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<br />SUMMARY OF FINDINGS
<br />(Continued)
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<br />A new population of the endangered humpback chub was discovered in Cataract Canyon as a result
<br />of this investigation. This brings the total number of populations in the upper basin to five: Black Rocks,
<br />Westwater Canyon, Desolation/Gray Canyons, Yampa Canyon, and Cataract Canyon. The Cataract
<br />Canyon population was distributed from the confluence of the Green and Colorado Rivers downst.ream
<br />to the Lake Powell Inflow, a dist.ance of about 16 miles, although the population was concentrated in
<br />a 4-mile reach of whitewater. The population was composed of all age categories, but the adults were
<br />relatively small geomorphs, perhaps the remnant of a larger more extensive population that once
<br />inhabited the 41 miles of Cataract Canyon prior to the closure of Glen Canyon Dam in 1963.
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<br />A form of chub suspected to be the rare bonytail (Gila elegans) was found in Cataract Canyon. The
<br />14 specimens captured exhibited the complement of morphologic and merist.ic features that more closely
<br />resembled this species than the other two congeneric Colorado Rivier chubs, humpback chub (G.
<br />cypha) and roundtail chub (G. robusta). Since these specimens did not exhibit definitive characteristics
<br />of G. elegans, and since a detailed taxonomic study of these forms was outside the scope of this
<br />investigation, a peer examination of the morphologic, meristic, genetic, and cytogenetic characterist.ics
<br />of the chubs from Cataract Canyon is recommended. The existence of G. elegans in this region would
<br />make Cataract Canyon very important to the recovery of this rarest. of the upper basin fishes.
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<br />Following the record high water years of 1984, 1985, and 1986, densities of three non-native
<br />cyprinids (red shiner, sand shiner, fathead minnow) were lower than in the more normal water year of
<br />1987 and the low water year of 1988. These species showed a 3 to 4-fold increases in densities in a
<br />2-year period during the normal and low water years. This implies that periodic and temporary control
<br />measures (i.e., floods) on these quickly maturing and rapidly reproducing species would be ineffective
<br />to long-term control. The only effective control of these non-native species would be long-lasting and
<br />persistent measures. The reasons for depressed populations in high water years was attributed to the
<br />elimination of large backwaters that harbored these species, a general reduction in quiet-water habitats,
<br />and delayed warming that prevented multiple spawns. The low water years were believed to have
<br />benefited these species because large backwaters maintained their permanency, reduced velocities
<br />provided more quiet water habitats, and prolonged warm temperatures allowed for multiple spawns.
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<br />viii
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