FISHERY BULLETIN: VOL. 74, NO. 4 



Table 2.-Average percentages of different taxa (by weight) in the food of juvenile chinook salmon 

 compared with percentages of those organisms in the day drift (in parentheses). H = heated stream, 

 C = control stream. 



30 



J FMAMJJASONDJ FMAMJ JASOND 

 1972 1973 



Figure 6.-Seasonal changes in the biomass of food organisms 

 present in the day drift. Each point is the mean of two triweekly 

 samples. ^ 



Moreover, seasonal patterns in drift differed 

 greatly between 1972 and 1973, with both streams 

 exhibiting higher drift biomasses during the 

 second year than during the first. Although 

 benthic biomasses were significantly greater in 

 the control than in the heated stream, (P<0.001, 

 paired ^-test), these differences were often not 

 translated into drift; in fact, during the latter part 

 of 1972 and spring 1973, more food was available in 

 the heated stream. No explanation was found for 

 increased drift in 1973 relative to 1972, but it 

 appeared that increased food availability in 1973 



E 



o 



25 



20 



A- Heated 

 D - Control 



J FMAMJ J A SON D J FMAMJ JASOND 

 1972 1973 



Figure 7.— Seasonal changes in the biomass of food organisms 

 present in the riffle benthos. Each point is the mean of two 

 triweekly samples. 



resulted in more growth, higher biomasses, and 

 increased production of fish in the heated stream. 

 Why production in the control stream population 

 did not reflect the greater abundance of food is not 



770 



