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291 offspring of one mature female. Our initial assump- tion was that, in fish of the slow-growing stock, growth rate, length at maturity, and age at maturity resembled these characteristics in upper-basin Col- orado squawfish, whereas in the fast-growing stock, squawfish growth resembled that in Os- mundson's pond (Fig. 2). First maturity of Col- orado squawfish in the upper basin occurs at a length of about 428 mm and an age of about 6 years (Seethaler 1978) - a size that pond-raised fish might conceivably reach in about half that time (Fig. 2). For computational convenience, we assumed that maturity in both simulated stocks occurred at a length of 410 mm and the age of 6 years in slow- growing fish and 3 years for fast-growing fish. There are no definitive data on the survival of Colorado squawfish in its natural environment. In the simulations, we therefore assumed annual mor- talities of 80, 90, 95 or 99% in the shortest length class (10-110 mm TL), and 20% in each of the three larger, 100 mm length classes. Although arbitrary, these rates were chosen because such rapid, early- life mortality and a reduced, constant rate for later ages is typical of many freshwater fishes (e.g. Weatherley 1972). Thus the important difference between our simulated populations was the length of time that fish remained in each length class, which was determined by growth rate. The number of fish that died in each length class was calculated as the product of the initial number of fish, mortal- ity rate, and duration of time spent within the length class. For periods longer than 1 year, the number of deaths during the first year was calculat- ed as described above, and deaths during the re- maining time period were similarly calculated for fish that survived the first year. Vital statistics for our simulated populations were calculated using equations provided by Krebs (1972): r = (IogeR°)lG, and f = e', where G = mean length of generation (the mean period between the birth of the parent and that of offspring); lx = age-specific survival; mX = age- specific effective birth rate; x =age in years; R° _ E lXmx =net reproductive rate (the number of ma- ture female offspring produced in the lifetime of a female parent); r = intrinsic rate of natural in- crease; and f =finite rate of increase (the multipli- cation factor by which the adult female stock will annually grow if that particular value of R° is main- tained). For these computations it was assumed that, beginning in the first year of maturity and continuing through age 10 (arbitrarily taken to be the age of last reproduction), each slow- and fast- growing female produces mature female offspring at an annual rate equal to the age-specific effective birth rate (mx) for our simulated stocks when early- life mortality was 95 or 99%. Development of the hypothesis Temperature regime analysis Although the temperature data used in our analys- es do not reflect the precise temperatures that Col- orado squawfish may experience throughout their life history, they nonetheless allow demonstration of the marked differences in suitability for Col- orado squawfish growth among the temperature regimes of present and historic habitats. If growth of Colorado squawfish occurs only when water temperatures exceed 13° C, growing seasons in the upper-basin river reaches are less than 6 months, whereas they were 7 to 9 months in historic, pre- development, middle- and lower-basin reaches (Fig. 3). Comparisons among these temperature regimes are more useful, however, if their relative suitability for Colorado squawfish growth is con- sidered. Such suitability was estimated as 3.2, 2.1, 3.9, and 4.9 for the Green River and the upper, middle, and lower Colorado River, respectively. If the bias in the Green River data (Fig. 3) was as large as our worst-case estimate of 1° C, and we reduced these data by that amount, the Green Riv- erwould continue to warm earlier and have warm- er temperatures during the growing season than does the upper Colorado. Suitability of the Green River temperature regime would be 2.7, an aver- age value 29% larger than that of the upper Col- orado.