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293 ~ 40 E L ~ 35 C Q 30 C 25 ^ Green River i980 ^ • Colorado River 1982 r2 = .91 1982 • ^ 1983 1983 1984 • 1.5 2.0 2.5 3.0 Suitability index Fig. 4. Relation between the mean total length of age-0 Col- orado squawfish infall in the Green and Colorado rivers and the relative availability of temperatures suitable for growth (suit- ability index) for the year of capture. Year of data collection is given. theoretical populations is much greater than in pre- sent-day, upper-basin Colorado squawfish. Be- cause that population is at best stable and perhaps declining (Holden & Wick 1982, Tyus et al. 1982), its net reproductive rate (R°, the number of mature female offspring produced in the lifetime of a fe- male parent) is no more than 1 - a condition far worse than that shown even by our simulated slow- a~ N .~ +` O .~ 0 ,z «I E m LL "Fast" (95% per year, early-life mortality) "Fast" (99% per year, early-life mortality) "Slow" (95% per year, early-life mortality) "Slow" (99°h per year, early-life mortality) Time Fig. 5. Comparison of theoretical growth of populations of fast-growing, early-maturing female Colorado squawfish (`fast') with those of slow-growing, late-maturing Ctsh (`slow'), in a limitless environment, under conditions of 95 or 99% early-life mortality. growing population when early-life mortality is 99% (Appendix). Although the sustained, geomet- ric growth indicated by our simulations (Fig. 5) does not occur in nature, the slopes of these curves provide an indication of the relative capacity of the Table 1. Estimated survival (number of fish) in simulated populations of 1000 female Colorado squawfish that grow at different rates (S = slow; F =fast) and are subjected to different annual mortalities (80-99%) while at total lengths of 10-110 mm and to equal annual mortalities of 20% while in each of three longer length classes.e Total length Time in (mm) length class (years) S F Assumed annual mortality of first length class, and growth-rate category 80% S F 90% S F 95% S F 10-110 1.9 1.0 56 200 19 100 7.3 SO 111-210 1.1 0.25 44 190 15 95 5.7 48 211-310 1.2 0.5 34 171 11 85 4.4 43 311-410 1.8 1.25 23 130 8 65 3.0 32 Production of mature fematesb - - - 466 - 712 - 967 99% S F 1.1 10 0.9 9.5 0.7 8.6 0.5 6.5 1200 e Initial population of 1000 female larvae 10 mm long were accepted as being the offspring of one mature female "Percentage by which fast-growing fish exceed slow-growing fish in producing mature females 410 mm long.