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TABLE 1. Comparison ~f survival for simulated populations of slow <br />and fast-growing Colorado squawfish that experience annual <br />mortality rates of 50, 70, 80, 90, 95 or 99~ in the 10-110-mm <br />length class, and 20~ in subsequent size classes. Initial <br />populations consist of 1000 female larvae IO mm long <br />(considered the offspring of one mature female). Numbers in <br />parentheses are percentage difference in production of mature <br />females (410 mm TL) between "slow" and "fast" populations. <br />Length class (mm TL) <br />Population 10-110 111-210 211-310 311-410 <br />Residence time (years) <br />"Slow" 1.9 1.1 1.2 1.8 <br />"Fast" 1.0 0.25 0.5 1.25 <br />Survivors that grow into next length class <br />Mortality of first length class 80~ per year <br />"Slow" 56 44 34 23 <br />"Fast" 200 190 171 130 (466) <br />Mortality of first length class = 90~ per year <br />"Slow" 19 15 11 8 <br />"Fast" 100 95 85 65 (713) <br /> Mortality of first length class = 95~ per year <br />"Slow" 7.3 5.7 4.4 3.0 <br />"Fast" 50 48 43 32 (967) <br /> Mortality of first length class = 99~ per year <br />"Slow" 1.1 0.9 0.7 0.5 <br />"Fast" 10 9.5 8.6 6.5(1200) <br />1984)--normally during July or August in the upper basin (Figure <br />3)--and fertilized eggs hatch 4-5 days later. Because the <br />subsequent first-year growing season is short, the age-0 fish are <br />small at its .end (Table 2). Moreover, these small fish are at a <br />marked size disadvantage when compared. to available prey species <br />(Table 2). Because Colorado squawfish are about 30 mm TL when they <br />begin to feed on other fishes (Charles McAda, USFWS, unpublished <br />data), many of these fish will not achieve piscivory until their <br />second year of life. Overwinter mortality of such small fish is <br />probably high, however. In the historic lower river, spawning <br />temperatures were reached in early May (Figure 3) and young fish <br />had most of the long growing season available for first-year <br />growth. Timing of Colorado squawfish spawning therefore is a <br />reproductive strategy that seems adapted to temperature regimes <br />provided by former lower basin habitats, rather than to those of <br />the upper basin. <br />-113- <br />