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t t t educated guesswork (i.e., an expert believes that velocities ranging 1.0 to 8.0 ft/sec are necessary for successful spawning of striped bass). Most category one curves are the result of a combination of sources; the final curve may include information from the literature, combined with field data, and smoothed or modifi ed usi ng profess i ona 1 judgement. Category one curves usually are intended to reflect general habitat suitability throughout the ent ire geographi c range of the speci es and throughout the year, un 1 ess they are identified as being applicable only to a given area or season. In the latter case, curves developed for a specific area or stream may not accurately refl ect habi tat uti 1 i zat ion in other areas. Curves meant to descri be the genera 1 habitat suitabil i ty of a va ri ab 1 e throughout the entire range of a species may not be as sensitive to small changes of the variable within a specific stream (i.e., rainbow trout will generally utilize silt, sand, gravel, and cobble for spawning substrate, but utilize only cobble in Willow Creek, Colorado). Category two curves are derived from frequency analyses of field data, and are bas i ca lly curves fi t to a frequency hi stogram. Each curve descri bes the observed utilization of a habitat variable by a life stage. Category two curves unaltered by professional judgment or other sources of information are referred to as utilization curves. When modified by judgment they then become category one curves. Utilization curves from one set of data are not applicable for all streams and situations (i.e., a depth utilization curve from a shallow stream cannot be used for the Missouri River). Category two curves, therefore, are usually biased because of limited habitat availability. An ideal study stream woul d have all substrate and cover types present in equal amounts; all depth, velocity, and percent cover intervals available in equal proportions; and all combinations of all variables in equal proportions. Utilization curves from such a perfectly designed study theoretically should be transferable to any stream within the geographical range of the species. Curves from streams with high habitat diversity, then, are generally more transferable than curves from streams with low habitat diversity. Users of a category two curve should first review the stream description to see if condi- tions are similar to those present in the stream segment to be investigated. Some variables to consider might include stream width, depth, discharge, gradient, elevation, latitude and longitude, temperature, water quality, substrate and cover diversity, fish species associations, and data collection descriptors (time of day, season of year, sample size, sampling methods). If one or more deviate significantly from those of the proposed study site, then curve transference is not advised, and the investigator should develop his own curves. Category three curves are derived from utilization curves which have been corrected for envi ronmenta 1 bi as and therefore represent preference of the species. To generate a preference curve, one must simultaneously collect habitat utilization data and habitat availability data from the same area. Habitat availability should reflect the relative amount of different habitat types in the same proporti ons as they exi st throughout in the stream-study area. A curve is then developed for the habitat frequency distribution in the same way as for fish utilization observations, and the equation coefficients of the availability curve are subtracted from the equation coefficients of the 39 , , , , , I il 'I