17 



INTRODUCTION 



The effects of flow releases from Clark Canyon Reservoir on trout 

 populations in the Beaverhead River have been investigated by the Montana 

 Department of Fish and Game since 1966. The purpose of this study, which 

 was funded by the Bureau of Reclamation, was to measure changes in the 

 fishery caused by varied flow patterns, particularly during the non- 

 irrigation season when irrigation water is stored in the reservoir. The 

 study will provide biological criteria for recommending flows that will 

 maintain a desirable river fishery. 



METHODS 



Trout populations in the Beaverhead River were sampled using a boat 

 mounted electrofishing unit. The weight and total length of each 

 captured fish were measured to the nearest 0.02 pound and 0.1 inch, 

 respectively. Scales were taken for age determination. Some young-of- 

 the-year and yearling trout were permanently marked by removing an 

 adipose or pelvic fin. Trout >_8 inches were tagged with numbered T-tags. 

 All trout were marked with a partial fin clip and released for mark- 

 recapture population estimates. As many as three marking and two 

 recapture runs were made per estimate. Estimates of numbers, biomass, 

 age structures, mean weights by age groups, and appropriate confidence 

 intervals were calculated using methods summarized by Vincent (1971) 

 and adapted for computer analyses. 



Average daily flows at each study section were derived from U.S.G.S. 

 records and known daily irrigation withdrawals. A plot of the average 

 daily flows for each study section was obtained using computer techniques, 

 For statistical purposes, the average daily flows between successive fall 



