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_ 4 0-Ty k5 North American Journal of Fisheries Management 9:68-73, 1989 Population Size and Status of the Razorback Sucker in the Green River Basin, Utah and Colorado STEVEN H. LANIGAN' AND HAROLD M. TYUS U.S. Fish and Wildlife Service, 1680 West Highway 40, Room 1210 Vernal, Utah 84078, USA Abstract.-The status of the razorback sucker Xyrauchen texanus in the Green River, Utah, was evaluated with capture-recapture data collected from 1980 to 1988. The razorback sucker popu- lation in the upper Green River (river kilometers 282-555) was estimated at 948 fish (95% con- fidence interval, 758-1,138), based on a total of 410 fish captured (68 recaptured). Razorback suckers in the lower Green River (km 0-211) were extremely rare; their numbers were too small (13 fish captured, 1 recaptured) to allow a reliable population estimate. Gray and lower Desolation canyons separated the upper and lower Green River razorback sucker populations. These canyons and a low diversion dam appeared to be barriers to fish movement. The absence of the razorback sucker in Gray and lower Desolation canyons (km 211-282) suggested a lack of suitable habitat. The Green River supports the largest known population of razorback suckers among the species' native riverine habitats. However, low population density and recruitment, as well as habitat alteration, mandate increased research and management efforts to save this population. The razorback sucker Xyrauchen texanus is en- demic to the Colorado River basin of western North America (Minckley 1983). This species, once abundant and widely distributed (Seethaler et al. 1979), is now threatened with extinction (McAda and Wydoski 1980; Wick et al. 198; Tyus 1987; USFWS 1987). In riverine habitats, it exists only in the mainstream and lower parts of tributaries of the Green and upper Colorado rivers (McAda and Wydoski 1980; Tyus et al. 1982; Tyus 1987). Razorback sucker populations remain in main- stream reservoirs in the lower Colorado River ba- sin below Lee Ferry, Arizona (Carlson and Carlson 1982), but these are relict subpopulations of very old individuals (Minckley 1983; McCarthy and Minckley 1987). The precarious existence of the razorback sucker in the upper Colorado River basin was noted by McAda and Wydoski (1980), Wick et al. (1982), and Tyus (1987), but no attempt at estimating the population size has been published. We evaluated the status of this rare species in the Green River, Utah and Colorado, using capture-recapture data collected from 1980 to 1988. Methods Personnel of the U.S. Fish and Wildlife Service, Colorado River Fish Project (USFWS) and the Utah Division of Wildlife Resources (UDWR) collected razorback suckers in the Green River Present address: Nez Perce National Forest, Route 2, Box 475, Grangeville, Idaho 83530, USA. Basin (Figure 1) primarily by electrofishing; a few fish were also collected in seine and trammel nets. The upper and lower boundaries for sampling were river kilometers 35-555 in the Green River (mea- sured from the river mouth), km 0-169 of the Yampa River, and km 0-235 of the White River. Data from two USFWS electrofishing sampling programs were used: data collected during April- June 1980-1981 as part of a basinwide "stan- dardized" program (Tyus et al. 1982) and data from a "spring" program conducted during April- June 1984-1988 (Tyus 1987). Sampling by UDWR personnel was done during April and May of 1986, 1987, and 1988, as part of a spring monitoring program for Colorado squawfish Ptychocheilus lu- cius. This program encompassed km 539-483, 418-394, and 185-154 of the Green River and km 34-0 and 153-167 of the White River. Although all reaches of the rivers described in this paper were sampled, effort varied among reaches be- cause of varying habitat, remoteness, logistics, and sampling goals among the years and programs. Sampling effort was reduced in areas (e.g., White and upper Yampa rivers) where no razorback suckers were found during the systemwide stan- dardized program of 1980-1981. Additional spring electrofishing was conducted in suspected razor- back sucker habitats during 1980-1988. Yearly capture-recapture data for razorback suckers for 1980-1987 were entered as a data ma- trix into the computer program CAPTURE (White et al. 1982). This program tests capture probability assumptions by comparing the absolute and rel- ative fits of eight population estimation models to 68 07