Abstract Individual-based modeling is a recent technique that is fast becoming an established discipline separate from more classical population models. Individual-based models (IBM's) incorporate 3 distinguishing characteristics. First, they reflect real life cycle complexity including mortality, fecundity, and spawning migrations that depend upon the age or size of individuals. Second, resources are explicitly modeled. Physical habitat availability, food abundance, temperature, and water quality are modeled using actual measurements at high, intermediate, and low flows. Third, and most important, is accounting for differences among individuals within a population. Rather than assume that mortality, growth, reproduction, movement, spatial distribution, and the effects of interference competition is the same for an average trout, IBM's model differences between individuals (e.g. size) providing a realistic prediction of the stream flows required to maintain salmonid populations. The instream flow incremental methodology (IFIM), is a straightforward technique of estimating potential fish abundance from field observations. Although IFIM is widely used, it is increasingly viewed as an inadequate oversimplification. A new approach (IBM's) will incorporate increased biological realism by considering how environmental factors influence fish energetics and growth.
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