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16 the hypothesis that fish in our sample of 119 cui-ui collected along the fishway (excluding the old fish) were representative of cui-ui that entered the FPB. Using a chi-square test, we compared the observed length frequency of the fishway sample with the length frequency of fish that entered the FPB. Males and females were tested separately. Both samples proved to be representative of the run. The age structure of cui-ui captured in the FPB was proba- bly representative of that of the young fish offshore. We then determined the relative age structure of the offshore population by estimating the relative number of old and young fish in this population in 1983. From this estimate, we deduced that 92% of the fish in the offshore population were of the 1969 year class, 3 % were young fish of year classes other than that of 1969, and 5% were old fish. The predominance of the 1969 year class in the cui-ui population in 1982-84 was further supported by records of cui-ui captured at the FPB in 1978-83, which showed that mean length of fish of both sexes increased progressively (Fig. 11). The impact of this dominant group on the population was also reflected in data from the systematic gill netting con- ducted in Pyramid Lake during 1977-83 (Fig. 12). The subpopulation of young fish increased in length, whereas that of old fish retained nearly the Females Males 70 E 60 I ' 1,1 \111,1 ~ ..<:: l T 1'''''' a, c 4 (8771 ~ 50 ' I" ,"00' :.:: (111 (rl1rl t 1> 1 c III Q) ~ 40 30 1978 1980 1982 Year 1978 1980 1982 Year Fig. 11. Length distribution of male and female cui-ui that entered the fish processing building during the 1978-83 spawning migrations. Data given are mean and range; sample size is shown in parentheses. same length frequency, but declined in numbers. This decline was probably due to natural mortality or sample size (also, the heads of some old fish may have become too large to permit their capture in the largest mesh size of the gill nets that we used). The reason for the domination of the population by the 1969 year class is open to conjecture . We Females 1977 20 83 10 1978 20 10 197920 10 198020 10 198120 10 198220 10 Males 20 80 10 20 10 20 10 ~ u ro u 20 '0 - 10 c Q) ~ Q) CL 20 10 20 198320 10 10 20 10 40 :50 60 40 50 60 Fork length (em) Fig. 12. Length distribution of male and female cui-ui gill-netted from the prespawning aggregation in Pyramid Lake off the mouth of the Truckee River, 1977-83. Numbers in the upper left corner of each panel show numbers of fish; stipled areas denote the 1969 year class.