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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



We recognize that such comparisons are less sensi- 

 tive than comparison by analysis of covariance 

 but feel, nevertheless, that the means, usually 

 derived from stratified samples, fairly represent 

 the age groups. 



Effect of Age Group Fecundity and Sex Ratio on Egg 

 Potential 



Our second specific objective is to assess the 

 effect of age and sex composition on the egg 

 potential of Okanogan River spawning escape- 

 ments. Using the estimated age and sex com- 

 position (table 5) ' and the age group fecundity 

 information, we have calculated the egg potential 

 of the Okanogan River spawning escapements of 

 1953 and 1956 through 1959 in three ways: 



Estimate (A) is based on the number of fish in the 

 escapement and on the assumptions of an even sex ratio 

 and a mean fecundity of 3,000 eggs per female. 



Estimate (B) is based on the number of females in the 

 escapement and on the assumption of 3,000 eggs per 

 female. 



Estimate (C) is based on the number of females in each 

 age group and the mean fecundity of the age groups 

 present. 



It is clearly evident from the results (table 6) 

 that the effect of age and sex composition on tlie 

 egg potential of the Okanogan River blueback 

 spawning escapements varies widely from year to 

 year. In 1957 and 1959 the individual variables 

 offset one another and resulted in little over-all 

 change in estimated egg potential. In sharp con- 

 trast, the combined influence of the two factors 



3 Because the 32's are rarely found in streams other than the Olcanogan. 

 we believe that the discrepancies between the number of small fish (32's) 

 counted at Rock Island Dam (table 1) and the 32's in the estimated Okano- 

 gan population (table 5) are due to (1) errors in the spawning ground esti- 

 mates, (2) errors in the large-small breakdown at Rock Island Dam, and 

 (3) mortalities that occur between the two sites. 



reduced the estimated egg potential by 43, 24, 

 and 60 percent in 1953, 1956, and 1958, 

 respectively. 



When we consider the variables separately to 

 determine tlieir relative importance we see that 

 sex ratio was the most important factor in 1956, 

 1958, and 1959, whereas age group fecundity was 

 the greater source of error in 1953 and 1957. 



To illustrate the combined effect of sex ratio 

 and age group fecimdity on egg potential and 

 ultimately on escapement-return relationships, 

 let us compare from table 6 the three measures of 

 egg potential for the years 1957 and 1958. Using 

 estimate (A), we note that the egg potential in 

 1958 was 1.24 times as large as that of 1957. 

 From these data we might expect the return from 

 the 1958 brood to be 1.24 times as great as the 

 return from the 1957 brood, all other factors 

 affecting the return being similar. 



Correcting for the numbers of female spawners 

 in each age group and the mean fecundity for each 

 age group (estimate C), we note that the egg 

 potential in 1958 is only about one-half the egg 

 potential in 1957. Other factors affecting the 

 return being similar, we might expect that the 

 return from tlie 1958 escapement would be only 

 one-half as large rather than 1.24 times as large 

 as the return from the 1957 escapement. 



By the same reasoning, we would expect that 

 the 1953 escapement would produce a return only 

 one-half as large as would an escapement of the 

 same size, but normal with respect to age and sex 

 composition. 



We full\ realize that the escapement-return 

 relationship of 1953-57 would have been poor 



Table 5. — Estimates of the age and sex composition of the Okanogan River blueback spaivning populations 1953, 1956-59 



