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. .1:, i qq3 ~m (en) ef at. Nonh Amertcan Journal of Fisheries Management 13:467-474. 1993 ~7'1'+IO'3S 9) 7g~. t Probabalistic Projections for Recovery of the Endangered Cui-ui JOHN M. EMLEN National Fisheries Research Center, U.S. Fish and Wildlife Service Naval Station Puget Sound, Seattle, Washington 98115, USA THOMAS A. STREKAL U.s. Bureau of Indian Affairs 1677 Hot Springs Road, Carson City, Nevada 89706, USA CHESTER C. BUCHANAN U.S. Fish and Wildlife Service 4600 Kietzke Lane, Building C. Reno, Nevada 89502, USA Abstract. - The cui-ui Chamistes cujus, a lake sucker found in only Pyramid Lake, Nevada, is listed as endangered under the U.S. Endangered Species Act, and a recovery plan has been for- mulated in an effort to save it. We used a population simulation model that incorporated envi- ronmental stochasticity in water availability to project the persistence of cui-ui over 200 years. Based on this approach, recovery of the species in terms of persistence was evaluated for the existing hydrological situation and for situations involving water supplementation in various amounts. Our calculations indicate that immediate (as of spring 1991) aCQuisition of 70,000 acre-feet per year of supplemental water to raise levels in Pyramid Lake would have assured recovery and persistence over 200 years with a probability of 0.95. Postponement of acquisition for 3 years followed by incremental increases of 3,000 or 10,000 acre-feet per year would require, respectively, 83,000 and 120,000 acre-feet of annual supplemental water to assure persistence. The approach is applicable to a wide range of species and conditions. The U.S. Endangered Species Act of 1973, as amended, provides that "the Secretary of the In- terior shall develop and implement plans for the conservation and survival of endangered species and threatened species.. .." To carry out this charge, recovery teams set up under the Act collate life history data and produce recovery plans. Ide- ally, the information available to these teams should be sufficient to project hypothetical future population densities under various feasible man- agement scenarios. In this way, the effectiveness and cost of specific potential recovery actions can be evaluated. The cui-ui Chasmistes cujus is a large omniv- orous sucker (family Catostomidae) unique to Pyr- amid Lake, Nevada, and is an obligate stream spawner in the lower portion of the Truckee River. Each spring, adults congregate in a prespawning aggregate near the mouth of the river and, ifinftow, river access, and water temperature are suitable, migrate into the river to spawn over gravel sub- strate (Scoppettone et al. 1983). Upon completion of spawning, adults return to the lake and do not spawn again until some subsequent spring. The fish appear to attempt spawning annually, but un- der present conditions may be able to ascend the river only two or three times a decade. Life span reaches 41 years, and females generally outlive males, resulting in an increasingly skewed sex ratio with age (Scoppettone 1988). Each cui-ui female produces 40,000-180,000 eggs annually, the number increasing with female size and generally with age. Fertilized eggs hatch in 1-2 weeks. Yolk-sac larvae remain in the gravel substrate an additional 5-10 days before emerging and then emigrate downstream into the lake. More information on cui-ui life history can be found in Koch (1972), Chatto (1979), Scoppettone et al. (1982, 1983, 1986), Sigler et al. (1985), and Bu- chanan and Coleman (1987). Information dis- cussed below is from these sources. Size of the historical cui-ui populations and spawning runs is not known; only general obser- vations have been reported. For example, Snyder (1917) noted that "at times cui-ui appeared in such large and densely packed schools [in the Truckee River] that considerable numbers were crowded out of the water in shallow places. . . ." From such descriptions, and the fact that spawning runs not only used to feed many people but provided a cultural basis for the local Paiutes, we may rea- sonably conclude that spawning runs probably 467 8pp.