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RESULTS FISHERY QUANTIFICATION To determine the role flows have on fish community structure it is necessary to quantify both the fishery and habitat availability and then to establish the relationship between habitat and flow. The fishery was characterized by three different attributes for each of the study sites: 1) species composition, 2) size distribution and 3) density and biomass estimation. Since a complete summary of the fish data was in the job progress report, Anderson (2002), not all fish data is presented in this report. In general, each annual progress report compares species composition, size and density data to the prior year's data. Length frequency histograms for each station sampled in 1998 and 1999 are available in Anderson and Stewart (2000). Histograms for the 2000 sample are given in Anderson and Stewart (2001) and histograms for the 2001 sampling are in Anderson (2002). Length data can also be obtained from the author in spreadsheet format. Seining data is presented in Anderson and Stewart (2000) and Anderson (2002). Data for the Yampa River was organized by two periods, the first two years (1998 and 1999) had above normal base flows, while base flows during 2000 and 2001 were well below normal. Differences in the fishery between these two periods probably were attributable to differences in base flows. At the Duffy station, fish population sampling was done in a 4.5-mile reach (Long Reach), but only the upper 1.8 miles of the long reach was modeled for habitat composition (habitat reach). The Long Reach fish data was used for making comparisons between years. Fish Over 15-cm in Length Native species composition was highly consistent between years at the Sevens station with native fish comprising 73% (four-year mean) of the fish over 15 cm. The four-year mean for flannehnouth-sucker composition at Sevens was 49% with a range of 46% to 53%. The four-year mean for bluehead sucker was 19% and it was 4% for roundtail chub (Table 5). Colorado pikeminnow were rare or absent at Sevens for the four-year period. The greatest inconsistency for a native species at Sevens was a drop in bluehead and a small increase in flannelmouth composition in the 2001 sample compared against the three prior years (Table 5). Native fish species were uncommon to rare at Duffy, averaging only 13% of fish over 15 cm for the four years. The data showed a downward trend in native fish at Duffy with highest native fish composition in 1998 (15%) and the lowest in 2001 (10%) (Table 5). Flannelmouth suckers were consistently at 5% in the first three years, but were only 2% in 2001. Bluehead suckers comprised about 4% and roundtail chub were 3% over the study period. Colorado pikeminnow were uncommon at Duffy. They comprised between 0.6 to 1.5% of the total catch. 34