42 



nonirrigation flows explain 81% of the annual variation in the fall 

 estimates of age IV and older rainbow trout. The other relationships 

 were not significant. 



Visual inspection of the plot of average daily flows for the 1965-75 

 period showed that low winter flows were not the principle factor 

 depressing numbers of yearling rainbow trout during the 16-month period 

 between fry emergence and the fall estimate. The 1975 estimate of year- 

 lings (796/6455 ft) followed the lowest winter flow period of the study. 

 The highest estimate (997/6455 ft) also followed low winter flows. 



Average daily flows during the 1965 through 1974 rainbow trout 

 spawning periods (March 1 - April 30) are compared in Figure 11. Visual 

 inspection shows that spawning flows devoid of violent fluctuations and 

 increasing from approximately 200 cfs to approximately 700 cfs during 

 the 61 day spawning period yielded the highest yearling estimates 

 18-months later. The estimated number of yearlings in 1966, 1967, and 

 1968 may reflect water quality problems in addition to spawning flows. 



Multiple linear regression analyses (Table 9) suggest the magnitude 

 of winter flows partially determines the numbers of spawning age rainbow 

 trout present in the spring. In combination, numbers of age I and older 

 trout in the fall and the mean flows between fall and spring estimates 

 explain 94% of the annual variation in spring estimates of numbers of 

 age II and older rainbow trout. When mean fall -spring flows were less 

 than approximately 275 cfs, estimates of numbers decreased between fall 

 and spring and increased when mean flows were greater than approximately 

 275 cfs (Table 10). The lowest mean winter flow (94 cfs) produced the 

 greatest decrease (33%). 



