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289 ~ 600 500 400 ~ 300 O ~ 200 N 100 ^ Colorado River (Seethaler 1978) • Green-Yampa Rivers (Vanicek & Kramer 1969) * Green-Yampa Rivers (Seethaler 1978) - Pond (Osmundson 1987) Northern pike :`~c ~] Largemouth ~ ~~ ~ bass ~ ~' ~ ~. ~[ • T v * 1 2 3 4 5 6 7 8 9 10 11 Year of f ife Fig. 2. Early-life growth of two widely distributed piscivores, northern pike and largemouth bass (shaded areas show the median 50% of the range of length-at-age values for these species provided by Carlander [1969,1977)), compared to that of Colorado squawfish from the upper Colorado River basin (data derived by Vanicek & Kramer 1969, and Seethaler 1978, using scale annuli). Also shown is early-life growth of Colorado squawfish in a pond near Grand Junction, Colorado (Osmundson 1987). The broken line on the curve for pond fish shows estimated growth used in our simulations. Methods Temperature regime analysis Black &Bulkley (1985a) studied the relation be- tween constant temperature and growth of 45- 100 mm long Colorado squawfish given excess food. They reported that growth was optimal at 25° C, and that growth at 15, 20, and 30° C was 18, 54, and 51% of optimum, respectively. Additional studies by Black &Bulkley (1985b) and Bulkley et al. (1981) indicated that 25° C was the preferred temperature, generally considered the optimum for many physiological processes including growth (e.g. Magnuson et al. 1979), for both yearling and adult Colorado squawfish. Our least-squares ana- lysis of the published data of Black &Bulkley (1985a) suggested that growth ceases at temper- atures below about 13° C, a value supported by observations on seasonal growth of Colorado squawfish in ponds (Osmundson 1987). We as- signed suitability indices to the growth-rate versus temperature relation of Black &Bulkley (1985a) according to the percent of optimum growth that temperatures provided. Thus the suitability indices for temperatures of 25,15, 20, and 30° C were 1.00, 0.18, 0.54, and 0.51, respectively, and those for 13° C or lower were zero. Indices for intervening temperatures were estimated by interpolation. The growth-rate versus temperature relation for Colorado squawfish reported by Black &Bulkley (1985a) is symmetrical and triangular in shape, whereas those for other species often have a flat dome around the optimum temperature and an absolute value of the slope to the left of the opti- mum less than that to the right (e.g. Magnuson et al. 1979). Because we were concerned that the unusual shape of the Black &Bulkley (1985a) curve might affect our subsequent analyses, we performed preliminary analyses using several dif-