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<br />Using a Schnabel population estimate, the number of COlorado pikeminnow <br />between RK 136.6 and 119.2 for the period between June 1991 and october 1995 <br />was estimated at 19 fish (based on 15 sampling trips). Ninety-five percent <br />confidence intervals place the range of this population at 10-42 COlorado <br />pikeminnow. Petersen index population estimates for the same time period and" <br />group of river miles place the Colorado pikeminnow population between 9 and 20 <br />fish (based on the 14 sampling intervals). ' <br />Five major instream diversion structures in the san Juan River between RK <br />178.5 and 142.0 impede fish movements to varying degrees. Of thsse <br />diversions, the San Juan Generating station (PNM) Weir is probably the <br />greatest barrier to native fish movements. However, the PNM Weir also appears <br />to be inhibiting the upstream spread of channel catfish. <br />Walleye and striped bass invaded the San Juan River from Lake Powell in <br />1995 when rising lake levels inundated the waterfall at RK 0.0. These species <br />have been documented to prey upon native flann~lmouth sucker. The presence of <br />these aggressive lacustrine predators coupled with the already large numbers <br />of nonnative fish species that compete with and prey upon native species, <br />complicates recovery efforts for endangered fishes in the San Juan River. <br />80th endangered Colorado pikeminnow and razorback sucker have been <br />collected within the San Juan River of Lake Powell and the lake influence zone <br />in the San Juan River. Both of these species have been documented moving <br />upstream from Lake Powell into the San Juan River when the waterfall at RK 0.0 <br />was not present. so, while the waterfall protected native fishes in the San <br />Juan River from predatory lacustrine fishes, it also kept rare fish species in <br />Lake Powell from communicating with ,riverine populations. <br /> <br />iv <br />