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<br />172
<br />
<br />D. J. ORTH
<br />
<br />. In the meantime, decisions that affect the future of many stream fisheries will be made without the
<br />benefit of predictions of fish population responses to alternative flow regimes. In the near future the
<br />IFIM process will continue to playa major role in instream flow assessments and components of IFIM will
<br />be modified to accommodate new situations (e.g. hydropeaking, Bovee, 1985). Therefore my purpose in
<br />this paper is to (1) review ecological concepts that may explain why WUA is an inconsistent predictor of
<br />fish population abundance, (2) suggest cautious applications of PHABSIM models, (3) encourage
<br />application of all available models and knowledge in the assessment process, and (4) suggest areas of
<br />needed research.
<br />
<br />STREAM ECOLOGY AND FLOW ALTERATION
<br />
<br />At least six primary factors influence the structural and functional characteristics of stream ecosystems
<br />with respect to distribution and abundance of stream fishes. These are energy source (food), water
<br />quality, temperature, physical habitat structure, flow regime, and biotic interactions (Karr and Dudley,
<br />1981). Too often, instream flow assessments focus only on changes in physical habitat structure (e.g.
<br />WUA) due to flow alteration and ignore potential temperature and water quality. changes despite the
<br />availability of water temperature and water quality models (Grenney and Kraszewski, 1981; Theurer and
<br />Voos, 1984). Variables related to energy source and biotic interactions are typically ignored even though
<br />such changes frequently occur following certain types of flow regulation (Ward and Stanford, 1983).
<br />Consequently, most instream flow assessments represent incomplete analyses of potential impacts of flow
<br />regulation.
<br />Improved instream flow models must address all of the six primary factors affecting stream ecosystems
<br />in order to avoid unanticipated effects. Assessment of potential effects of regulation requires a
<br />knowledge of how these factors interact and_ th~Uime .scale requ~a: before the full effects may be
<br />recognized. Petts' (1984) hierarchial framework (Figure" 1) descri6es these effects in terms of three orders
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<br />Figure 1. Hierarchial framework for examining the impacts of river impoundment. From Petts (1984). Copyright by John Wiley &
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