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<br />;' <br /> <br />(. 0 Jo _~ J <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />Seasonal changes in length frequencies ofYoY in Glen Canyon showed effects of <br />hatch timing, growth, survival variation, and movement from low to steep angle shorelines. <br />Substantial decreases in density following the early-September reduction in the minimum <br />daily flow from 10 to 5 kcfs were observed in both 2003 and 2004 and a very big drop in <br />density in steep angle habitats following the November 2004, 42.5 kcfs beach habitat building <br />flow was also seen. Catch rates obtained at the minimum daily flow were 3- to 5-fold higher <br />compared to those during the daily maximum and these differences were statistically <br />significant. A weekly striping pattern was evident in at least 51 % of the 255 otoliths examined <br />in 2003 but in only 5% of the 334 otoliths in 2004. The atypical weekly increment was 25% <br />wider compared to the other increments and indicated enhanced growth during Sunday steady <br />flow periods. There was little weekly striping in 2004 because daytime flows were not steady <br />on Sundays during the summer of 2004. Age determinations based on analysis of otolith <br />microstructure were made from 237 and 318 fish in 2003 and 2004, respectively. Variation in <br />length-at-age was very low with logistic growth models predicting 86-87% of the variation in <br />forklength as a function of the number of days from hatch. Yo Y in steeper habitats were <br />significantly larger at age than those in low angle habitats for fish that were at least 3 months <br />old. Hatch date distributions for the total YoY catch in 2003 (n=966) and 2004 (n=4,647) <br />were computed by length back-calculation. The correspondence between the back-calculated <br />hatch date distributions and those inferred from redd counts was very strong, indicating that <br />there was limited variation in mortality rates over the incubation period. The observation that <br />Yo Y generally remain at the daily minimum flow elevation, and the post-September density <br />reductions documented in Glen Canyon, coupled with the substantial literature on stranding <br />impacts, support the need to evaluate a 'stranding' flow operation from GCD targeted at <br />reducing Yo Y recruitment. <br /> <br />Estimates ofYoY weekly survival rate from the stock synthesis model in both low and <br />steep angle habitats were approximately 0.85 and were not dependent on the form of the hatch <br />timing distribution. The constant and variable survival rate models provided good fits to the <br />length frequency data and the improved fit of the latter models was useful in untangling <br />recruitment and survival effects in the length frequency data. Survival rates for the period <br />between the August and September samples for low and steep angle habitats of 0.84-0.85 <br /> <br />III <br />