study results suggest that flow reductions affect rain- 

 bow trout more severely than brown trout. A 25% reduction in , 

 the minimum summer flow in Section I (from 5"2 3 cfs in 1976 

 to 393 cfs in 1977) coincided with a 27% reduction in the esti- 

 mated biomass of adult rainbow trout, but had no adverse ef- 

 fect on brown trout. The rainbow trout population was also 

 highest in the least dewatered study section (Section I) , 

 considerably reduced in Section II, and nearly absent in Sec- ^ 

 tion III, where dewatering is severe. A comparative measure 

 of the abundance of rainbow trout is provided by the April 

 1977 electrofishing runs in which 627, 72, and 5 rainbow trout 

 were captured in Sections I, II and III, respectively. Brown o- 

 trout appear to be adversely affected by summer flows of 

 250 cfs as indicated by the elevated summer mortality in ■ 

 Section II. These results suggest that the optimum flow for 

 adult rainbow trout exceeds 52 3 cfs while the optimum for 

 brown trout is lower, lying between 250 and 39 3 cfs. 



-' r ,. : .. i 1 J ,' . ^ ! 



Standing crops of mountain whitefish were estimated in 

 the upper 8,000 ft of Section I in September 1976 and 1977 

 (Table 11) . The estimated number and biomass of age III+ and 

 older whitefish in 1977 were 68 and 67%, respectively, of 

 those in 1976. This 33% reduction in the biomass of white- 

 fish coincided with a 25% reduction in the minimum summer '^ ': 

 flow (from 523 cfs in 1976 to 393 cfs in 1977). .• ■ 



Table 11. Estimated niombers and biomass (lbs) per 8,000 ft 

 of mountain whitefish in Section I of the 

 ,. Gallatin River in September 1976 and 1977. 

 -,. Approxmate 80% confidence interval in parenthesis. . 



-; , , ■ .. SECTION I - MOUNTAIN WHITEFISH JJ -■ ■ 



v^— - September 1976 September 1977 . 



Age -Group ..' N/8,000 ft lbs/8,000 ft N/8,000 ft lbs/8,000 ft 



III+ & Older 3,993(+737) 3,796(+739) 2,714(+382) 2,559(+343) 



Between April and September 19 77, the estimated number 

 and biomass of age III and older whitefish in Section II in- 

 creased by 62 and 9 3%, respectively. These increases probably 

 reflect the upstream movement of whitefish from the severely 

 dewatered downstream reaches of the Gallatin River. . 



43 



