74 



(1968) measured the rate of change of cover (overhanging vegetation and 

 undercut banks) and current velocity with respect to flows. He concluded 

 that cover was not greatly reduced by high levels of dewatering, while 

 current velocity showed the greatest rate of change of the physical 

 parameters measured. When flows are reduced, the preferred habitat of 

 older rainbow trout in the Hildreth Section may be diminishing at a 

 faster rate than that of older brown trout. This may account for the 

 elevated rates of population decrease of older rainbow trout during low 

 flow periods. 



Poor reproductive success was the major factor limiting the total 

 numbers and biomass of trout in the Hildreth (2) Section. Reproductive 

 success throughout much of this study was inadequate for providing suf- 

 ficient numbers of trout to fill all available living space. It is 

 doubtful populations of brown trout of age groups I, II and III and 

 rainbow trout of age groups I and II reflect the carrying capacity of 

 the Hildreth Section, even during low water years. The simultaneous 

 increases in the estimated total numbers and biomass of brown and 

 rainbow trout, which occurred in the Hildreth Section during the last two 

 years of the study, demonstrate the inadequacy of reproductive success 

 in previous years. These increases, which coincide with some of the 

 lowest mean nonirrigation flows of the study, were the result of two 

 strong year classes (1973 and 1974) in succession. With flow releases 

 favorable to both reproduction and the survival of older trout, the 

 Hildreth Section is capable of supporting greater numbers and biomass 

 of trout of all age groups than those which existed throughout much of 

 this study. 







