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1 1 ii 1 Structure and Components of The Mechanistic Simulation Model The conceptual life-history model illustrates the larger framework within which the mechanistic simulation model was used to evaluate the effects of environmental factors on survival and growth ofage-0 Colorado squawfish during their first summer. Age-0 fish were chosen as the subject of study for the simulation model for several reasons. First, recruitment from eazly life stages has been shown to be an important factor in determining year-class strength in other fishes (Houde 1987). Second, eazly life stages of Colorado squawfish have been the subject of numerous field and laboratory studies. Third, we intended to collect empirical data for construction of the model by conducting pertinent experiments, and hatchery-reazed age-0 Colorado squawfish were readily available. During model construction, results of these experiments were integrated with field observations; thus the model has a foundation grounded in empirical data which increases the likelihood that its predictions aze ecologically relevant. An individual-based model (IBM) structure was selected as the fundamental basis for the simulation model because we suspected that individual differences in size and growth of larvae may affect recruitment patterns as much or more than average differences of populations (Rice et al.1993; DeAngelis and Rose 1992). The IBM that was ixutially proposed had components that (a) linked river dischazge to prey availability for larvae, (b) incorporated starvation effects on larvae, (c) allowed multiple species to prey on larval Colorado squawfish, and (d) simulated the influence of temperature regime on growth of larvae (Figure 2). Every effort was made to identify pre-existing data that could be used to derive quantitative relationships relevant to each component of the IBM. However, during eazly stages of IBM development, it became evident 6