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 formulated in an effort to save it. We used a population simulation model that incorporated environmental 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.
