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2 The cui-ui (Chasmistes cujus) is a federally listed endangered species endemic to Pyramid Lake, Nevada. The species is believed to be the last remaining genetically pure member of the genus Chasmistes (Miller and Smith 1981). This taxonomic distinction, the severe decline of the cui-ui popula- tion, and the favorable outlook for recovery of the species resulted in its receiving a top priority rating for recovery (U.S. Fish and Wildlife Service 1983). Because much natural history information was lacking, the Service supported a 4-year study of the species as outlined in the Cui-ui Recovery Plan (U.S. Fish and Wildlife Service 1978), We present the results of that research. The primary cause of the decline of the Pyramid Lake cui-ui population was clearly the disruption of the reproductive cycle. During the past 50 years, spawning adults have had only restricted access to the Truckee River, the only permanent tributary of Pyramid Lake, An extensive and shallow delta that formed at the mouth of the river in the 1930's has been a barrier to migrating adults in most years since then. The delta resulted from a drastic lowering of the water level of Pyramid Lake over a relatively short period, as a result of water diversion from the Truckee River. The water level of the lake dropped about 25 m between 1906 and 1982. The comple- tion of the Newlands Project in 1905 (Townley 1977) and the associated construction of Derby Dam diverted an average of 311 million m3 annually, or about half the river flow. A series of measures were taken, beginning in the mid-1970's, to attempt to restore the Pyramid Lake sport and Indian food fishery. In 1976, Marble Bluff Dam and Pyramid Lake Fishway, a canal 4.8 km long that bypasses the delta, were completed. The dam diverts water down the fishway, enabling migrating fish to bypass the delta. Also, portions of the water stored in Stampede Reservoir (an impoundment on a tributary of the Truckee River) were reserved to augment flows required for cui-ui spawning. As a further measure against the possibility of extinction, hatchery propagation of cui-ui for release into Pyramid Lake was begun in 1973, based on techniques developed by Koch and Contreras (1973). A fully equipped cui-ui hatchery, managed and maintained by the Pyramid Lake Paiute Indian Tribe, produced an average of 7 million cui-ui larvae annually in 1980-84 (Alan Ruger, Pyramid Lake Fisheries, personal communication). These larvae were released into the lower Truckee River or directly into the lake, In 1981, the U.S. Fish and Wildlife Service (Seattle National Fishery Research Center and Reno Great Basin Complex Office) began the present study to provide life history information that might facilitate recovery of the species. Adult cui-ui, taken on their spawning migration, were confined in a natural braid of the lower Truckee River (Scoppettone et al. 1983). Data developed on cui-ui reproduction here, under controlled conditions, served as the basis for the rest of the work in the main stem of the Truckee River. The phase of the life history spent in the river was emphasized because of its obvious importance in determining survival of the species. Included in the work were studies of spawning migration, reproduction, emigration of larvae, age and growth, population size, and food. Each spring, adult cui-ui congregate at the south or river end of Pyramid Lake before they migrate upstream to spawn (La Rivers 1962). The concen- tration is greatest near the mouth of the river (Koch 1972). Catches per unit of effort in gill nets fished in Pyramid Lake near the Truckee River and the Pyramid Lake Fishway were greatest when river flows were high (SonnevilI981). Historically, cui-ui spawned from mid-April to late May, primarily in the lower 40 km of stream (Snyder 1918). In more recent years, spawning was observed in Pyramid Lake itself from May through July Oohnson 1958; La Rivers 1962; Koch 1973). However, the high salinity in the lake is lethal to incubating eggs (Chatto 1979). Even the hatching success of eggs deposited at a freshwater inlet was questionable (Koch 1973). In the river, adult cui-ui spawned in riffles over predominantly gravel substrate, burying their adhesive, demersal eggs at depths as great as 10 cm (Snyder 1918; Scoppettone et al. 1983). Cui-ui spawning behavior was described by Scoppettone et al. (1983), early life stages by Koch (1972), and embryology by Bres (1978). At lOoC, eggs hatch in 13 days and larvae swim-up about 5 days after hatching (Koch 1972; Koch and Contreras 1973). Bres (1978) observed that the mouth of cui-ui larvae opened 16 days after hatching. When not congregated at the river end of the lake, adult cui-ui live primarily in littoral waters less than 23 m deep Oohnson 1958; Vigg 1980), Zooplank- ton (not further identified) was reported to be the primary food of adults Oohnson 1958; La Rivers J