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<br />INSTREAM FLOW-HABITAT MODELS
<br />
<br />175
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<br />habitats. In a similar experiment Flecker (1984) found that the presence of fish (mainly dace Rhinichthys
<br />and sculpins COitus) depressed the densities of midges (Chironomidae) and the stonefly Leuctra, but not
<br />other invertebrate taxa. These studies suggest that stream fish production may be strongly influenced by
<br />invertebrate production or availability. In fact, Warren et aI. (1964) dem~nstrated that an increase in
<br />biomass of aquatic insects following sucrose enrichment resulted in a seven-fold increase in trout
<br />production in experimental streams.
<br />However, other studies demonstrate that fish predators do not always exert an influence on
<br />invertebrate densities in natural streams (Allan, 1982; Reice, 1983; Culp, 1986). Allan (1982) reduced
<br />trout densities between 10 and 25 per cent of initial levels for four years and observed no increase in
<br />invertebrate densities. Reice (1983) found that stream invertebrate communities were unaffected by the
<br />exclusion of fish. CuIp (1986) increased the densities of coho salmon (Oncorhynchus kisutch) fourfold
<br />and found no measureable effect on macro invertebrate density and suggested that salmonid predators are
<br />only weak interactors in the food web. The effect of fish predation on invertebrate densities is expected to
<br />be slight at all but the highest fish densities (Brocksen et aI., 1968) and in silt-sand (i.e., structurally
<br />simple) substrate (Angermeier, 1985); therefore, substrate complexity or lack of precision in estimating
<br />densities may explain why density reductions were not detected in some field experiments.
<br />A second approach to study the relative roles of food availability and habitat structure involves
<br />experiments on microhabitat choice. Theoretically, the optimum habitat is one' th~t minimizes the ratio of
<br />mortality (predation risk) to growth rate. Concerning growth rates, Fausch (1985) demonstrated that
<br />position choice in three juvenile salmonids maximized the potential for net energy gained (available prey
<br />energy minus energy costs for swimming). Bachman (1984) suggested that availability of preferred
<br />foraging sites was a major limiting factor for salmonid populations at high population densities. When not
<br />feeding, trout select resting microhabitats that provide cover from predators and minimize energy
<br />expenditure for swimming; availability of the~e ~~~ting microhab!tats_may also act as a limiting factor
<br />(White, 1975). The uniwue characteristics of f-6r~ging sites and r~~m.g'Sites are probably not adequately
<br />described by present habitat suitability criteria which do not distinguish between activities or seasons.
<br />The relative importance of food and cover in determining position choice varies with body size,
<br />temperature, and time of day (predator activity) or. light lev~l; this variability further complicates the
<br />process of developing suitability criteria for instream flow aSSessme:nts. Smith and Li (1983) found that,
<br />for juvenile steelhead trout, increased fish size -:.,ot increased;. Jemperature resulted in selection of
<br />microhabitats with higher water velocities, presumably due to increased metabolic needs. This would
<br />suggest that availability of prey and preferred foraging sites is probably mos(critical during summer, a
<br />time when invertebrate prey availability is declin.iJ).g (Angermeier, 1985). In fact, Wilzbach (1985)
<br />documented that food abundance was more important than cover in determining the numbers of
<br />cutthroat trout remaining in laboratory stream channels at sumri:l(ir~temperatures. However,
<br />microhabitats for resting and protection fr()m predation or displacement may be more critical at other
<br />times of the year since salmonid abundance is often related to cover alone (Boussu, 1954; Bjornn, 1971;
<br />Hunt, 1976). Rainbow trout show dramatic seasonal shifts in habitat use (Campbell and Neuner, 1985),
<br />which suggest that habitat and food requig:ments change seasonally:"~ ~. .
<br />The importance of prey availability suggests that increas~d,~fforts be made to incorporate criteria for
<br />stream invertebrates into instream flow assessments. Efforts'must include riffle invertebrates, which are
<br />well studiM, as well as inhabitants of woody debris that are often the preferred food items for fish in
<br />low-gradient streams (Angermeier and Karr, 1984; Benke et aI., 1985). Habitat variables that adequately
<br />describe food availability must be sought since abundance of invertebrates is related to the detritus food
<br />base (Culp et aI., 1983) and quality of food (Brown and Brown, 1984) as well as to substrate or velocity
<br />characteristics of the microhabitat. ;;'1'.';. c' ~c.
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<br />PREDATION RISK
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<br />Consideration of predation risk in instream flow assessments is important because (I) microhabitat
<br />: : utilization may vary due to the presenCe of predators and their activity pattern (i.e. time of day) and (2)
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