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<br />18 . Executive Summary <br /> <br />fish of these species made up a relatively small <br />proportion of their populations. Of total numbers of <br />humpback chub (6,294), tlannelmouth suckers <br />(2,775), and bluehead suckers (1,040), sub adults <br />(YOY and juveniles) composed 72%, 21%, and <br />34%, respectively, indicating that reproductive <br />success of other native species was considerably less <br />than that of humpback chub. These species <br />appeared to be reproducing primarily in tributaries, <br />but survival of larvae drifting into the cold <br />mainstem is probably low from thermal shock, and <br />predation on survivors is probably high. <br /> <br />LIFE HISTORY SCHEDULE <br /> <br />The life histOlY schedule of the LCRI aggregation is <br />depicted in Fig. 16, based on observations during <br />1990-93. Adults between RM 57.0 and RM 65.4 <br />were typically fOlUld in or near large eddy <br />complexes from about July through January. In <br />February and March, adults congregated locally in <br />a few large eddy complexes before moving to stage <br />at the LCR inflow. Adults staged primarily in <br />March, April, and May, with individuals remaining <br /> <br />Final Report <br /> <br />in the LCR inflow an average of 17 days and <br />ascending primarily when flows in the LCR were <br />decreasing, clearing, and warming. Staging and <br />ascent into the LCR, and presumed spawning, <br />occwred primarily over a period of about 3 months <br />(i.e., April, May, June). Adults remained in the <br />LCR from March through June, returning during <br />and after this period to mainstem eddy complexes, <br />often within 2 km from their original location before <br />the spawning movement. Eggs deposited in the <br />LCR at about 220C probably hatched in about 5 <br />days (Hamman 1982). <br /> <br />Large numbers of young were seen in the mainstem, <br />primarily downstream of the LCR inflow, during <br />and immediately after heavy summer rainstorms in <br />the LCR drainage. The timing of these "monsoon <br />rains" determined the appearance of these YOlUlg <br />chubs in the mainstem, indicating that dispersal was <br />concurrent with LCR floods; floods occurred in <br />September 1991, May 1992, and July 1993. Large <br />numbers of sub adult humpback chub descended <br />from the LCR into the mainstem in September 1991 <br />and May 1992, concurrent with LCR floods, but in <br /> <br />Life History Schedule for Humpback Chub <br />Colorado River in Grand Canyon <br /> <br />Fish Dispersed <br />in Main Channel <br /> <br /> <br />Pre-spawning <br />Aggregations <br /> <br />V///1 <br /> <br />Stage at <br />LCR Inflow <br /> <br /> <br />V////1 <br /> <br /> <br />Return to <br />Main Channel <br /> <br />YOy\JUV <br />Leave LCR <br /> <br />V///////~ <br /> <br />V////1 <br /> <br />V////1 <br /> <br />o <br /> <br /> <br />N <br /> <br />Fig. 16. Life history schedule for the LCRI aggregation of humpback chub in the mainstem Colorado River in <br />Grand Canyon. <br />