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<br />" <br /> <br />,. 00620 <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />any young-of-year rainbow trout in Nankoweep Creek during the April 2004 survey and saw <br />only limited spawning activity. <br /> <br />Relative to Glen Canyon, we found very few YoY rainbow trout between Lees Ferry <br />and the LCR confluence during the June and August 2004 surveys. Across gear types, <br />densities ofYoY rainbow trout in Glen Canyon were 12-fold higher than in Marble Canyon in <br />June and IS-fold higher in August. Densities ofYoY in Marble Canyon were highest in the <br />IS-mile reach immediately below Lees Ferry and increased between June and August in the <br />first 30 miles downstream of Lees Ferry. Length-at-age and hatch date distributions for YoY <br />trout caught in Glen and Marble Canyons were very similar. These data suggest that the <br />limited number of Yo Y caught in Marble Canyon in 2004 likely originated from Glen <br />Canyon. The reach between river mile 4S and 60 had the lowest densities ofYoY fish of all <br />the reaches in Marble Canyon with only 14 and 4 YoYs caught in June and August, <br />respectively. Although this reach had the greatest amount of spawning habitat in the mainstem <br />and a tributary that could support a substantial amount of spawning (Nankoweep Creek), these <br />habitats clearly did not produce a significant number of viable young in 2004. <br /> <br />If our surveys in Marble Canyon are representative of reproductive conditions for <br />rainbow trout in future years, it is unlikely that flows from GCD can be used to reduce the <br />survival rate of young trout in this reach. There was virtually no spawning habitat above 8 <br />kcfs that could potentially be dewatered. The limited area of spawning habitat that was <br />observed was well below S kcfs. The very low densities ofYoY observed in 2004 indicate <br />that recruitment to the Marble Canyon population is already quite low, thus the incremental <br />effect of destabilizing shoreline habitats would be very small. Our results cannot be used to <br />definitively determine the origin of the current population of rainbow trout in Marble Canyon. <br />It is possible that these fish originated in Marble Canyon and that the limited reproduction we <br />observed in 2004 was a unique occurrence. Continued documentation of Yo 'y absence in <br />, Marble Canyon over the next few years, combined with observations of a constant or <br />increasing adult population above the mechanical removal reach, would provide strong <br />evidence that this population is supported by downstream dispersal from Glen Canyon. <br />Alternatively, if downstream dispersal is not a significant process, and if the lack oflocal <br /> <br />v <br />