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~ , Colorado (1980). The Upper Colorado River Basin (above Lee Ferry, Arizona) currently supports the last stronghold of the Colorado squawfish, which has been extirpated from the Lower Basin. Endan- gered fishes in the Lower Basin survive only as remnant, non-repro- ducing populations of bony tail and razorback sucker, which persist in large mainstem reservoirs (Minckley 1973, 1984), and the humpback chub, a species which appears to survive in relatively unaltered habi- tats in the Grand Canyon (Kaeding and Zimn,erman 1984). This paper is an update of Fishes of the Upper Colorado River Basin: Distribution A')undance and Status, (Tyus et al. 1982), pre- sented at a symposium of the Annual Meeting of the American Fisher- ies Society (Miller et al. 1982). The earlier distribution paper summarized current knowledge of all fishes of the Upper Colorado River Basin from 1975-1981. We present updated information for only the four endangered forms in the intervening four year period 1981-35. METHODS The Upper Colorado River was divided into three major systems, or hydrologic sub-basi~s: Green, Upper Mainstem and San Juan-Colo- rado (Fig. 1), after the work of Iorns et al. (1965). Information for each fish species is presented separat~ly, using the above geo- graphical separations. The distribution and abundance of the four fish species was evaluated from unpublished data. The U.S. Fish and Wildlife Service (USFWS) data for the Green sub-basin was obtained from Tyus and Jones (1985), and from the upper mainstem Colorado sub-basin from Archer et al. (1985). Bureau of Reclamation data were obtained for the San Juan-Colorado sub-basin (TullY and Williams 1985). The terminology used in 1981 (Tyus et al. 1982) was retained in this presentation: The relative abundance terms used are: abundant - a species occurring in large numbers and consis- tently collected in a designated area; common - a species occurring in moderate numbers, and fre- quently collected in a designated area; r~~e - a species occurring in low numbers either in a re- stricted area or having sporadic distribution over a larger area; or incidental - species occurring in very low numbers and known from only a few point collections. -21- l t I # I ~ I I I ~ i I I 1