Laserfiche WebLink
<br />33% in 2003 and 2004, respectively. Mortality in 2004 was higher because of the <br />implementation ofa daytime Sunday steady flow of8 kcfs between January and March. <br />Under nonnal Record of Decision (ROD) operations from January to March with a similar <br />total volume released from Glen Canyon Dam (OCD) to the volumes in 2003 and 2004, the <br />model predicted a redd loss of 19% in January and 33% from February to March when the <br />majority of spawning occurs. Thus, there was likely very little additional incubation mortality <br />associated with the higher experimental fluctuations in January to March of 2003 and 2004. <br />We estimated that between 1988 and 1991, when daytime low flows during the spawning and <br />incubation period averaged 1-3 kcfs, total redd loss likely exceeded 75%. We predicted that <br />redd loss rates could be increased to over 50% if a daytime Sunday steady flow of 5 kcfs was <br />implemented. <br /> <br />The spawning habitat preference models we developed for rainbow trout in Glen <br />Canyon were useful for evaluating the extent to which increased discharge during the January <br />to March experimental flow period altered the elevations where spawning occurred. Depths <br />of 0.5 - 1.5 m, velocities of 0.3 - 1 m/sec, and 085 values of 15-45 mm were preferred. <br />Weighted useable area computations showed that higher discharges increased total spawning <br />habitat availability at sites that had spawning habitat located at higher stages such as Four <br />Mile and Powerline Bars, and reduced spawning habitat availability at deep-water redd sites <br />such as Ferry Swale. The model also showed that the stages of preferred suitable spawning <br />habitat at Four Mile and Powerline Bars were increased under higher discharges. Such <br />changes in spawning habitat availability would increase the proportion ofredds that would be <br />dessicated and increase the duration of exposure. The redd hypsometry study showed that <br />there was a significant proportion of redds excavated in deep-water that would not be <br />dewatered at flows as low as 5 kcfs. The large decline in spawning habitat availability at Ferry <br />Swale under high discharge suggests that spawning at deep-water sites could be suppressed <br />through maintenance of high flows through the entire spawning period; however, this <br />conclusion needs to be validated by direct field observations. This uncertainty is important as <br />40-50% of the redds in Glen Canyon were located below 5 kcfs. <br /> <br />II <br /> <br />" <br /> <br />. <br /> <br />. <br /> <br />. <br />