47 



and the estimated biomass for any group of brown trout were not demon- 

 strated. The biomass of age I brown trout did explain 83 and 79%, 

 respectively, of the annual variation in the biomass of age II and 

 age III brown trout in succeeding years. 



TABLE 12. Estimated biomass (lbs/6455 ft) of brown trout in the 1965 

 through 1974 year classes in the Hildreth Section of the 

 Beaverhead River. 



Rainbow Trout 



Spring and fall biomass estimates (lbs/6455 ft) of rainbow trout 

 by age groups are given in Table 13. Fall biomass estimates of age I 

 and older rainbow trout are compared in Figure 9. The biomass increased 

 slightly between 1966 and 1973, increased substantially between 1972 and 

 1974, then decreased slightly in 1975. The elevated biomass in 1974 and 

 1975 primarily reflect strong 1973 and 1974 year classes. 



The biomass of the 1965 through 1973 year classes are shown for a two 

 year period in Table 14. Significant relationships between the magnitude 

 of mean flows during the irrigation and nonirrigation seasons and the 

 estimated biomass of any group of rainbow trout were not demonstrated. 

 However, the biomass of age I rainbow trout did explain 83% of the annual 

 variation in the biomass of age II rainbow trout the following fall. 



