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I I I I I I I I I I I I I I I I I I I DISCUSSION Species composition, density and size structure for fish over 15 cm were very similar between years on the Yampa River. This indicates stable and consistent population parameters at both Sevens and Duffy between 1998 and 1999. Significant differences in species composition, density, and size were found between Duffy and Sevens in both years, 1998 and 1999. The differences identified between the two Yampa sites could be related to biological interactions, differences in macro habitat availability (temperature, water quality) or differences in meso-habitat availability (gradient, substrate panicle size, rime/run ratios). A lack offish under 30 em, higher mean lengths for vinually all species, and a much lower density estimate of only 410 ftsh/km at the Duffy site could suggest increased predatory pressures there compared to the Sevens site. DuffY had the higher density of large predator fish, northern pike, small mouth bass and Colorado pikeminnow. Unfortunately it is not possible to compare habitat availability between the Sevens and Duffy sites at this time. Channel catfish catch abundance was similar between both Yampa stations and the 15- Mile Reach. There were large differences found in habitat composition between Duffy and the 15- Mile reach. At Duffy the dominant habitat type was shallow runs at 49% and that habitat was , obselVed to be mostly unproductive in regards to adult fish. Shallow runs were only 5% of the habitat in the IS-mile reach. Another habitat type that is unproductive for adult fish is the wetted-sand pool or low velocity areas with depths less than 0.2 m. This habitat was 11% at DuffY at a flow of 600 cfs. The dominant habitat type in the IS-mile reach was rimes over 0.5m in depth. Areas of deep fast current comprised 64% of the surface area in the IS-mile reach study site. It is believed that the high amount of deep and fast habitat accounts for the high percentage (38%) 43