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Y - Y <br />of the catostomid sample in disturbed areas. Neither white <br />suckers nor their hybrids occurred in the White River. <br />Flannelmouth X bluehead suckers were quite rare but <br />occurred in both rivers. <br />Bluehead sucker caudal peduncle/standard length <br />ratios diminished at successive downstream areas of the <br />Yampa River. No narrow-pe&incled types were found in the <br />upstream areas, and only one broad-peduncled type was found <br />at the downstream areas. <br />Hybrid sucker intermediacy supported the theory that <br />aril hybrid suckers found in both rivers were non-reproductive. <br />High numbers of h;:brid suckers in the tipper Yampa River were <br />probably due to recent environmental disturbances and pre- <br />sc-nce of the introduced white sucker, causing losses of re- <br />productive isolation. <br />Polymorphism of bluehead sucker populations was <br />attributed to morphologic adaptations to swifter and more <br />unavoidable current in the downstream areas. Distinct popu- <br />lations of bluehead suckers at the various Yampa River <br />stations have resulted from isolation within areas exhibitin; <br />progressii-ely higher water velocities. Theories of develop- <br />mental control of the. polymorphism were discounted. <br />Charles G. Prewi.tt <br />Department of Fishery and <br />Wildlife Biology <br />Colorado State University <br />Fort Collins, Colorado 80523 <br />June, 1977 <br />IV