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Life History and Status of the Endangered Cui-ui of Pyramid Lake, Nevada by G. Gary Scoppettone and Mark Coleman us. Fish and Wildlife Service Great Basin Complex Office 4600 Kietzke Lane Reno, Nevada 89502 and Gary A. Wedemeyer u. S. Fish and Wildlife Service National Fishery Research Center Building 204, Naval Station Seattle, Washington 98115 Abstract Prespawning adult cui-ui (Chasmistes cujus) congregate off the mouth of the Truckee River in Pyramid Lake, Nevada, from mid-March to early June. The spawning runs have been largest during years when spring flows in the Truckee River were relatively high. In 1982-84 upstream migration lasted 48 to 59 days and peaked when daily mean water temperatures were 12 to 150C. Radio-tagged males remained in the river 6,5 to 16.5 days and females 4,0 to 10.5 days, The fish spawned over predominately gravel substrate at depths of 21 to 110 cm, where water velocities were 21 to 140 cm/s, Fecundity ranged from 24,000 eggs for a female 432 mm long (fork length) to 196,000 for a fish 657 mm long, The ratio of males to females in the spawning run was 1:1.3 in 1982 and 1983 and 1 :2.5 in 1984. Most cui-ui larvae emigrated downstream to Pyramid Lake soon after swim- up, primarily at night. The interval between the peak of the adult spawning migration and peak emigration of the young was 26 days in 1982 and 29 days in 1984. Marble Bluff Dam, constructed in the Truckee River in 1976, did not appear to cause substantial mor- tality of emigrating larvae, Opercle bones were used to determine cui-ui ages and growth rates; the aging method was validated by following individual year classes through time. Eleven year classes that were identified included fish from 1 to 4 I years old. In 1983, about 92% of the fish in the reproductive population were from the 1969 year class and 5% were from the 1950 or earlier year classes; the rest were divided among six year classes (1967, 1971, and 1973-76). Males and females grew at the same rate until 6 years of life, the age at which they first entered the spawning run; thereafter, females grew faster and lived longer than males, Growth in length virtually ceased by age 20. The estimated num- bers of cui-ui in the prespawning aggregation off the mouth of the Truckee River were 187,000 in 1982 and 103,000 in 1983, Larvae in the Truckee River fed primarily on chironomids and zooplankton; yearlings in Mud Lake Slough (near the mouth of the Truckee River) and adults in the prespawning aggregation off the mouth of the river ate mostly zooplankton. Predation by American white pelicans (Pelecanus erythrorhynchos) caused substantial mortality of adult cui-ui during their spawning migration in 1982 and 1983,