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YOUNG COLORADO PIKEMINNOW RECRUITMENT effects of a single predator, red shiners. Simulations also provide a means to evaluate potential outcomes of management scenarios (e.g., predator removal, variable temperature regimes) to enhance recruitment in wild populations before they are implemented. Integrated studies that include experiments, detailed field obser- vations, and simulation modeling are well-suited to gain understanding of complex recruitment processes in fishes. The body of information we gathered supports the thesis that the recruitment variation observed fo; early life stages of wild Colorado pikeminnow may be structured by interacting effects of physical factors and predation from nonnative fishes. Consistency of growth and recruitment patterns we observed in field studies combined with expectations from laboratory experiments and predation-driven ffiM simulations led us to this conclusion. Predation by a novel and gape- limited predator, the small-bodied red shiner, may play a particularly imponant role, along with a host of other nonnative taxa that occur in backwaters of the Green River. Below we discuss components critical to this recruitment assessment for age-O Colorado pikemin- now and their relevance to management. Laboratory Experiments The red shiner is not usually viewed as an important piscivore. However, experimental data showed that red shiners are effective at capturing Colorado pikeminnow larvae and the number of capture attempts by red shiners was relatively high. Large red shiners were generally the best predators, capturing and consuming pikeminnow that were up to one-third of their length (23-rrml-TL pikeminnow captured by a 69-mm-TL red shiner), but a red shiner as small as 38-rrml TL consumed larvae. Because red shiners are gape-limited, growth rate of pikeminnow larvae determines their window of predation vulnerability. Compared with laboratory experiments, mesocosm studies had a broader range of natural conditions, which included fluctuating water temperature, sand substrate, turbidity, and alternative prey. Even under these more natural and challenging conditions, meso- cosm studies confirmed that red shiners were effective predators on Colorado pikeminnow larvae. Presence of turbidity and alternative prey greatly reduced attack and predation rate and made simulation results more representative of field conditions. An unexpected outcome of mesocosm experiments was that large larvae were attacked more frequently than small ones, even though probability of capture declined as pike- minnow size increased. Large larvae may provide more visual stimuli or have behavior or swimming speed differences that increase the likelihood of encountering 1737 predators. Another unexpected outcome was that different groups of red shiners demonstrated diffetent levels of predation ability, which were independent of predator length. Additional study of differences in predatory behavior among red shiners may have theoretical and practical application. The predation demonstrated by red shiners in experiments suggested that other small-bodied fishes in Green River backwaters may play important roles as predators on early life . stages of fish, including Colorado pikeminnow. This was supported by Johnson and Dropkin (1992), who found that 15 relatively small-bodied fishes, including cyprinids, darters, and killifish that are not generally considered as piscivo- rous, consumed larval American shad Alosa sapid- issima stocked in the Susqueharma River. Others have also documented predation by small-bodied fishes, including fathead minnow, which is an abundant nonnative resident of Green River backwaters (Mc- Govern and Olney 1988; Dunsmoor 1995). Predation by small-bodied fishes, such as the widely introduced and abundant red shiner, deserves further recognition as a factor that may influence recruitment and recovery of rare native fishes. Expanding populations of small- mouth bass Micropterus dolomieu may exert an even greater predation pressure on a wide range of life stages of native fishes in Green River backwaters. Field Studies Intra-armual survival of cohorts of wild Colorado pikeminnow larvae in the field was highly variable, and the highest number of autumn juveniles did not originate from the largest cohorts of larvae. Distribu- tions of hatching dates and growth rates of wild fish showed that the few early hatched larvae that survived to autumn were among the fastest growing individuals, suggesting that a strong size-selective mechanism was operating. Early-hatched, wild Colorado pikeminnow larvae were also among the largest autumn juveniles because of their relatively longer growing season. Absence of autumn juveniles from large cohorts of early hatched larvae was probably not due to adverse physical conditions like cold water temperatures because water was warm when the first armual cohorts of larvae arrived in nursery habitats. It is possible that backwater food supply was scarce because the backwaters formed after high flows receded, which was just before colonization by Colorado pikeminnow larvae. This may be why growth was low and subsequent mortality was relatively high. Compared with early hatched individuals that grew relatively fast and were larger in autumn, later-hatching fish survived at higher rates but grew relatively slowly and were smaller in autumn.