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YOUNG COLORADO PIKEMINNOW RECRUITMENT co '!~ :> .~ :: l (/) * co :> .~ ::l '" :;: 0 45 B c: -Warm -Cool c: s- '6 40 Q) g -'<: 's.. .....:l 35 0 E-< ~ co 30 ~ c: 0 ~ "0 25 u 20 Hun lI-Jun 21-Jun I-Jul lI-Jo1 21-Jul 31- Jul Hatching date FIGURE 8.-Simulated effects of cool and warm thermal regimes and hatching date on (A) swvival and (B) final toIal length (TI..) of Colorado pikeminnow larvae that hatched from 1 Iune to 1 August in the Green River, Utah. Simulations ended 1 October, and predator density was 6 red shiners/m2. Colorado pikeminnow length at the end of the growing season was strongly affected by hatching date (Figure 8B). In both of the warm and cool thermal regimes, early hatching larvae reached almost twice the size of late hatching ones because they had more time to grow. Total lengths in simulations with the warm thermal regime were 12-26% larger than those of fish with the cool thennal regime. Changes in length of Colorado pikeminnow over time showed that larvae hatched in June became invulnerable to red shiner predation by late July, whereas fish that hatched in late July were still small enough for large red shiners to consume at the end of the season. Effect of predator density.-Predator density had a strong effect on the survival of larvae with a 1 July hatching date in both cool and warm thermal regimes. Survival in both cool and warm thermal regimes was over 50% at a red shiner density of 1 fish/m2 but 10% or less at a density of 6 fish/m2 (Figure 9A). The curvilinear response was due to the rate of change in the number of anacks and capture rates of larvae (from equation 3) as predator density increased. Water temperature had a strong effect.; at the highest predator density about twice as many larvae survived !he simulation period in warm water temperatures com- pared to cold. Effect of growth rate of larvae.-In simulations, the mean growth rate of Colorado pikeminnow larvae had a large and positive effect on survival (Figure 9B). 1735 I~ 12345 6 Red shiner density/m2 60 50 40 30 20 10 o 0.2 B ........ Wann -0- Cool Q3 Q4 Q5 Baseline growth rate (mm TLld) 0.6 FIGURE 9.-Simulated effects of (A) red shiner predator density and (B) Colorado pikeminnow baseline growth rate on swvival .of age-O cohorts of pikeminnow larvae in the Green River, Utah, using warm and cool thermal regimes. Coloni- zation date was 1 Iuly. Those growth rates, 0.2 to 0.6 mm/d TL, reflected the range we found for the wild age-O Colorado pike- minnow (0.15--O.65-mm/d TL). Survival was relatively high (40-50%) in cold and warm scenarios at the highest mean growth rate but less than 10% at the lowest mean growth rate. Water temperature had a substantial effect, especially at the lowest growth rate, where about twice as many larvae survived the simulated period in warm conditions. Selection for fast growers.-In all simulations, growth rate varied atnong individual fish as it does in natural populations. Similar to growth rates observed in field studies, the simulated growth rates of Colorado pikeminnow that survived to the end of the growing season shifted to fish with faster growth rates compared with !he initial distribution (Figure 10). This was because size-selective predation by red shiners tended to remove the smaller individuals with relatively low growth rates faster than fish with higher growth rates. Comparison of model and field patterns.-Predic- tions of cohort relative survival from mM simulations were similar to those observed for wild Colorado pikeminnow (Figure 6). Relative survival indices calculated for wild fish from the first arumal cohort and for those simulated were I or less and were lower than relative survival indices in second, !hird, and fourth cohorts (Table 1). Similarly, relative survival indices were usually highest for fish in the last cohorts