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1 <br />1 <br />1 <br />1 <br />1 <br />1 <br /> <br />Effect of Larval Growth Rate <br />Analysis of growth rates in wild fish showed that natural growth rates range from <br />0.15 to 0.65-mm TL/d (Bestgen 1997). A variety of factors besides temperature may affect <br />growth rates. For example, longevity of backwaters may be related to production of food for <br />larval fish and consequently, growth rates. To examine the effect of mean growth rate on <br />survival, we conducted a series of simulations with baseline growth rates ranging from 0.2 to 0.6 <br />mm/d. <br />Mean growth rate of larval Colorado squawfish had a major effect on cohort survival <br />(Figure 11). Survival declined from 40-50% at the highest mean growth rate to <10% at the <br />lowest mean growth rate. Results of this scenario suggest that management actions that increase <br />larval growth rates may enhance recruitment of Colorado squawfish. <br />Selection for Fast Growers <br />In all our simulations, growth rate varied with individual as it does in natural populations. <br />Although all the larvae in our simulations started at the same size, these growth rate differences <br />generated variation in size among individuals over time. Because size-selective predation by red <br />shiners tended to remove relatively small individuals within a cohort, the distribution of growth <br />rates of squawfish that survived to the end of the growing season was shifted noticeably higher <br />than the initial growth rate distribution (Figure 12). This same pattern was noted in field studies <br />in the Green River, Utah, where size-selective mortality of age-0 Colorado squawfish was <br />presumed due to predation by non-native fishes (Bestgen 1997). <br />21 <br />