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174 D. J. ORTH Orth and Maughan, 1982, 1986). Temporal variations in stream flow can limit populations of stream fish by affecting emigration, growth, survival, or recruitment. Consequently, population size at a given time is determined by past habitat limitations for any life stage and instantaneous measures of microhabitat availability will not be related to instantaneous population size. It is, therefore, not surprising that many attempts to correlate instantaneous WUA and instantaneous fish standing crop have been unsuccessful (Scott and Shirvell, in press). For stream fishes, survival during the early life stages is critical in determining adult population size, and for many species, these stages can tolerate only near-zero water velocities (Larimore, 1975; Ottaway and Clark, 1981; Ottaway and Forrest, 1983). Therefore, fish densities may be strongly related to habitat conditions during the critical early life stages. Anderson and Nehring (1985) found strong negative relationships between juvenile trout (Salmo gairdneri and S. trulla) numbers and peak flows during the time when fry have emerged from the gravel, presumably due to excessive water velocities. Loar el al. (1985) found that rainbow trout densities were correlated with minimum incubation habitat (minimum WUA during incubation period) in southern Appalachian streams without brown trout. However, the lack of a significant relationship for sites where rainbow trout occurred with brown trout suggests a potential competitive interaction. In populations where recruitment is highly responsive to flow conditions, which may be unpredictable, population densities may exhibit stochastic annual variability. Consequently, adult population densities cannot be predicted on the basis of present WUA measures alone; knowledge of past limiting habitat events and past population densities would be needed to predict population density. It is crucial that those involved in negotiating stream flow regimes in regulated rivers recognize that microhabitat availability is not the only regulatory factor and does not operate continuously to limit stream fish populations. The prevailing limiting factors vary widely among streams and may vary temporally within the same stream. Moyle and.Vondracek' {1985-l_fourid that despite variable flow conditions in a small stream, stream fish ass;mblage 'structure -Wa~Ppersistent, suggesting that biotic interactions play an important role in fish population regulation. However,in other streams, stochastic factors (e.g. variable flow) may be equally important~ resulting in more variable fish assemblage structure (Grossman el al., 1985; Schlosser, 1985).-. . '._ ._ ENERGETICS AND-FOOD LIMlTATION The usual omission of aquatic invertebrates from instream flow assessments presumes that either food availability does not normally limit fish production in_~treams or adequate flows for fish result in suitable flows for invertebrates. At the present time, there is insufficient evidence to support either of these assumptions. Furthermore, evidence indicates that some aquatic insects are quite sensitive to small changes in flow and shoulci be included in instream flow asseSSS1ents (Gore, in press). Stream hydraulics has been proposed as a major determinant affecting stream invertebrate assemblages (Statzner and Higler, 1986). In recent years several studies have described the microhabi!at requirements for several stream insects for application in instream flow assessments (Gor~ and Judy, -1981; Orth and Maughan, 1983; Teague el al., 1985). Morin el al. (i986) has critically ~valuated methods for developing habitat suitability criteria for stream insects and Newbury (1984) described the measurement of important hydraulic characteristics. One line of evidence that supports a regulatory influence of prey availability on biomass of stream fish populations is the indication that fish predators depress the density of their invertebrate prey, subsequently resulting in reduced growth of individual fish (Brocksen et al., 1968; F1e~ker, 1984; Anderson, 1985; Angermeier, 1985). In laboratory streams, Brocksen et al. (1968) found that reticulate sculpins (COllus perplexus) and cutthroat trout (Salmo clarki) could overexploit their invertebrate prey. Increased fish biomass reduced the density of food organisms, which directly reduced growth rate of trout and sculpins. Anderson (1985) also observed that growth rates in wild populations of sculpins were inversely related to population density. In exclosure experiments, Angermeier (1985) demonstrated that - predation by stream fish depressed invertebrate densities in silt-sand habitats but not in gravel-cobble