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continues through life (Simmons et al. 1970), and thus the growth of opercle width and body length was disproportionate. For fish in the 1950 year class we constructed two growth curves (Fig. 10)-one for the proportional growth of opercle width to body length throughout life, and one based on the assumption that growth ceased in fish 20 years old or older. (It was also near the 20th year that annuli began to coalesce.) Growth rates of fish of the 1969 year class were virtually the same for both sexes in their first 6 years of life (Fig. 10); thereafter, the growth of females was the faster. (Six-year-old fish were also the youn- gest we found in the spawning run.) The growth pattern in cui-ui was thus somewhat similar to that in longnose suckers (Catostomus catostomus) in Yellowstone Lake, among which growth also diverged-the females growing faster after age 4, when males became sexually mature (Brown and Graham 1954). We believe that our modified curve for 33-year- old females (Fig. 10) better depicts cui-ui growth 600 E E - 400 .<: 0, c: Q) '" 200 o u. - - -- Males - Females 2 4 6 8 10 12 14 Age (years) 600 -------- - ....- - /. ~ .& h # - - 20 Annuli - 33 Annuli E E :S 400 OJ c (l) ~ 200 u. 5 10 15 20 Age (years) 25 30 Fig. 10. Back-calculated growth of cui-ui in Pyramid Lake. Upper graph: Females (n = 93) and males (n = 73) of the 1969 year class collected in 1983-84. Lower graph: Female (n = 50) of the 1950 year class, based on 33 annuli and 20 annuli (see text for discussion). 15 than does the curve based on direct proportion. (This hypothesis can perhaps be tested as fish of the dominant 1969 year class increase in age in future years.) We were unable to find comparable growth curves for catostomids in the literature. Much of the voluminous literature on catostomid growth either concerned short-lived species, did not include validation of the aging method, or combined the growth rate of the sexes (Brown and Graham 1954; Harris 1967; Bailey 1969; Hauser 1969; Dauble 1980; McAda and Wydoski 1980; Vondracek et al. 1982). The growth rate of females up to 14 years old appeared to be somewhat faster in the 1969 year class than in the 1950 year class, as shown by the solid curves in the upper and lower graphs of Fig. 10. To us, the most appealing explanation for the difference is that the substantially larger cui-ui population of the early 1950's described by Koch (1976) resulted in greater population density and perhaps greater competition for food and space, and thus reduced growth. Age Structure of Population An understanding of the age structure of the cui-ui population in Pyramid Lake is facilitated if one con- siders the prespawning aggregation as consisting of two subpopulations, categorized simply as young fish and old fish. Of the 11 cui-ui year classes iden- tified (Table 3), young fish were considered to be those of the 1967 and later year classes and old fish those ofthe 1950 and earlier year classes. Members of the two subpopulations were generally easy to distinguish. Most old fish were females, in about 10% of which the anal fin was abraided or missing, presumably as a result of spawning in previous seasons (Scoppettone et al. 1983). The presence of large brown spots, usually around the anal fin, in the stomach region, or on the head, was typical. However, the most distinguishing morphological characteristic was the large ratio of head length to body length. Behaviorally, old fish concentrated at the terminal ladder of the fishway, but seldom migrated beyond that point. Cui-ui that moved through the fishway were almost exclusively from the subpopulation of younger fish. Of the 5,994 fish that passed through the FPB in 1983 (the year in which our largest sample of oper- cles was taken), more than 97% were of the 1969 year class. Before making this estimate, we tested