in the fall estimates of bioraass of age 11+ .iTainbow trout, the 

 number of average daily flows less than 150 cfs and the esti- 

 mated biomass of age 11+ rainbow trout the previous fall ex- 

 plain 77% of the annual variation in the fall estimates of 

 biomass of age II 1+ rainbow trout, and the number of average 

 daily flows less than 300 cfs and the estimated biomass of 

 age III+ and older rainbow trout the previous fall explain 59% 

 of the annual variation in the fall estimates of biomass of 

 age IV+ and older rainbow trout. 



Only the standing crops of older brown trout appear to 

 be influenced by the magnitude of the flow releases during 

 the study (Table 9) . In combination, the number of average 

 daily flows less than 300 cfs between successive spring esti- 

 mates and the estimated nimibers of age III and older brown 

 trout the previous spring explain 71% of the annual variation 

 in the spring estimates of numbers of age IV and older brown 

 trout, and the number of average daily flows less than 300 cfs 

 and the estimated biomass of age III and older brown trout 

 the previous spring explain 55% of the annual variation in the 

 spring estimates of biomass of age IV and older brown trout. 



During the study, fall estimates of numbers of age 1+ 

 brown trout ranged from 39 to 908 and those of rainbow trout 

 ranged from 10 to 997. Flows were examined to determine if 

 this extreme variation in numbers could be attributed to flow 

 variations. 



The flow and population data suggest that average daily 

 flows less than 250 cfs favored the survival of rainbow trout 

 up. to age I+. This relationship was not evident for brown 

 trout. 



Spawning flows produced the most consistent relationship 

 with numbers of age 1+ brown trout. The data suggest that 

 the pattern and magnitude of the flow releases during the 

 brown trout spawning period influenced reproductive success 

 which in turn led to the extreme variation in numbers of age 

 1+ brown trout. Flow fluctuations during spawning appear to 

 have a greater impact on reproductive success than the magni- 

 tude of the spawning flows and decreasing flows appear more 

 favorable than constant flows. In general, spawning flows 

 devoid of violent fluctuations and gradually decreasing to a 

 minimum of 150 cfs during the 47-day brown trout spawning 

 period (September 15 - October 31) appear to maximize repro- 

 ductive success. 



Much of the fluctuation of the spawning flows that oc- 

 curred during the study can be attributed to the Montana De- 

 partment of Fish, Wildlife and Parks requesting lower flow 

 releases to facilitate the completion of the fall population 

 estimates. This practice was discontinued in 1974. 



36 



