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, I: Z'C c'o: ;I~ '0 iu I I I , I )( 1 r----l :Wyo"""' : .---1 1 j :--r-- -- \ ; Uta h i Colo1'odo:\ I : :-~._..l.-...___.......l ~ j I '-l I ..-.J ,..... ! "0 Ililo......n 20 --'1 I L__.j L-__________. -.--------~-------.~ FIGURE 1. Dinosaur National Monument study area. (Salmo gairdneri) (Vanicek, Kramer, and Franklin 1970) . Between 1968 and 1972, the Utah Cooperative Fishery Research Unit assessed the distribution and relative abundance of fish throughout the upper Colorado and Green Rivers, including major tributaries (Holden 1973). Between 1974 and 1976, the Unit investigated the movement, habitat, and possible spawning areas for the Colorado squawfish and humpback sucker in the lower Yampa and upper Green, rivers. These studies by the Unit have indicated that these four indigenous fish species are declining in abundance. In most areas, only large adults have been captured, suggesting a lack of reproduction. FISH FAUNA The native fish fauna of the Colorado River basin is unique, in that 74% of the native species occur only in this river system (Miller 1959). This high degree of endemism is linked to a long period of isolation reflected in the geologic his- tory of the basin. The Colorado River Wildlife Council listed 20 species (40%) as native to the river and 3D species (60%) as introduced (Rich- ardson 1976). Holden and Stalnaker (1975a) re- corded 10 native fish species (34%) and 19 intro- duced species (66%) in the main stems of the upper Colorado River system. In the present study we have updated the list of fishes to reflect condi- 'tions as we now pe,ceive them in the vicinity of Dinosaur National ~, nument. ~ 606 \ .. Holden (1973) found all 10 of the native spe- cies and all but 4 (15 of 19) of the introduced sp~cies in the lower Yampa. Our recent ~bserva- tions were similar to his, although we d~d not find largemouth bass (Micropterus salmoide~), bluegill (Lepomis macrochirus), green sunf~sh (Lepomis cyanellus), or walleye (Stizostedion vitreum). Holden considered these fish to be occasional or rare. More significantly, we failed to collect any specimens of the native bony tail chub. We collected three introduced species from the Yampa River that were not documented previ- ously: One plains killifish (Fundulus kansae) was captured near Red Rock Creek in July 1975; one Utah chub (Gila atraria) was collected at Box Elder in April 1976 (see Fig. 1 for locations); and sand shiners (Notropis stramineus) were seined in Lily Park and in the Little Snake River just above the Monument boundary. Researchers from Colorado State University have also collected this species at Lily Park and points further upstream in the Yampa River. (C.G. Prewitt and D.E. Snyder, personal communication). In the Green River we collected one smallmouth bass (Micropterus dolomieui) at Horseshoe Bend (about 40 kID below the Monument) in October 1975, and the Colorado Squawfish Recovery Team captured one northern pike (Esox lucius) below Horseshoe Bend in May 1976. Indeed the species composition of the fish population in the Green River below the Monument appears to differ from that of the Yampa in several notable respects: The centrar- chids and walleye, only occasionally found in the