Standing crops of trout in a 6,455 ft section of reach #2 

 of the Beaverhead River were estimated in th"e fall and spring 

 between October 1966 and October 1978. The section begins 

 1.8 miles below Clark Canyon Dan and 1.4 miles below a USGS 

 gage (Figure 7) . Fall estimates were made between September 20 

 and October 28. Spring estimates were made between March 1 

 and April 2. Age 1+ (yearling) and age II trout were the 

 youngest group estimated in the fall and spring, respectively. 

 Fall estimates of age 11+ and older brown trout and spring 

 estimates of age II and older rainbow trout are generally in- 

 flated due to the upstream movement of spawners into the study 

 section. These estimates were eliminated from the analysis 

 since most do not reflect standing crops of resident trout. 

 Fall estimates of age 1+ brown trout are assumed to be valid 

 estimates of residents. 



During the study, spring estimates of numbers and biomass 

 of age II and older brown trout ranged from 317 - 1,749 and 

 721 lbs - 2,623 lbs, respectively. Fall estimates of numbers 

 and biomass of age 1+ and older rainbow trout ranged from 

 112 - 1,338 and 224 lbs - 1,857 lbs, respectively. 



Flows varied considerably during the study. Between 1966 

 and 1978 the mean flovvs during the irrigation season (approxi- 

 mately April 16 - October 14) ranged from 320 - 870 cfs and 

 mean flows during the non-irrigation season (approximately 

 October 15 - April 15) ranged from 97 to 467 cfs. Average 

 daily flows ranged from 5 7 - 1,36 5 cfs. 



Two variables, the year-class strength during the previous 

 estimate and the magnitude of the flow releases between suc- 

 cessive estimates, were found to explain much of the annual 

 variation in the estimated numbers of the various age-groups 

 of rainbow trout (Table 8) . In combination, the number of 

 average daily flows less than 100 cfs between successive fall 

 estimates and the estimated numbers of age 1+ rainbow trout 

 the previous fall explain 96% of the annual variation in the 

 fall estimates of nxombers of age 11+ rainbow trout, the number 

 of average daily flows less than 150 cfs and the estimated 

 numbers of age 11+ rainbow trout the previous fall explain 90% 

 of the annual variation in the fall estimates of numbers of 

 age III+ rainbow trout, and the number of average daily flows 

 less than 300 cfs and the estimated numbers of age III+ and 

 older rainbow trout the previous fall explain 81% of the annual 

 variation in the fall estimates of numbers of age IV+ and 

 older rainbow trout. 



Similar analyses were conducted for the biomass estimates 

 (Table 8) . In combination, the number of average daily flows 

 less than 100 cfs and the estimated biomass of age 1+ rainbow 

 trout the previous fall explain 9 8% of the annual variation 



34 



