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