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I I :1 I I I I I I I I I I I I I I I I METHODS Fish Samoles Fish were electro-shocked and netted from an Achilles raft between 1998 and 2000. The self-bailer rafter was equipped for electrofishing in 2001 using the same Smith-Root electro- fisher, 5000-watt generator and anode array mounted on a forward boom as in the three earlier years. The boat was maneuvered by either oars or by a battery powered 40 pound trolling motor. Two netters caught as many fish as they could while the shocker was in operation. All fish were measured to the nearest millimeter. Only fish over 150 mm were marked and therefore used for mark and recapture population estimates. Density estimates were made for the each study site on the Yampa, Colorado and Dolores Rivers. The Darroch multiple mark method (Everhart and Youngs 1981) was used to make the population estimate with ninety-five percent confidence intervals. An estimate was made for the total fish collectively and for each species. Recapture rates generally vary between species and size-groups. In this study, larger suckers had highest recapture probabilities and species with appreciably lower recapture probabilities were catfish, bass, pike and carp (the lower group). The total fish estimate blends recapture rates, but should produce a reliable comparison of fish abundance at a given station between years, when species and size composition was consistent. For rare species (pikeminnow, etc) with zero or one recapture in the sample, abundance was estimated by dividing the number in the sample by the mean recapture probability of the lower group. The z-test with an alpha of 0.05 (z = 1.96) was used to test for significant differences in density estimates between years at each station. At stations with three or more years of sampling, the Bonferroni inequality was also used to control the overall significance level (.05) for the simultaneous comparison of all pairs of years (Dr. David Bowden, CSU, pers. communication). At stations with 4 years of data, (Duffy and Sevens) the z value (2.631) corresponds to an alpha of 0.05 divided by six (0.0083). The fish data was further summarized into sampling sub-units referred to as polygons for future habitat suitability analysis. A polygon refers to a specific section of river and could vary in size, but typically a polygon is 25m to Sam in length depending on habitat homogeneity. Each polygon has a set of fish attribute data, which allows for a qualitative assessment of species composition and relative abundance within subsets of the study area. Following completion of hydraulic modeling, the physical attributes of each polygon will be determined for the analysis between fish and habitat characteristics. On the Yampa River, a different mark was used for each run-riffle sequence, which allowed for determining if recaptured fish had moved up, down or had not moved between captures. Fish sampling was earlier on the Yampa in 2001 than the previous year. Flows were again very low in the 2001 field season, but the Achilles shocking boat was replaced with a self- bailer Hyside raft which made sampling at low flows somewhat easier. Flows below 120 cfs were highly problematic for the Achilles boat since the boat had to be frequently dragged, and this became the case for the Hyside at flows less than 100 cfs. 11