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600 ~- 500 Z LLJ 400 _! Q 300 F- ~ 200 Z Q io0 W • Colorado River (Seethaler 1978) 0 o Green-Yampa Rivers.(Vanicek & Kramer 1969) +-+ Green-Yampa Rivers (Seethaler 1978) ~ 1 2 3 4 5 6 7 8 9 10 11 YEAR OF LIFE ` Figure 2. Growth of Colorado squawfish from the Green River near its confluence with the Yampa River and from the Colorado River near Grand Junction, Colorado. and increase survival) and mature at early age (thus increasing reproductive potential). Such life-history characteristics are shown, for example, by northern pike Bsox lucius and largemouth bass Micropterus salmoides (Figure 3). Colorado squawfish occur today only in the upstream regions of their former range (Figure 1). The high latitude and elevation at such limits of range can result in a relatively short growing season. In such areas, widely distributed species often have populations that show relatively slow growth and late age of first reproduction (Nikolsky 1963, Weatherley 1972). We believe the seemingly unusual life-history characteristics of Colorado squawfish in the upper basin result from t:he short growing season there, and that these characteristics are quite different from those of squawfish in the historic lower basin. Unfortunately, Colorado squawfish were extirpated from the lower basin before life-history information that could support this contention were collected. Nevc:rthele~ss, it is possible for us to objectively speculate on the probable life-history characteristics of these former, lower- basin fish. Life History of Colorado Squawfish in the Historic Lower Colorado River: A Probable Scenario Growth rate of fishes is largely dependent upon water temperature and food availability (see discussion in Weatherley 1972). Behnke and Benson (1983} suggested that the slow growth of Colorado squawfish in the upper basin might be due to limited prey availability. This seemed unlikely to us because 149