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8/11/2009 11:34:00 AM
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UCREFRP
UCREFRP Catalog Number
7229
Author
Kaeding, L. R. and M. A. Zimmerman
Title
Life History and Ecology of the Humpback Chub in the Little Colorado and Colorado Rivers of the Grand Canyon
USFW Year
1983
USFW - Doc Type
Transactions of the American Fisheries Society
Copyright Material
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592 <br />KAEDING AND ZIMMERMAN <br />TASt,E 5.-Frequency (percent) of dorsallanal fin-ray counts for humpback chubs longer than 100 mm collected from <br />the Little Colorado River, confluence, and Colorado River. <br /> Dorsal/anal fin-ray counts <br />Reach N 9/10 10/10 9/9 9/11 10/11 8/10 10/9 11/10 <br /> Little Colorado <br />LC 2 168 81.0 7.1 7.1 1.8 0 1.8 1.2 0 <br />LC 3 296 80.4 7.1 7.4 2.0 2.0 0.7 0.3 0 <br />LC 4 57 70.2 17.5 7.0 1.8 1.8 1.8 0 0 <br /> Conjinence <br />C4 216 67.6 16.7 7.4 3.2 3.2 0.9 0.5 0.5 <br /> Colorado <br />C 1 2 50.0 0 50.0 0 0 0 0 0 <br />C2 8 87.5 0 12.5 0 0 0 0 0 <br />C3 l9 78.9 5.3 10.5 5.3 0 0 0 0 <br />C 5 42 69.0 11.9 14.3 2.4 2.4 0 0 0 <br />C6 5 100.0 0 0 0 0 0 0 0 <br />C 7 3 33.3 0 66.7 0 0 0 0 0 <br />Total 816 75.7 10.4 8.1 2.3 1.8 1.0 0.5 0.1 <br />tive physicochemical environments, and marked <br />differences in physicochemical characteristics <br />exist between the Little Colorado and the Col- <br />orado (Table 1; Fig. 2). Young humpback chubs <br />that move from the Little Colorado into the <br />cooler waters of the Colorado might form more <br />fin rays than do humpback chubs that remain <br />in the Little Colorado. Such relationships be- <br />tween temperature experienced by embryos and <br />early larvae and the development of fin rays <br />have been demonstrated for several fish species <br />(Blaxter 1969). However, the nearly complete <br />mortality of embryonic and larval humpback <br />chubs at the temperatures of the Colorado <br />(Hammon 1982) seems to us to preclude tem- <br />perature-induced ecotypic variation as a possi- <br />ble explanation for the observed differences in <br />ray counts. We believe that a more plausible <br />hypothesis is suggested by the work of Holden <br />and Stalnaker (1970). They found many Gila <br />from the Colorado of Glen Canyon, about 100 <br />km upstream from our study area, with mor- <br />phologies that integraded between the hump- <br />back chub and the bonytail. Holden and Stal- <br />naker speculated that the presence of these <br />intergrade forms is an indication of incompete <br />speciation and of subsequent interbreeding. The <br />high frequency of the 10/10 ray-count com- <br />bination-more characteristic of the bonytail <br />than of the humpback chub (Miller 1946; Hol- <br />den and Stalnaker 1970)-could indicate the <br />occurrence of some bonytail genes in the hump- <br />back chub population from the lower Little Col- <br />orado. <br />Analysis <br />The Little Colorado immediately upstream <br />of our study area was formerly a perennial <br />stream. Colorado squawfish, bonytail, and <br />roundtail chub are among the fishes that may <br />have then occurred in the river as far upstream <br />as Grand Falls, a barrier to upstream movement <br />about 120 km above the confluence (Miller 1963; <br />Smith et al. 19'79). Changing land-use practices <br />and water impoundments in the drainage led <br />to seasonal dewatering of this reach of the Little <br />Colorado River about the turn of the century <br />(Miller 1961), and thereby reduced the region <br />where perennial flow occurs to that of our study <br />area. Seasonal dewatering of the upstream reach <br />might have eliminated use of the Little Colo- <br />rado by species other than those tolerant of its <br />present physicochemical conditions. <br />The alteration of the Colorado River envi- <br />ronment that resulted from closure of Glen <br />Canyon Dam was too rapid for adaptation by <br />species unable to persist under the new condi- <br />tions. We believe that the humpback chub per- <br />sisted, whereas other endemic species were <br />eliminated, because a portion of the humpback <br />chub population spawned in the Little Colo- <br />rado. Because the modified Colorado River <br />thermal regimen is not entirely unlike that of <br />
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