Laserfiche WebLink
<br />"'~ <br /> <br />INSTREAM FLOW-HABITAT MODELS <br /> <br />177 <br /> <br />COMPETITIVE INfERACfIONS <br /> <br />Competitive interactions complicate instream flow assessments because: (1) species-specific analyses of <br />microhabitat availability ignore the effects of crowding at low flows and (2) presence of competing <br />species can influence microhabitat selection. Although multiple species and life stages are considered in <br />most instream flow assessments, effects of flow regulation on the outcome of competitive interactions is <br />seldom considered. However, it is generally agreed that many fishes in small streams are habitat <br />specialists (Gorman and Karr, 1978) and reductions in flow will decrease the range of habitats available. <br />Consequently, low flows, especially those of unprecedented duration and magnitude, may increase the <br />intensity of competitive interactions. Because fish are crowded into the same limited physical space at low <br />flows, the influence of resource sharing on microhabitat availability must be considered. The degree to <br />which habitats are partitioned among similar species varies for Cyprinidae (Gibbons and Gee, 1972; <br />Mendelson, 1975), Percidae (Page and Schemske, 1978; Smart and Gee, 1979; Paine et ai., 1982; Wynes <br />and Wissing, 1982; Englert and Seghers, 1983; Schlosser and Toth, 1984) and Cottidae (Finger, 1982; <br />Matteson and Brooks, 1983). <br />The most critical concern for determining habitat suitability criteria is that the abundance and presence <br />of competitors may influence the microhabitat selection. If microhabitat shifts occur in presence of <br />competitors, habitat suitability criteria are not transportable to sites with different assemblages of <br />competitors. Habitat utilization by sculpins (Cottus spp.) was strongly influenced by the presence of other <br />sculpin species in an Oregon stream (Finger, 1982); microhabitat shifts were also documented in <br />manipulative experiments in laboratory streams. Fausch and White (1981) demonstrated that removal of <br />brown trout allowed brook trout (Saivelinus fontinaiis) to occupy more advantageous resting positions. <br />Baltz et ai. (1982) observed high similarity in microhabitat use between two morphologically dissimilar <br />species (riffle sculpin C. guiosus and speckled dace R. oscui~) in,sYI;Ilpatry. Abundance of dace was <br />negatively correlated with abundance of sculpms in:the field, ana sCQ:lpw.abundance affected utilization of <br />limited refugia by dace in laboratory experiments. Since the outcome of the competitive interaction <br />between the sculpin and dace was influenced by temperature, habitat suitability criteria developed for <br />either of these species would be limited to the 'site of development. <br /> <br />SELECTION OF TARGET SPECIES <br /> <br />Selection of appropriate species and life stages on ~p.ich to base analyses of instream flow needs is a <br />critical step, especially in warm water streams which have diverse fish faunas. Microhabitat preferences <br />vary greatly among the fish species and life stages in a givenstream (Moyle aria Baltz, 1985). Species and <br />life stages with the narrowest range of habitat preference will generally be most sensitive to flow <br />alterations. Target species with restricted habitat preferences should be selected to encompass the range <br />of habitat-use guilds (or reproductive guilds) represented by the faunal assemblage in the stream. Fish in <br />different habitat-use guilds exhibit dissimilar habitat respoI?-ses' to discharge and, consequently, <br />recommended flows will represent a compromise between the needs of inhabitants of slack water and <br />inhabitants of fast water (Leonard et ai., 1986). Therefore, spe~ies most restricted to fast water and slow <br />water would be most useful as target species. <br /> <br />SUMMARY OF IMPLICATIONS <br /> <br />Consideration of other ecological factors that affect stream fishes must be made during the IFIM process. <br />The development of new models either d~pendent upon or independent of IFIM must incorporate the <br />critical determinants of stream fish population dynamics. Because of the long time scales involved in <br />second- and third-order impacts of flow regulation, the use of field manipulations of low flows to <br />determine an appropriate flow regime may not be an appropriate alternative to the use of some type of <br />