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<br />1734 BESTGEN ET AL. <br /> 0.6 MGR 1991 <br />---- 0.5 0.6 <br />'"0 <br />~ 0.4 0.5 <br />0.3 0.4 <br /> 0.3 <br />S 0.2 0.2 <br />S 2 3 <br />'--' <br />ll) <br />..... MGR 1992 <br />Ctl 0.6 <br />..... 0.6 <br />..c: 0.5 0.5 <br />..... <br />~ <br />0 0.4 0.4 <br />..... <br />0 0.3 0.3 <br /> 0.2 0.2 <br /> 2 3 <br /> Cohort number <br /> <br />LGR 1991 <br /> <br /> <br />2 <br /> <br />3 <br /> <br />LGR 1992 <br /> <br /> <br />2 <br /> <br />3 <br /> <br />4 <br /> <br />FIGURE 7.-Presumed effects of size-selective predation on mean growth rates (total length) of summer and autumn juvenile <br />Colorado pikeminnow for each of three or four within-year cohorts, comparing summer (white bars) and autumn (black bars) in <br />the middle (MGR) and lower Green River (LGR), Utah, in 1991 and 1992; whiskers = 5Ds. The single summer sample was <br />collected before fish in the third cohort (fourth cohort in the case of the lower Green River in 1992) hatched, so there was no <br />corresponding summer sample for those cohorts. <br /> <br />August. Mean growth rate was only 0.33 mrn/d TL (N <br />= 7) for fish that hatched in the third cohort during 31 <br />July to 11 August, compared with growth rates of 0.43 <br />mrn/d or greater for auturmI juveniles hatched in the <br />first or second cohorts. Surrumer juveniles in the lower <br />Green River, 1992, which recruited poorly, were small <br />upon capture and had the lowest growth rates observed <br />in the study. Mean growth rate of auturmI juveniles in <br />the first, second, and third cohorts sampled in autumn <br />was 50% or more higher than for summer juveniles in <br />corresponding cohorts. Hatching dates for the first <br />three cohorts preceded an extended period of higher <br />discharge (53.5-100.8 m3fs; 9 July until early August) <br />from heavy and prolonged precipitation in the lower <br />Green River basin. Most of the auturmI juveniles from <br />the fourth cohort that had relatively high survival <br />hatched after discharge subsided. <br /> <br />IBM Simulations <br /> <br />We implemented the ffiM to evaluate the relative <br />effects of red shiner predation on the survival and mean <br />TL of Colorado pikeminnow in cohorts at the end of <br />the first growing season as a function of thermal <br />regime, hatching date, larval growth rate, and predator <br />density. The goal was to better understand interactions <br />of red shiner predation and environmental factors on <br />Colorado pikeminnow growth and recruitment and to <br />determine whether patterns observed in simulations <br />were consistent with those observed in the field. Most <br />simulations were conducted with turbid water and <br />alternative prey present because those conditions were <br /> <br />typical for backwater nursery habitats in the Green <br />River. Recall that those conditions produce the lowest <br />attack rate, so predation results may be conservative <br />compared with conditions where backwaters were clear <br />or food was scarce. Unless otherwise specified, we <br />used a moderate mean baseline growth rate of 0.3 mrn/ <br />d for larvae, and a moderate red shiner density of 3 <br />fish/m2. <br />Effects of thermal regime and hatching date.-We <br />ran scenarios with daily inputs of 500 larvae and <br />hatching dates beginning on 1 June and ending on 1 <br />August (6 red shiners/m2) to exanaine the effect of <br />hatching date and thermal regime on the survival of <br />larvae. . Red shiner predation was significant because <br />survival of Colorado pikeminnow larvae was only 1.3- <br />8.5% in the warm thermal regime (Figure 8A) and 0.8- <br />4.3% in the cool thermal regime. Survival was <br />relatively low for larvae with early or late hatching <br />dates compared with midseason hatching dates. Larvae <br />that hatched early experienced rapid growth because <br />backwater temperatures were relatively warm, but the <br />larger and more efficient red shiner predators present <br />during this time reduced larval survival. Larvae that <br />arrived midseason in backwaters encountered the best <br />conditions for survival because temperatures were <br />warm and predators were relatively small. fu these <br />conditions, larvae experienced up to 30 d of rapid <br />growth to sizes that were not susceptible to predation. <br />fu contrast, later-hatching larvae encountered smaller <br />predators but experienced slower growth because of <br />declining water temperatures. <br />