i'' J F M A M J J A S O N D MONTH FIGURE 3. Comparison of temperature regimes (average mean-monthly temperatures) of the historic lower (near Yuma, Arizona, 1917- 1924, from Dill 1944) and middle Colorado River (near Grand Canyon, Arizona, 1943-1947, 1957), and of the present upper Colorado River (near Colorado-Utah border, 1972-1978). Middle and lower-river data were taken prior to modification of temperature regimes by upstream dams. Horizontal lines are temperature thresholds for growth (14 C) and the onset of spawning (20 C) of Colorado squawfish. Temperatures from middle and upper river are from the United States Geological Survey. See Figure 1 for sites of temperature-data collection. provided a six-month growing season. On the basis of growing season length, the potential for growth of Colorado squawfish in these former downstream habitats was great. Moreover, based on Colorado squawfish growth in the pond, it is reasonable to assume growth in these former downstream areas was rapid, typical of a piscavore. From the perspective of piscavore evolutionary theory, the growth physiology of Colorado squawfish seems adapted to temperature regimes provided by former lower basin habitats, rather than to those of the upper basin. CONSEQUENCES OF LIFE AT THE UPSTREAM LIMITS OF RANGE Si ow growth in upper basin rivers has important effects on Colorado squawfish populations there. Because maturity generally is more dependent upon fish size than age, slow growth delays maturity. The immature fish therefore are exposed to the causes of mortality for a longer period and fewer of them reach maturity (cf. Nikolsky 1963, Weatherley 1972). We simulated this effect using -111-