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weighted usable areas for selected fish species at unmeasured discharges, using "electivity" curves (curves which graphically express the suspected preferences of the species for parameters such as depth, velocity, substrate and temperature. If some concensus of opinion regarding the preferences of adult,juvenile and larval squawfish or humpback chubs is reached in the near future, the present data base will be easily input, and the various trend zones compared by the weighted usable areas at a normal range of flows. Fish Collection A total of 18,772 fish were processed in 130 collections made at 41 sites throughout the five trend zones and at special sites. A summary of total numbers and percentage composition of fishes collected on all trend zones is found on Table 6. Tables 7-12 provide total numbers and percentage composition of fish species collected at each sampling site (all habitats combined). Table 8 summarizes the special sites on the Yampa River. Tables 13-17 provide percentage composition of fish species collected within generalized habitats for each trend zone (all sampling sites combined). In all tables, the fish are categorized as less than or equal to 20 mm TL or greater than 20 mm TL. The presence of fishes in the former size category at that time of the year indicates probable spawning within approximately five weeks prior to capture. No threatened or endangered fishes were collected on scheduled sampling trips. However, a Gila c ha x robusta. hybrid was caught.by Per. Charles Prewitt while fishing on the Colorado River in the Black Rocks area of Ruby Canyon in August 1977. The fish was caught by hook and line (grasshopper as bait) behind a large boulder (depth, 1 m; velocity, 1.8 ft/sec; substrate, cobble). A slide of the fish is included in Appendix C. Also, prior to the sampling reported herein, during April through July 1977, Colorado squaw- fish were captured at three trend zone sites and one special collection site.