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Table 17. Lincoln-Peterson population estimates and associated 95% confidence intervals (CI) or <br />humpback and roundtail chubs at each of the three study sites. <br />Humpback 1993 1994 1995 1996 <br />Chub <br />RK N CI N CI N CI N CI <br />198.6 848 902 3216 6191 3618 6974 3564 6868 <br />194.4 NR* 1680 2242 5880 11374 1050 857 <br />192.0 NR 725 950 650 685 572 600 <br />Roundtait <br />Chub <br />RK N CI N CI N CI N CI <br />198.6 1760 1221 3131 6007 NR 721 416 <br />194.4 3831 4212 1210 895 2370 3188 1303 1509 <br />192.0 1218 1123 1392 1598 744 979 527 550 <br />~ NR - no recaptures <br />DISCUSSION <br />Reproduction and Eardy Life History <br />Hatching Dates /Spawning period <br />Hatching dates were positively correlated with pre-peak flows in excess of 6000 cfs and <br />negatively correlated with pre-peak degree days in excess of 10°C. The greater flow on the ascending <br />limb of the peak and the associated cooler temperatures delayed hatching times. The spawning period <br />typically commenced in June; early June during the low flow years 1992 and 1994, and late June in 1993. <br />In 1996, the most successful year of chub reproduction during the present study, spawning commenced <br />on 19 June in Westwater Canyon and continued steadily for a seven week period through O 1 August. <br />The few YOY collected in 1995 were apparently spawned between 27 July and OS August. These <br />hatching and spawning times were determined through back calculations. Researchers recently have <br />been ageing YOY through examination of otoliths to more accurately determine ahatch /spawn date. <br />The results of their work with pikeminnow (Bestgen et al. 1998 }and razorback sucker (Muth et al. <br />1998) often compress annual differences in spawning period by taking into account the environmental <br />conditions and resultant differing growth rates larvae /early juveniles experienced prior to capture. That <br />technique could be applied to the YOY chubs of Westwater to validate the findings of this study. <br />Humpback chub in the Green and Upper Colorado River Basin have previously been reported to <br />spawn in the spring and early summer during the descending limb of the hydrograph shortly after peak <br />flows (Valdez and Clemmer 1982; Valdez et al. 1982; Kaeding and Zimmerman 1983; Karp and Tyus <br />1990; Valdez and Ryel 1995). Tyus and Karp (1991) reported that humpback chub in Dinosaur National <br />Monument (Yampa and Green rivers) spawned at a water temperature of about 20°C. The findings of <br />this study generally agree with those of previous upper basin researchers. The overwhelming majority <br />(99.8%) of YOY chubs collected in this study were spawned on the descending limb at temperatures <br />ranging from 17-20°C. There was a notable exception to this trend. Seven of 24 YOY chubs collected <br />above the canyon on 25 June 93 at RK 201.1 were spawned prior to the annual peak. The largest YOY <br />20 <br />