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1 1 1 ii INTRODUCTION Understanding the role of biotic and abiotic factors that affect timing and success of reproduction is central to ecology because abundance of young individuals often drives recruitment dynamics of subsequent life stages (Roughgarden et al. 1988). Mechanisms of recruitment variation in animals such as fishes with multi-phase life cycles are particularly difficult to assess because larvae typically disperse, sometimes long distances, away from juveniles and adults and individuals in each life history phase are constrained by different factors. Moreover, most aquatic organisms with early life stages that disperse have highly variable recruitment because their high fecundity, coupled with small variations in regulating processes, cause large differences in survival and recruitment of larvae (Hjort 1914, Thorson 1950, Fogarty et al. 1991). Thus, factors that regulate distribution, abundance, size-structure, and survival of early life history stages are integrated into processes that structure recruitment (Thorson 1950, Gaines et al. 1985, Houde 1987, Miller et al. 1988, Underwood and Fairweather 1989, Johnston et al. 1995). Most populations of endangered Colorado squawfish Ptychocheilus lucius of the Colorado River basin may be recruitment limited (Tyus 1991). In the Green River, where the largest remaining population occurs, annual estimated density of juveniles in fall (recruits) ranged from near zero to 75 fish/100 mZ in backwater habitat (Tyus and Haines 1991). However, the relative effects of discharge regime, habitat alterations, introduced fishes, and annual abundance of Colorado squawfish larvae on recruitment of juveniles remain poorly understood. This study was initiated in 1990 and was part of the Five-Year Flaming Gorge Flow ii Recommendations Investigations (FGFRI); specifically Element 1 (Reproduction) of the Systematic Data Collection and Research (SDCR) program. The SDCR was composed of four interrelated elements and was intended to assess effects of water regulation by Flaming Gorge Dam on annual reproduction and long-term recruitment of fishes in the Green River system. The four elements included 1) reproduction, 2} age-0 survival to fall (age-0 recruitment), 3) over- winter survival of young fish (age-1 recruitment), and 4) links between recruitment of young fish and recruitment to adult stocks. This study addresses the first two elements. Integrated studies under FGFRI (i.e., SDCR studies and hypotheses-testing studies, which were intended to aid in