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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 />