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chub lost most of its best habitat to reservoirs such is that this species is not as close to extinction <br />as Lake Powell and Flaming Gorge, but the prognosis as was commonly believed a few years ago. <br />1982 UPDATE <br />Intensive sampling since 1980 failed to discover <br />any new concentrations of humpback chub except for <br />the Colorado River in Westwater Canyon, Utah a few <br />miles below the state line. Movement of a tagged <br />humpback chub from Westwater Canyon to Ruby Canyon, a <br />distance of 13 miles, indicates that the humpback <br />chub of Ruby Canyon and Westwater Canyon can be con- <br />sidered as a single population because of interchange <br />between the two habitats. Most humpback chub, how- <br />ever, exhibited little movement as revealed by tagged <br />fish. Most of the tagged fish that were recaptured <br />moved less than half a mile from the point of original <br />capture. Evidently, all life history needs can be <br />met in the relatively restricted zone of the Colorado <br />River in the Black Rocks area of Ruby Canyon. <br />Humpback chub were found in deep areas of the <br />Green and lower Yampa rivers in 1981 and 1982 but <br />only sporadically. There has been no indication that <br />any other sites contain a high abundance of humpback <br />chub comparable to their numbers in the Colorado <br />River at Black Rocks. <br />Laboratory studies demonstrated that the hump- <br />back chub is a hardy species. It proved more resist- <br />ent to the effects of organic and inorganic toxic <br />compounds than did non-native fishes such as channel <br />catfish, fathead minnow, and bluegill. Humpback <br />chub tested for tolerance to total dissolved solids <br />(salinity) showed no avoidance of the highese levels <br />used in the experiments, 11,600 parts per million, or <br />about one-third the salinity of the ocean. <br />The hardiness of the humpback chub as revealed <br />by the laboratory studies indicate why it is so <br />abundant in the Little Colorado River, Arizona. The <br />environmental extremes characterizing the Little <br />Colorado River are so harsh for fish life that few <br />other species are able to tolerate these conditions. <br />The peculiar body shape of the humpback chub <br />essentially restricts high population abundance in <br />the upper basin to the unique deepwater habitat sites <br />of the Colorado River in Westwater and Ruby Canyons. <br />A flow regime for the Colorado River, necessary to <br />maintain the unique habitat characteristics in the <br />canyon areas, is, as yet, unknown. The U.S. Fish <br />and Wildlife Service stocked several thousand <br />hatchery-raised humpback chub into the Colorado River <br />in Cataract Canyon, Utah, in 1980 and 1982 in an <br />attempt to increase humpback chub abundance in this <br />section of the Colorado River. <br />19