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<br />D. J. ORTH
<br />
<br />predation pressure, rather than microhabitat availability, may act to regulate population density. Several
<br />studies indicate how predators can affect microhabitat use. Power and Matthews (1983) demonstrated
<br />that stonerollers (Campostoma anomalum) and piscivorous bass (Micropterus spp.) had complementary
<br />distributions due to either elimination of stonerollers by bass predation or active avoidance by
<br />stonerollers. Power (1984) found that armoured catfish (Loricaridae) avoided shallow areas of stream
<br />where they were susceptible to avian predation even though their food, attached algae, was abundant
<br />there. At night, these catfish tended to utilize shallower water. Juveniles of two minnows (Semotilus
<br />atromaculatus and Rhinichthys atratulus) also actively avoided locations that contained fish predators
<br />(adult S. atromaculatus) (Fraser and Cerri, 1982). In that study, the structural complexity (i.e. presence of
<br />shade, plastic pipes, and dendritic roots) and time of day affected the response of prey fish to the presence
<br />of predators. Presence of structure was the most important determinant of patch choice by these two
<br />minnows (Fraser and Cerri, 1982), presumably because predation rates were reduced in structurally
<br />complex habitats (Newsome and Gee, 1978).
<br />Changes in habitat use by some forage species with time of day may reflect inactivity of predators and
<br />reduced predation risk. Cerri (1983) demonstrated that, in daylight, prey fish were more aggregated and
<br />predator fish were concealed in refuges; therefore, predation rates were lowest in daylight. Furthermore,
<br />prey fish had a greater reactive distance to predators, but this advantage was reduced under low light
<br />conditions. Therefore, prey fish must alter their behaviour and habitat selection in low light conditions.
<br />These cover seeking and die! activity patterns are common in stream fishes and likely represent adaptive
<br />responses to predation. In some stream fishes the role of predation in influencing habitat choice may be
<br />independent of food availability (Cerri and Fraser, 1983; Power, 1984). Presence of fish predators
<br />reduced the patch utilization by juvenile R. atratulus by the same proportions at low and high food levels;
<br />therefore, the benefits of increased food were not balanced against risk of predation (Cerri and Fraser,
<br />1983). Power (1984) concluded that avoiding. pr~dators was.am_oread~Qtive strategy than obtaining
<br />maximum energy intake because fish can witlista1ia periods ofst~ation (Brett and Groves, 1979).
<br />Therefore, improved assessments of instream flow changes must be based on site-specific habitat
<br />suitability criteria, which reflect the local adap!ation~ of fishes to'the indigenous predators. Also, cover
<br />availability must be included in instream flow 'assessments w!'1ere cover is strongly influenced by river
<br />stage. -
<br />The other important consideration of predation-t6.instreamflow assessment is whether predators,
<br />rather than microhabitat availability, limit the population densities of fishes at the site under study. Lemly
<br />(1985) showed with removal experiments that green sunfish (Lepomis cyanellus) predation on
<br />young-of-year fishes was a dominant force in deteJ1I!ining the fish assemblage structure in first-order
<br />streams; when green sunfish were removed, most native fishes increased,in numbers and biomass.
<br />Anderson (1985) hypothesized that factors that regulate stream fish populations vary with stream size
<br />and found that sculpin population densities were lower in huger stream sites that also contained rock bass
<br />and suggest~d that predation limited sculpin population density' at these sites. In smaller stream sites
<br />without rock bass, sculpin densities were higher arid growth rates and.fecundities were lower suggesting
<br />that food availability played a more dominant role than predation in liiilitinrpopulation size in smaller
<br />streams. If available microhabitat was liniifing, one would expect growth, fecundity, and density to be
<br />similar among populations 'within patches of suitable microhabitat. .
<br />. In summary, predation risk will influence instream flow assessments and development of new models in
<br />four ways. First, habitat suitability criteria may not be applicable to streams with different predation risks.
<br />Second, structural complexity is an important characteristic to incorporate into models for stream fishes.
<br />Third, the effects of flow regulation on fishes will likely depend in part on the type of predators
<br />(terrestrial and aquatic) that occur in the riparian/stream ecosystem and how the predation rate is
<br />influenced by flow. Finally, if predation is limiting, availability of microhabitat will not be directly related
<br />to population density.
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