73 



the upper Beaverhead River as a high quality, trophy rainbow trout fishery 

 will require average daily flows greater than approximately 250 cfs. 

 This corresponds to a flow release of approximately 200 cfs at Clark 

 Canyon Dam. 



Positive relationships between the magnitude of flows during the 

 nonirrigation season and the survival and numbers of older brown trout 

 were not demonstrated during this study. However, the period (1971-72) 

 producing the lowest rate of population decrease of age III and older 

 brown trout and having the highest number of age IV and older brown 

 trout at the end of the period has the fewest average daily flows in 

 the 101-300 cfs interval (Appendix Table 34). 



The annual rates of population decrease of older rainbow trout 

 during low flow periods were greater than those of older brown trout. 

 During the period containing the lowest nonirrigation flows of the study 

 (1974-75), age III and older rainbow trout showed a 94% rate of decrease, 

 while that of age III and older brown trout was only 50%. The relative 

 stability of the estimates of numbers of age IV and older brown trout 

 and the instability of estimates of age IV and elder rainbow trout 

 throughout this study primarily reflect a differential rate of decrease 

 during low flow periods. 



Habitat preferences may explain the differential rates of population 

 decrease documented in this study. Although the components of the pre- 

 ferred habitat of brown and rainbow trout are poorly understood, some 

 difference have been measured. Lewis (1969) found cover (primarily 

 overhanging vegetation, undercut banks, and brush) to be the most 

 important factor influencing numbers of brown trout in stream pools, 

 while current velocity was most important for rainbow trout. Kraft 



