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82 <br />actually less numerous in the population than the <br />collections indicated. <br />The occurrence of no hybrids and only one white <br />sucker at Station Y-4 indicated the relative lack of in- <br />fluence from either exotic species introduction or habitat <br />degradation. Cross Mountain Canyon, located 2.5 km up- <br />stream from Station Y-4, may impede up-and downstream move- <br />ments of fish, although it does not act as a barrier. Also, <br />abundance of white suckers was greater near population <br />centers, where likelihood of introduction as baitfish was <br />greater. Station Y-4 and the remainder of the lower Yampa <br />are rarely visited by fishermen. <br />Distribution of broad- and narrow-peduncled bluehead sucker <br />Polymorphism of the caudal peduncle in the bluehead <br />sucker has been noted by many authors. Smith (1966) noted <br />caudal peduncle/standard length ratios ranging from 5.4 to <br />9.1 percent within a large sample representing much of this <br />fish's distribution. He attributed some of the polymor- <br />phism in New Mexico specimens to a headwaters stream cap- <br />ture, introgression between the bluehead sucker and the <br />Rio Grande Mountain sucker (C. [P.] plebius), and subsequent <br />retention of characters. He concluded that subspecies <br />designation would not be of value at that time as a means <br />of more adequately describing or delineating the forms.