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<br />/ <br /> <br />~ <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />OO""~ <br />Oi.\J <br /> <br />Executive Summary <br /> <br />An experimental alteration of the hydrograph from Glen Canyon Dam (GCD), targeted <br />at reducing the survival rate of young rainbow trout through increased daily fluctuations in <br />flow, was implemented from January through March in 2003 and 2004. This report describes <br />the impact of the experimental flow regime on the early life stages of rainbow trout below <br />Glen Canyon Dam. The study consisted of five components. We measured the timing ofredd <br />excavation and the distribution of redds across elevations (i.e., redd hypsometry) in Glen <br />Canyon to estimate the potential egg and alevin mortality caused by the experimental flow <br />regime. We quantified the relationships between spawning habitat preference and depth, <br />velocity, and substrate in Glen Canyon to evaluate the feasibility of controlling spawning <br />elevations through changes in discharge. We obtained monthly samples of young-of-year <br />(YoY) from Glen Canyon to compare length-frequencies over time and among habitat types to <br />make qualitative inferences regarding the seasonal timing of hatch, and YoY survival, growth, <br />and movement among habitat types. We analyzed the microstructure of otoliths from a <br />subsample of fish to establish length-age relationships and evaluate the effects of dam <br />operations on YoY growth. These data were used in a stock synthesis model to estimate <br />seasonal trends in the number of rainbow trout emerging from the gravel, and their subsequent <br />survival rates and movement among habitat types. Finally, we conducted rainbow trout <br />spawning habitat, redd, and fry surveys from Lees Ferry to the confluence of the Little <br />Colorado River (LCR) to evaluate the extent of natural reproduction in this reach. <br /> <br />There was minimal spawning prior to mid-January in both 2003 and 2004 and peak <br />counts ofapproximately 1,000 redds were obtained by late-March/early-ApriI. We estimated <br />that 4,000 and 2,100 redds were excavated in 2003 and 2004, respectively. The average <br />percentage ofredds above 12, 8, and 5 kcfs at high elevation spawning sites was 27%,54%, <br />and 82%, respectively. The system-wide redd survey documented a total of 27 spawning <br />locations in the Glen Canyon with the majority ofredds located at elevations below 8 kcfs. <br />Intergravel water temperatures at Four Mile and Powerline Bars increased with elevation and <br />exceeded the lethal egg incubation limit of 16 C by mid-March at higher elevations. Estimates <br />of the percentage ofredds that did not produce viable young for Glen Canyon were 23% and <br /> <br />1 <br />