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POPULATION SIZE OF RAZORBACK SUCKER 71 TABLE 1.-Catch of razorback suckers in the Green River, Utah and Colorado, by U.S. Fish and Wildlife Service and Utah Division of Wildlife personnel, 1980-1988. NT = total number of fish caught; NQ = number of fish caught with known effort; effort = hours of electrofishing; CPUE = catch per hour of electrofishing. Year NT Upper rivera NQ Effort CPUE NT NQ Lower ri verb Effort CPUE 1980 17 10 78.7 0.13 1 1 22.9 0.04 1981 68 62 95.1 0.65 0 0 13.4 0.00 1982 35 32 32.2 0.99 0 0 0.0 0.00 1983 0 0 19.8 0.00 3 3 76.0 0.04 1984 91 87 110.6 0.79 2 2 32.3 0.06 1985 18 6 91.3 0.07 0 0 15.2 0.00 1986 80 35 84.6 0.41 0 0 39.7 0.00 1987 61 53 95.4 0.55 6 6 38.7 0.15 1988 59 53 75.6 0.71 1 1 39.6 0.02 Total 429 338 683.3 0.49c 13 13 277.8 0.050 a River kilometers 282-555. b River kilometers 0-211. c Total NQ divided by total effort fishing effort in 1980-1988 consisted of 112.8 It in the Yampa River and 83.7 h in the White River). No razorback suckers have been captured in the Green River above km 555 in the last 8 years; therefore, we used this location as the upper boundary of the upper Green River. The assump- tion of geographical closure is thus supported by field data, and we feel justified in estimating the population of razorback suckers in the upper and in the lower sections of the Green River. The assumption of demographic closure also ap- pears to be met, even though several years of cap- ture-recapture data were used. The presence of larval razorback suckers has been documented be- low suspected spawning sites in the upper Green River (Tyus 1987), and ripe razorback suckers have been captured at several suspected spawning sites in the Green River basin (McAda and Wydoski TABLE 2.-Summary statistics for 9 years of Green River razorback sucker capture data, 1980-1988. C = first-time captures; R = recaptures; P = probability of capture determined by program CAPTURE. Year C Upper rivers R P Lower riverb C R 1980 17 0 0.02 1 0 1981 65 0 0.07 0 0 1982 34 1 0.04 0 0 1983 0 0 - 3 0 1984 75 6 0.09 2 0 1985 12 6 0.02 0 0 1986 59 21 0.09 0 0 1987 43 12 0.06 5 1 1988 37 22 0.06 1 0 Total 342 68 12 1 a River kilometers 282-555 b River kilometers 0-211. 1980; Wick et al. 1982), but no razorback sucker juveniles (fish larger than 20 mm but smaller than 400 mm) have been captured in the Green River, even though effort in the past 9 years has been substantial: sampling for juvenile fishes, 1979- 1986, has included seining over 167,921 m2 of backwaters, 231,185 m2 of shoreline, 58,439 m2 of eddies, and 2,530 m2 of riffle habitats through- out all of the mainstream Green River and its major tributaries with seine mesh sizes ranging from 0.5 to 6.35 mm. (unpublished U.S. Fish and Wildlife Service data, Vernal Utah). Several authors (Miller 1961; Minckley and Deacon 1968; Behnke and Benson 1980; Minckley 1983; Tyus 1987) hypothesized that (1) large num- bers of introduced predator species in the Green River prey on young razorback suckers so heavily that there is no recruitment to the adult population and (2) habitat alteration (e.g., after the impound- ment of Flaming Gorge Reservoir in 1962) has adversely affected recruitment. The first hypoth- esis is supported by Marsh and Brooks (in press) who reported heavy ictalurid predation on stocked juvenile razorback suckers and speculated this could prevent recruitment. Minckley (1983) noted that populations of large, presumably old razor- back suckers persisted for many years in reservoirs of the lower Colorado River basin before disap- pearing and cited lack of recruitment as the reason for disappearance. We hypothesize the same se- quence of events is now taking place in the upper Green River. Tyus (1987) reported presumed ages for two Green River adult razorback suckers as over 19 years (otolith annulus count) and 12 years (annular vertebral centrum count). We inspected vertebrae