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6 <br />on a spawning migration although no data are available on migrations of this <br />? : -fish- species.-- If-suitable spawning habitat ism ,longer ravailable,:in the <br />Green River above the mouth of the Yampa, it will only be a matter of a few <br />years before the humpback sucker will disappear from this section of the Green <br />River unless those taken in Dinosaur National Monument have come from more <br />suitable environments elsewhere in the river system. <br />The-decreased growth-rate of-fishes observed during the 4 post-impoundment <br />years was most likely a direct result of the altered water temperature patterns <br />rather than of a food shortage since the quantity of invertebrate food organisms <br />present after closure of the dam appeared to be unchanged or increased. If <br />growth rate continues to decrease, mortality rates from predation can be ex- <br />pected to increase and age at sexual maturity will increase. <br />The study of invertebrates was seriously handicapped by the lack of tax- <br />onomic information necessary to enable groups to be identified to species. <br />Sampling techniques were such that several of the nine species not collected <br />in the present study but reported before impoundment would not have been <br />taken. The only significant change in species composition is the disappearance <br />e :_,._ _ -6f =a?stonef3y Clan"sepia, whit'h apparently was commoir-before 1962 and vn&'s r- of <br />found in the present study. If it were present, the sampling methods used <br />should have taken this insect. Little is known of its life history and eco= <br />logical requirements, but is is known to exist in many other waters in the <br />Intermountain States at the present time, notably in the Rocky Mountain Range <br />(Sessions and Gaufin, 1960). <br />CONCLUSIONS <br />The river environment changed in four major aspects after the the closure <br />.r_ ._ cif F`7airiirig°GOrge'?3at2:'l} sesonalr'iltietutio?is'nVist-harge,are` r?ec3cr;...ra; <br />i <br />daily fluctuations in discharge are increased; 3) summer water temperatures <br />are lower and winter water temperatures are higher; and 4) turbidity is re- <br />duced. These changes are the most extreme immediately below the dam. They <br />are moderated by the flow of the Yampa River entering the Green River at <br />Echo Park, 65 miles below Flaming Gorge Dam, but can be detected as far down- <br />stream-as Jensen, Utah, 118 miles below the dam. <br />Distribution of native fish species at present is closely related to the <br />resultant environmental changes effected by closure of Flaming Gorge Dam. <br />All species previously taken in pre-impoundment surveys from the dam site <br />downstream to Ouray, Utah were taken in the present study, but only below <br />the mouth of the Yampa River. Above this point, only the flannelmouth sucker <br />(Catostomus latipinnis), bluehead sucker (Pantosteus delphinus), speckled <br />dace (Rhinichthys osculus), and Colorado chub (Gila robusta) were taken and <br />were more common when discharge from the dam was reduced and water tempera- <br />tures approached pre-impoundment levels. One fish species currently considered <br />as endangered (Bureau of Sport Fisheries and Wildlife, 1966), the Colorado <br />squawfish (Ptychocheilus lucius).and another whose status is undetermined, the <br />humpback sucker (Xyrauchen texanus), were not found above the mouth of the <br />Yampa River in 1964-1966 but both species were common in the Green River above <br />this point in pre-impoundment years. The endangered humpback chub Gila cypha) <br />was not collected in the study area in pre-impoundment surveys. Three speci-