50 



Annual fall to fall rates of population change of rainbow trout by 



age groups are given in Table 16. The annual rate of population change 



of age III and older rainbow trout was significantly correlated with 



the magnitude of mean flows during the nonirrigation season (Figure 12). 



Mean nonirrigation flows explain 66% of the annual variation in the rate 



of decrease of age III and older rainbow trout. The annual rates of 



decrease for all age groups of rainbow trout were elevated during the 



period (1974-75) containing the lowest nonirrigation flows of the study. 



TABLE 16. Annual rates of population change {%) of rainbow trout by age 

 groups in the Hildreth Section of the Beaverhead River between 

 1966 and 1975. 



Rainbow Trout Age Group 

 Fall to Fall I- 1 1 II-III I & Older- II & Older- III & Older- 



II & Older III & Older IV & Older 



-49 

 -78 

 -36 

 -42 

 -28 

 -81 

 -94 



Tag returns by fishermen were consistently low throughout the study 

 (Table 17). Although the river was open to year-round fishing, most of 

 the tagged fish were caught between April and September. Angler tag 

 returns were not elevated when mean April - September flows were lowest, 

 suggesting excessive harvesting by anglers was not the major cause of 

 the elevated rates of population decrease of older trout in low water 

 years. 



ft 



