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PROBABILITY-OF-USE CURVES FOR KOKANEE SPA WNlNG IN TWO TRIBUTARIES TO FLAMING GORGE RESERVOIR, WYOMING-UTAH: COMPARISON TO PREVIOUS CURVES AND AVAILABLE HABITAT BRADFORD G. PARSONS AND WAYNE A. HUBERT WYOMING COOPERATIVE FISH & WILDLIFE RESEARCH UNIT UNNERSITY OF WYOMING LARAMIE, WYOMING 82071 ABSTRACT Probability-of-use curves were developed for nose current velocity, water depth, and substrate for kokanee (Oncorhynchus nerka) spawning in the Green River, Wyoming, and Sheep Creek, Utah. The Green River was a large river (discharge 850 cfs during the 1985 study period), while Sheep Creek was smaller (discharge 55 cfs in 1985) with a higher gradient. The Green River and Sheep Creek nose velocity probability-of-use curves showed optimums of 2.4 and 2.0 ft/sec, respectively, with ranges of 0.8-3.7 ftlsec and 1.2-3.9 ft/sec, respectively. For water depth, the Green River curve had an optimum of 1.1 feet, range 0.5.;.2.3 feet, while the Sheep Creek curve had an optimum of 0.8 feet, range 0.4-2.4 feet. The substrate curves for both streams center around gravel. The nose velocity and depth curves differed between the Green River and Sheep Creek, and both were different from probability-of-use curves previously developed by the U.S. Fish and Wildlife Service Instream Flow Group. Differences in available habitat between streams is discussed as the most probable cause. . IN1RODUCTION With the effort to get the instream flow initiative on the ballot in Wyoming, instream flow has become a greatly discussed issue. Loosely defmed, instream flow requirements refer to that amount of water flowing through a natural stream course needed to sustain instream values for fish and wildlife at an acceptable level (Bayha 1978). The Instream Flow Group (IFG) of the U.S. Fish and Wildlife Service Western Energy and Land Use Team in Fort Collins, Colorado, has developed the Instream Flow Incremental Methodology (IFIM) to generate values of habitat area for fish to help make decisions about necessary streamflows (Bovee 1982). A critical component of the IFIM is the probability-of-use curve (Bovee and Cochnauer 1977). The authors of the probability-of-use concept (Bovee and Cochnauer 1977) described the basis of the curves as the assumption that individuals of a species will tend to select areas within a stream having the most favorable combination of hydraulic conditions, termed the optimum. It was further assumed that individuals of a species will also utilize areas with less favorable conditions, with the probability-of-use decreasing with diminishing favorability of one or several hydraulic conditions (Bovee and Cochnauer 1977). The focus of this paper was to compare probability-of-use curves for current velocity, water depth, and substrate in two different spawing streams to Flaming Gorge Reservoir for kokanee (Oncorhynchus nerka). The questions addressed were: did the curves developed differ between streams, and if so, why? 2Lj.