Introduction Relations between physical habitat and fish abundance are the foundation for fisheries management in lotic systems. Stream habitat quality and quantity are known to influence the population size, species composition, and size structure of fish in streams (Chapman 1966; Binns and Eisermann 1979; Scarnecchia and Bergersen 1986, 1987; Wesche et al. 1987a,b). Identifying fish habitat requirements is therefore a prerequisite to mitigating damage to fish populations from water development activities.
Water diversion can have a major impact on stream habitat by reducing flows which in turn reduce depths, modify velocities and decrease overall stream volume (Bovee 1982; Deacon 1988). These changes can then alter the microhabitats available to various life-stages of fish species. Long term impacts of reduced flows are less well understood. Stream channel morphology may change (Wesche et al. 1985) leading to an accumulation of fine sediments in spawning gravels and interstitial crevices used by young fish. Thermal regimes may become altered, and species abundance or composition may ultimately change.
The North Fork Little Snake River in south central Wyoming is presently being affected by water diversion as a result of the Cheyenne Stage II water diversion project. The project diverts 23,00 acre-feet of water from the headwater streams in the drainage at an elevation of 2621 meters, transports it to the east slope of the Continental Divide and discharges it to Hog Park Reservoir (U.S.D.A. Forest Service, 1981).
Minimum stream flows were established in 1979 by the U.S. Forest Service (Jespersen 1979, 1980) to protect the native Colorado River cutthroat: trout (Oncorhynchus clarki pleuriticus). The species is considered sensitive by the Wyoming Game and Fish Department because its original distribution throughout most of the Colorado River headwater streams has been reduced to several isolated populations in Wyoming. Protecting the cutthroat trout in the North Fork Little Snake River drainage is particularly important because these populations are among the genetically purest Colorado River cutthroat trout remaining and are the source of broodstock for recovery efforts (Wyoming Game and Fish Department 1987).
To assess the immediate effects of reduced flows from water diversion projects on fish populations, the United States Fish and Wildlife Service has developed the Instream Flow Incremental Methodology (IFIM) (Stalnaker 1979; Bovee 1982; Reiser et al. 1989). This assessment methodology uses habitat curves that relate the suitability of a measured habitat variable to the observed optimal conditions for that species (Pajak and Neves 1987). The habitat suitability index (HSI) is a number between 0 and 1 used to rate overall habitat quality and when multiplied by the area of stream under consideration, it yields the total habitat units for the species being considered. Changes in these habitat units are then compared under different flow regimes to determine potential impacts to the fish population using the Physical Habitat Simulation model (PHABSIM). Use of the model assumes that standing stock is proportional to the weighted usable area (WUA) in the stream and that reduced WUA results in a proportionately reduced fish population. PHABSIM is considered to be the best tool available to assess flow-related impacts to fish populations particularly in cold water streams (Orth 1987; Gore and Nestler 1988).
While minimum flows have been set to protect populations of Colorado River cutthroat trout, little is known about the habitat requirements of the Colorado River cutthroat trout and in particular, newly-hatched stages (Hickman and Raleigh 1982). The objectives of this study were to:
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