40 



1972, increased slightly in 1973, increased substantially in 1974, then ^ r 



decreased slightly in 1975. Strong 1973 and 1974 year classes were 



responsible for the elevated numbers in 1974 and 1975. 



Strengths of the 1965 through 1974 year classes are shown for a two 



year period in Table 7. The data suggests that some movement of rainbow 



trout into the Hildreth Section occurred following a poor yearling crop. 



Although movement may have occurred, the number of yearlings still had 



a strong influence on the number of age II rainbow trout in the following 



fall (Figure 10). Numbers of yearlings explain 83% of the annual 



variation in numbers of age II rainbow trout one year later. Numbers of 



yearlings had little influence on numbers of age III rainbow trout two 



years later. Numbers of yearlings explain only 14% of the annual 



variation in numbers of age III rainbow trout. ^ 



TABLE 7. Estimated numbers of rainbow trout in the 1965 through 1974 

 year classes in the Hildreth Section (6455 ft) of the 

 Beaverhead River. 



Multiple linear regressions were computed to evaluate the relationship 

 between mean irrigation and nonirrigation flows and numbers of age III 

 and age IV and older rainbow trout (Appendix Table 8). In combination, 

 the numbers of age III and older rainbow trout the previous fall and mean 



