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<br />were significantly lower than in the previous (0.91 and 0.95) and following intervals (0.87 and <br />0.90). This change may have been caused by the reduction in the daily minimum flow from 10 <br />to 5 kcfs in early-September. However, it is also possible that the decrease in survival we <br />estimated was a natural occurrence, possibly driven by a normal ontogenetic habitat shift <br />and/or density-dependent mortality. In steep habitats, weekly survival rates dropped from 0.90 <br />for the period spanning early-September to early-November, to 0.59 for the period between <br />November and December. This change was likely related to the 42.5 kcfs Beach Habitat- <br />Building Flow (BHBF), although it could also have been caused by a natural or flow-induced <br />habitat shift rather. Uncertainty in survival rate estimates were generally larger in low angle <br />habitats because of lower sample size. Hatch timing-patterns estimated from the model were <br />highly uncertain when survival rates were allowed to vary among sample periods. The <br />juvenile stock synthesis modeling approach we have developed represents a potential advance <br />in monitoring the response ofsalmonid populations to habitat alterations. However, we <br />caution that it does not replace the need to track trends in the adult population. Without this <br />supporting data, it will be uncertain whether a measured change in the mortality at an early <br />juvenile stage will be compensated by a subsequent change in density-dependent mortality. <br /> <br />Exposed gravels and cobbles above 8 kcfs were generally much too coarse and poorly <br />sorted to support spawning between Lees Ferry and the confluence with the Little Colorado <br />River. We observed a handful of very small areas (10-400 m2) that contained appropriate <br />grain sizes for spawning, however no redds were obseJVed at any of these locations. Due to <br />high turbidity we were unable to survey for spawning habitat or redds in Marble Canyon <br />below the daily minimum flow elevation of 8 kcfs on the April 2004 trip. In June 2004 water <br />clarity was adequate and suitable spawning substrate below 8 kcfs was found at a number of <br />locations in Marble Canyon, with the majority oflarger sites located between the confluence <br />ofNankoweep Creek and Kwagunt rapid (river miles 52 and 56, respectively). The vast <br />majority of these sites were located at the downstream end of rimes or small rapids at depths <br />of3-10 m at a discharge ofca. 9 kcfs. Nankoweep Creek was accessible to fish in the <br />Mainstem Colorado River in 2004 and could support at least a few thousand spawners <br />between January and early-February when water temperatures are suitable. We did not catch <br /> <br />IV <br /> <br />" <br /> <br />. <br /> <br />. <br /> <br />. <br />