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Reprinted from Proc. Symp, and Spec. Con£. on Instream F1 w Needs, Vol. II, p. 267-284. West.-Div. Am. Fisheries Soc., and Am. Soc. Civil Eng. Boise, Idaho, May 3-6, 1976. VALIDITY OF METHODOLOGIES TO DOCUMENT l STREAM ENVIRONMENTS FOR EVALUATING FISHERY CONDITIONS William S. Platts t Intermountain Forest and Range Experiment Station Forest Service, U.S. Department of Agriculture Ogden, Utah 844011 ti ABSTRACT Studies were conducted from July 1970 through September 1972 of (1) the relationship of the physical structural components of aquatic environments to each other, (2) the relationship between stream structure and fish populations, (3) the importance of multiple variables in controlling fish populations, and (4) the validity of using present methodologies to evaluate fishery productiv- ity. A 397-square-mile area in the upper South Fork Salmon River watershed was evaluated for aquatic environment-fishery relationships by analyzing data from 2,482 transects in 38 streams for physical aquatic and streamside environments, with 291 areas for fishery conditions. Certain valid interpretations could be made concerning aquatic variable control of fish populations, but the overall observed variation was low. In-stream conditions controlled the density of fish populations and the compo- sition of fish species. Control was not isolated to any one variable. Stream depth, width, and the elevation of the stream channel were the most important evaluated variables controlling fish populations. INTRODUCTION Most aquatic-fishery methodologies now used to provide land, water, and fisheries managers with information and analysis for decisionmaking have been based mainly on opinion, intuitive thinking, and relationships of variables derived empirically. Seldom has any aquatic methodology been tested to determine if the data are reliable and provide the basis for a valid analysis. Managers often are apathetic to aquatic environment studies or use them ineffectively because (1) it is presently difficult to relate aquatic method- ologies to fishery resources, (2) the manager has a difficult time meshing these data and analyses with those of other disciplines, (3) the masses of data do not offer an analysis to fit decisionmaking needs. Difficulties arise in developing valid methodologies because of the prob- lems encountered in quantitatively describing the true state of an aquatic system. A stream is dynamic, changing from day to day and especially from year to year. Aquatic scientists usually collect data during the warmer months 'Aquatic ecologist located at the Intermountain Station's Forestry Sciences Laboratory, Boise, Idaho. 267 Purchased by USDA Forest service for official Use.