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1962). Robertson and Koch (1978), who evaluated techniques of aging cui-ui by means of scales, otoliths, and opercles, concluded that all three methods were reliable. Although Johnson (1958) found it extremely difficult to identify definitive scale annuli, Koch (1976) used the scale method for aging. Study Area Pyramid Lake is in western Nevada and lies totally within the boundaries of the Pyramid Lake Paiute Indian Reservation. It is a terminal lake (area in 1983 about 467 km2) from which water is lost solely through evaporation. As a result, it is moderately saline; the concentration of total dissolved solids in 1983 was 4,900 to 5,000 mg/I. Galat et al. (1981) described the lake's general limnological characteristics. Pyramid Lake is the largest vestige of pluvial Lake Lahontan (Russell 1885; Hubbs and Miller 1948). The fact that lacustrine cui-ui are endemic to Pyramid Lake adds credence to Benson's (1978) assertion that it is the only Lake Lahontan vestige that has not completely desiccated during the past 9,000 years. Early in this century, cui-ui did occur in Winnemucca Lake, which once lay east of Pyramid Lake. The two lakes were connected at a I~) Winnemucca (I, Lake r I I ( :) Mud Lake Slough 'V I \ \' Marble Bluff Dam Pyramid Lake Fishway Stampede Reservoir \ Independence Lake/ Donner Lake/ t N I o 10 ~ km 3 their southern ends by Mud Lake Slough (Fig. 1). The primary source of water to Winnemucca Lake was overflow from Pyramid Lake; this overflow virtu- ally ceased with the completion of Derby Dam in the early 1900's. Winnemucca Lake had desiccated by 1938 (Sumner 1940). By 1983, Mud Lake Slough still received a modest amount of agricultural return water, but none of it reached Winnemucca Lake. The Truckee River, which is Pyramid Lake's only permanent tributary, originates at Lake Tahoe (elevation 1,898 m above mean sea level), flows 190 km northeasterly, and then discharges into Pyramid Lake (elevation 1,156 m in 1984). The river's primary water sources are the Sierra Nevada and the Carson mountain ranges. Flow is largely controlled; Lake Tahoe, which once overflowed into the Truckee River, is regulated by a dam, as are Donner and Independence lakes; three of the larger tributary systems also have one or more reservoirs (Fig. 1). The lower Truckee River, which includes the historical cui-ui spawning area (La Rivers 1962), is a low-gradient stream descending at a rate of about 1.5 m/km. In the early 1980's, the stream channel had only a sparse riparian corridor, dominated by Fremont cottonwood (Populus fremontii). In 1982-84, average daily flows ranged from 1 to 206 m3/s, and average daily spring flows from 18 to 206 m3/s. b Pyramid lake Truckee River Delta t N I Pyramid Lake Fig. 1. (a) Truckee River, Pyramid Lake, and connected waters. (b) Lower reaches of the Truckee River, showing the location of Pyramid Lake Fishway (PLF) and its four ladders, Marble Bluff Dam, and the fish processing building (FB); the mouth of the Truckee river in 1982 and 1983 (upper sketch), advanced 2 km upstream in 1984 (lower sketch) .