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[\11"- ,:'[0- rC- :cd [st lef ,,1- m- ,lie 'he ,eS ,liS ~e- :n- IS ,ld ,I'e 'n- ,he ky iy .nl n ,'W :2. ',Iy 'tiS ';Il ",I "[l ,If .s "11 'lJ~ 1I ,m Ii ',d 'it: ,'r, ,re ,!- k .[e '\- :r 'a~ "r -~ HOLDEN AND STALNAKER--COLORADO RIVER FISHES 229 Green River-areas 4 and 5--now harbor these forms. Reproduction of the rare fishes was evident only in these areas. Therefore, any alteration of the Green River system below Flaming Gorge Dam will undoubtedly be harmful to these rare fishes, acutely so in the Yampa River and Desolation Canyon. The reasons for the decline in native fish abundance are numerous. The most obvious factor is high dams with their re- sultant reservoirs and cold tail waters. Neither reservoirs nor tail waters provide suitable habitat for the reproduction of en- demic species. Dams have been described as the major decimating factor for fish in the lower Colorado basin (Minckley and Deacon 1968) and the 104 km below Bam- ing Gorge Dam (Vanicek, Kramer, and Franklin 1970). The present study indicates the Colorado River of Glen, ~1.arble, and Grand Canyons should be added to this lis 1. Lowered temperatures of the Colorado River in Glen, Marble, and Grand Canyons as a result of cold discharge from Glen Canyon Dam, and in conjunction with little solar warmirlg because of high canyon walls and no large, warm tributaries, have made it unfavorable for most native fishes. Spawning temperatures, especially for rare forms, seldom occur. A daily 120--150 cm fluctuation in river level precludes the availability of warm, rich backwaters pre- ferred by juvenile fish in the upper basin. Secondary production appears very limited in the main river; few aquatic insects arc seen in, what appears to be, a very sterile environment. It is probable that members of the humpback chub complex collected here were remnants of small populations existing heforc dam closure. It is likely that other adult "large-river" species exist, but once they are gone no young will be available to repopulate the area. Perhaps the decrease in chubs in area 11 between 1967 and 1970 was indieative of this change. Only those native species adapted 10 tributary streams (speckled dace, 1,Iuehead and flannelrnouth suckers) are likely to survive. Native fish abundances In the Green River of Dinosaur National Monument below the mouth of the Yampa River has changed since 1966. This study has shown a decrease in young Colorado squawfish and adult bony tails chubs from that re- ported by Vanicek, Kramer, and Franklin (1970). We believe the reason for this is an unnatural deerease in mid-summer river temperature, especially since 1965, due to Flaming Gorge Dam, but this requires further documentation. Another reason for the decline in en- demic fish numbers in the upper basin is competition from introduced species, as has been documented in the lower basin. The Colorado squawfish and humpback sucker have bccome rare during the first half of the 20th Century. During this same period the channel catfish became well es- tablished. The addition of an abundant carnivore to the fish fauna has created ad- ditional competition for space and food. More recent introduction of now abundant small cyprinids (specifically the red sbiner) have created rnore competition for space and food with juvenile fish. This hypothe- sis may partly account for the lower abun- dance of endemic fishes in the Colorado River as compared with similar areas in the Green River. 1\lost introductions in the upper basin apparently have occurred in the Colorado River, probably from released baitfish. The future does not look bright for the Colorado Hiver and its endemic fIshes. Increased human population and use of Colorado system water will undoubtedly continue to deteriorate the quality of the environment. Only a small portion of the system now retains the requirements for successful populations of indigenous fishes. Little additional alteration may be all that is needed to push these species to extinc- tion. Perhaps it is the Colorado River envi- ronment whieh should be considered "Endangered. " ACKNOWLEDG\lENTS The authors are especially indebted to the late Robert H. Kramer who providc\!