Four general categories of instream flow methods were evaluated to determine their biases relative to each other. The categories included (1) the Termant method, (2) wetted perimeter curves, (3) habitat retention models, and (4) physical habitat simulation (PHABSIM) models.The Tennant method (30% of average flow) was one of the least biased methods, but it does not include biological data and is incapable of identifying trade-offs. No wetted perimeter methods were significantly unbiased, and methods relying on subjective identification of inflection points were biased upwards. Two habitat retention methods were significantly unbiased. These methods included (1) the mean recommendation of all riffles in a study reach where all three criteria are met, and (2) the recommendation for the single most "critical" riffle in a study reach where two of the three criteria are met. No PHABSIM models were unbiased. The IFG-4 model was biased upwards for small streams and low for large streams.

An assessment was made of the biological validity of weighted usable area (WUA) from the physical habitat simulation (PHABSIM) model based on standing crops of trout (Salvelinus and Salmo spp.) measured in Wyoming streams and standing crops predicted by the habitat quality index (HQI). Tests were made in trout streams for (1) validity of the HQI, (2) relationships between WUA and measured standing crops in different streams, and (3) relationships between WUA and the HQI within streams. Significant correlation (r = 0.934; P < 0.05) was found between HQI scores and trout standing crop during the low-flow period. No significant correlation was found for WUA and the measured standing crop among different streams; correlation coefficients for all tests were either near zero or moderately negative. Significant positive correlations (P < 0.05) did exist for 7 of the 60 within-stream analyses of WUA versus HQI; 19 other positive correlations were strong (r > 0.90), but statistical significance was limited by the number of data points at each site. Although positive correlations were expected for all 60 cases, 18 tests showed a negative correlation, 3 of which were significant. Analyses indicated that trout species, stream size, and stream gradient influence the validity of the within-stream relationship between WUA and the trout standing crop predicted by the HQI. Among test streams with steeper gradients and where velocity exerted the greatest influence on the HQI score, a positive correlation was observed in all cases, regardless of stream size or dominant species. When an attribute other than velocity has the greatest influence on trout density with change in discharge, WUA estimates may be invalid. This observation indicates that a relationship between WUA and trout standing crop may exist, but the nature of the relationship is likely to be unique for each stream. A variety of methods is available to establishinstream flow recommendations for fisheries American Fisheries Society 19:35-43. (Colorado State University, Fort Collins.) Orth, D., and O. Maughan. 1982. Evaluation of the incremental methodology for recommending instream flows for fisheries. Transactions of the American Fisheries Society 111:413-445. Shitveil, C., and R. Dungey. 1983. Micro-habitats chosen by brown trout for feeding and spawning in rivers. Transactions of the American Fisheries Society 112:355-367. Shitveil, C., and D. Morantz. 1983. Assessment of the instream flow incremental methodology for Atlantic salmon in Nova Scotia. Transactions of the Canadian Electrical Association, Engineering and Operating Division 22:83-H-108. Stalnaker, C. 1979. The use of habitat structure preferenda for establishing flow regimes necessary for maintenance of fish habitat. Pages 321-337 in V. Ward and V. Stanford, editors. The ecology of regulated streams. Plenum, New York. U.S. Department of the Interior. 1979. Instream flow guidelines. Bureau of Land Management Washington Office Instruction Memorandum (WO-78-534), . 1981. Response of fish and fish-food organis...