In conclusion, annual variations in the. -reproductive suc- 

 cess of brown trout appear to be the major factor influencing 

 the variations in total standing crops of brown trout during 

 the study. Reproductive success was probably related to the 

 magnitude and pattern of the flow releases during the fall 

 spawning period. Flows had little direct influence on the 

 total standing crop of brown trout even during the final years 

 of the study when densities of brown trout were highest and 

 flows were among the lowest. The one exception is age IV and 

 older brown trout whose numbers and biomass appear to be par- 

 tially limited by flows less than approximately 300 cfs. 



The magnitude of the flow releases directly affected all 

 age groups of rainbow trout. Results indicate that standing 

 crops of age II+, III+, and IV+ and older rainbow trout were 

 partially limited by flows less than approximately 100, 150, 

 and 300 cfs, respectively, while numbers of rainbow trout up 

 to age 1+ appear to be limited by flows greater than approxi- 

 mately 250 cfs. During low flow years, the higher numbers and 

 presumably higher survivial of age 1+ rainbow trout partially 

 compensated for the elevated losses of older rainbow trout, 

 resulting in little change in the total standing crops. The 

 high numbers of age 1+ rainbow trout in turn greatly influenced 

 year class strength in succeeding years. Yearling strengths 

 in previous years and flows were the major factors regulating 

 standing crops of age 11+ and older rainbow trout during the 

 study. In general, older trout were more affected by flow re- 

 ductions than were younger trout, and rainbow trout were more 

 affected than brown trout. 



Age IV+ and older rainbow trout are highly desired for 

 the sport fishery of the Beaverhead River due to their trophy 

 size. During the study, this group averaged 4.97 lbs with 

 specimens as large as 13.25 lbs captured. The results of the 

 study suggest that continually managing the flows solely for 

 trophy-size trout may eventually result in low densities of 

 rainbow trout by providing unfavorable conditions for age 1+ 

 rainbow trout and, thereby, limit recruitment into the popu- 

 lation. A minimum instream flow less than the optimum needed 

 for a trophy fishery may be desirable in terms of providing 

 higher densities of rainbow trout but not necessarily of 

 trophy size. 



Gallatin River - Reach #2 



Three study sections were established within a 22-mile 

 portion of reach #2 of the Gallatin River in 1976 to evaluate 

 the impact of summer irrigation withdrawals (dewatering) on 

 trout populations (Figure 11) . Section I began near the canyon 

 mouth at river mile 44 and extended 15,000 ft downstream. 

 This section is upstream of the majority of irrigation diversions 



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