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~ 100 ~ 50 2 100 O (r 50 C') ~ 100 j~ 50 Q LL 100 O 0 50 Colorado squawfish Bluegill Yellow Perch Sockeye 5 10 15 20 25 30 35 TEMPERATURE (C) 1.0 D.5 W Z J m Q F- fA Figure 5. Growth-rate vs. temperature relations for Colorado squawfish (Black and Bulkley 1985a) and representative warm (bluegill Lepomis macroc_hirus), cool (yellow perch Perca flavescens), and cold-water (sockeye Oncorhynchus nerka) fishes when fed excess rations (data for latter species were summarized by Magnuson et al. 1979). stocked 5-month-old Colorado squawfish 50-75 mm long into a pond that had abundant age--0 common carp Cygrinus carpio. When about this size, Colorado squawfish begin to feed on other fishes (Vanicek and Kramer 1969). Because of abundant food and the warm temperatures of the pond, the stocked fish averaged 231 mm TL one year later. The largest. was 304 mm long. Their average growth rate was 3 times that of Colorado squawfish in the adjacent Colorado River (Figure 6). We estimated the pond-raised fish will be of mature size (428 mm TL) in their third year of life if this growth rate is maintained. Using data provided by Osmundson (1985), we estimated the relative availability of temperatures suitable for Colorado squawfish growth during the six-month growing season of the pond as 3.6, which is similar to the historic middle river (Figure 4). On the basis of these observations, we concluded that Colorado squawfish in the historic middle and lower Colorado River probably grew much more rapidly and, consequently, had a younger age of first reproduction than do Colorado squawfish in the upper river. Furthermore, Colorado squawfish in the middle and lower river probably spawned earlier in the year than do those in the upper river. This is particularly true for the lower river where temperatures exceeded ZU C in early May, 2 months before they do in the upper river (Figure 4). Earlier spawning would have allowed greater first-year growth in the middle and lower river. 152