The April 1977 estimates provide a comparative measure 

 of the abundance of the resident mountain whi'tefish in the 

 study sections. Section I, the least dewatered study section, 

 supported the highest population. The estimated number of 

 age III and older whitefish per 1,000 ft in Sections I, II, 

 and III in April 1977 was 467, 433, and 289, respectively. 

 The estimated biomass of whitefish per 1,000 ft in Sections 

 I, II, and III was 374, 335, and 255 lbs, respectively. These 

 estimates followed three successive above average water years 

 (1974, 1975 and 1976). The minimum summer flows measured in 

 1976 in Sections I, II and III were 523, 396 and 198 cfs, 

 respectively. 



The data suggest that summer flows of approximately 523 cfs 

 and greater would sustain the highest standing crops of adult 

 mountain whitefish. Flows of approximately 39 3 cfs are judged 

 undesirable on the basis of the 33% reduction in the biomass 

 of adult whitefish in Section I between 1976 and 1977. On 

 the basis of the 32% reduction in the biomass of adult white- 

 fish between Sections I and III in April 19 77, flows of ap- 

 proximately 19 8 cfs are judged undesirable. A minimum instream 

 flow of about 19 8 cfs is, therefore, suggested for adult white- 

 fish. The optimum flow for adult whitefish probably exceeds 

 523 cfs. 



Big Hole River - Reach #1 



Standing crops of brown and rainbow trout in a 4.5-mile 

 section of reach #1 of the Big Hole River were estimated in 

 September 1969, 1970, 1977 and 1978 (Table 12). The 1969 and 

 19 70 estimates are for rainbow trout greater than 7 inches 

 and brown trout greater than 10 inches. The 1977 and 1978 

 estimates are for rainbow trout greater than 10 inches and 

 age 11+ and older brown trout. It is assumed that the Sep- 

 tember estimates of trout standing crops primarily reflect the 

 magnitude of the dewatering that occurs in reach #1 during 

 the summer irrigation season. This assumption is supported 

 by Kozakiewicz (19 79) who measured fishermen use and harvest 

 during 1977 and 1978 on a 10-mile section of reach #1. He 

 concluded that angler harvest did not appear to be an immedi- 

 ate threat to the well-being of the trout populations and the 

 fishery resource would best be served by efforts to maintain 

 and enhance the habitat, especially stream flows. 



The distributions of the average daily flows during the 

 summer (June-September) preceding each estimate are given 

 in Table 13. These four summers include both below and above 

 average water years in which the level of dewatering ranged 

 from mild to severe. The minimum average daily flows mea- 

 sured in the summers of 1969, 1970, 1977 and 1978 were 208, 

 248, 173 and 408 cfs, respectively. 



44 



