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<br />380 <br /> <br />}, KORMAN, S, M. WIELE AND M, TOR1ZZO <br /> <br />In the Colorado River in Grand Canyon. nine aggregations of humpback chub have been idcntified, The Lillie <br />Colorado River (LCR)fmainstem aggregation is the largest with an estimated population size of 3500 adult chub <br />>200 mm total length (TL), A large portion of this aggregation moves into the LCR to spawn between March and <br />May and then returns to the mainslem in late June-July. Their distribution in the mainstem rnnges from river km 91 <br />to 110 (Figure I; Valdez and Ryel, 1995), Temperatures of water released from GCD are too low to support main- <br />stem spawning and are hypothesized to severely limit larval survival (Valdez and Ryel, 1995; Converse el al,. <br />1998), ~Iany humpback chub hatched in the LCR descend into the mainstem beginning in May (Valdez and Ryel, <br />1995; Robinson et af.. 1998), The proportion of fish leaving the LCR as larvae (< 15 mm TL), young-of-the-year <br />(yay; <90 mm TL), or juveniles (9l}-200 mm TL) relative to those that remain is unknown, but it is likely to be <br />highly variable and dependent on floods and fish densities in the LCR (Robinson e/ ai" 1998). Survival rates of <br />yay and juveniles that enter the main stem are difficult to estimate; Valdez and Ryel (1995) and Robinson et aI, <br />(1998) suggest that fish entering at sizes <52mm TL do not survive, <br />Ju\'enile humpback chub in Grand Canyon are highly aggregated, The majority of fish are found in the vicinity <br />of the LCR from its confluence at river km 99 downstream to river km 136 (Valdez and Ryel, 1995), Juveniles <br /> <br /> <br />N <br /> <br />+ <br /> <br /> <br />Colorado River <br /> <br />River Km 99 <br /> <br /> <br />Little Colorado River <br /> <br />R3 <br /> <br />R4 <br /> <br />R5 <br /> <br />o <br /> <br />1000 <br /> <br />1000 <br /> <br />2000 Meters <br />I <br /> <br /> <br />ALC <br /> <br />PAL <br /> <br />River Km 107- <br /> <br /> <br />Figur~ t. ~h.y of lhe Colorado River in Grand Canyon nc:ar ils confluence with the Lillie Colorado River showing study reach loCations. The <br />Q\'erall smdy localion is shown in the bOlt highlighted Wilh an asterisk within the inset map <br /> <br />Copyright -t. 2004 lohn Wiley & Sons. Ltd. <br /> <br />Rilfu Res_ Applic. 10; 319-400 (1004) <br />