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112 FIGURE I.-Green River basin and the seven areas (A-G) sampled for razorback suckers. Sections A-F were sampled April-November 1979-1981. Sections A-G were sampled April-June 1984-1986. and involved continuous downstream coverage with a pulsed DC unit. The upper 40 km sampled during the spring program (G in Figure 1) and some local collections at the mouths of tributary streams were not included in comparisons be- tween the two sampling programs, but they are included in the general information presented. In addition, some opportunistic spring electrofishing was conducted in suspected razorback habitats during 1980-1986. Each adult razorback sucker was tagged with a Carlin dangler tag (Wydoski and Emery 1983), individually weighed, and measured for total length (TL). Reproductive condition was determined by visual examination for breeding tubercles and sex products. If eggs or sperm could be expressed by manually stripping a fish, the fish was classified ripe. Larval fish collections were made downstream of areas where ripe razorback suckers were col- lected in 1984. All quiet shoreline habitats ex- tending for about 1 km downstream of these areas were sampled with I-m2 seines of0.5-mm2 mesh. All larval fishes collected were preserved in 5% formalin solution and shipped to the Larval Fish ( ~. '., I TYUS Laboratory, Colorado State University, Fort Col- lins, for identification. Radiotelemetry. - Radio tracking was done only in 1980, 1981, and 1985. Transmitter modules, manufactured by the A VM Companyl and weigh- ing about 11 g in air, were surgically implanted in razorback suckers in spring. The implanted fish were monitored in the 40-MHz range, as previ- ously described for Colorado squawfish Ptycho- cheilus lucius (Tyus and McAda 1984). Surgical technique and fish recovery were evaluated by holding fish for a week in 1980; there were no apparent ill effects. Transmitters were implanted and fish were released on the same day in the other years. Attempts were made to locate fish at least three times weekly in 1980, but at only 2-week intervals in 1981 and 1985. Fish were tracked by boat during daylight hours and their general lo- cation was determined by bidirectionalloop an- tennas (Winter 1983). If a tracked fish remained in one location for 30 min, habitat information was recorded. Vari- ables measured included the general habitat and substrate type, water depth, and water velocity. Water depth was recorded by direct measurement with a wading rod, and water velocity was mea- sured at 0.6 of the depth below the water surface with a Marsh-McBimeyI current meter. Substrate type (silt, sand, gravel, rubble, or boulder) was determined by direct observation and by probing with a wading rod (Orth 1983). . Results and Discussion Distribution, Abundance, and Size The razorback sucker was widely distributed in flat-water sections of the mainstem Green River and lower portions of the Yampa River. The cen- ter of distribution of 323 fish was in the upper Green River (from the mouth of the Duchesne River to the mouth of the Yampa River), where the species was most abundant. Six fish were also collected in the lower sections of the Green River (A and B in Figure 1). None were collected in Split Mountain Canyon in DNM or in Desolation or Gray canyons; presumably, the razorback sucker does not prefer such white-water canyon habitat. Total lengths of the 323 razorback suckers cap- tured ranged from 426 to 608 mill. Ripe males averaged 503 mm (N = 37) and ripe females av- eraged 544 mm (N = 23). Females were signifi- 1 Reference to trade names does not imply U.S. gov- ernment endorsement of commercial products.