BISSON and DAVIS: PRODUCTION OF JUVENILE CHINOOK SALMON 



Table 3.- Annual average biomasses (cal/m^), drift rates (cal/m^), and drift ratios of selected aquatic taxa, excluding 

 winged adults. Drift ratios were calculated according to the formula (day drift/riffle biomass) x 10-''. Asterisks denote 

 values for the heated stream that were significantly different (P<0.05, single classification analysis of variance) from 

 the control. 



'Gyraulus sp. 



known, although severity of infestation by Nano- 

 phyetufi was not compared over the 2 yr and may 

 have been more serious in 1973. 



In 1972, drift ratios (the ratio of drift to 

 biomass) of several invertebrate taxa were higher 

 in the heated stream than in the control (Table 3). 

 The next year some of the drift ratios increased, 

 and although many were higher in the control 

 stream, the differences were not statistically 

 significant. Of taxa showing increased drift ratios 

 in the heated stream, Ephemeroptera and Plecop- 

 tera were most consistently influenced by elevated 

 temperature. In 1972, Chironomidae also exhibited 

 a significantly greater tendency to drift in the 

 heated stream than in the control. These three 

 groups were important components of both the 

 day drift and the diet of juvenile salmon and often 

 contributed to the greater availability of food in 

 the heated stream than in the control during 

 certain periods. 



Fewer macroinvertebrate taxa were present in 

 the heated stream than in the control. Paired 

 f-tests indicated that number of taxa were 

 significantly different in both riffles (treatment 

 mean = 21, control mean = 34; P<0.001) and pools 

 (treatment mean = 16, control mean = 19, 

 P<0.01). Most of those taxa that were unique to one 

 stream or the other were very rare and contributed 

 little to fish production. Major biomass differences 

 arose because many taxa had greater population 

 densities in the control while only a few fared 

 better in the heated stream. The several taxa that 

 did exhibit higher biomass in the heated stream 

 were very abundant and tended to dominate the 

 bottom fauna to a greater extent than did common 

 taxa in the control. The two most abundant in- 

 vertebrates in the heated stream were Oxytrema 

 silicula in the riffles and Limnodrilus sp. in the 

 pools. Neither of these two species was consumed 



in significant quantities by the young salmon; 

 thus, increased dominance in the heated stream 

 did not give rise to greater food availability. 



Periphyton Biomass and Sedimentation 



Plants and detritus were significantly more 

 abundant in the heated stream (P<0.001, paired 

 f-test) than in the control (Figure 8). The greater 

 amounts of plants and detritus in the heated 



D 



Meo'ed 



Control 



RIFFLES 



O 

 O 



- 1 



o 



^ 20 



c 

 



r 15 



10 



POOLS 



I  I ' 1 I I — 1_ 



JFMAMJJASONDJFMAMJJ A S O N D 

 1972 1973 



Figure 8.-Biomasses of plants and detritus in riffles and pools of 

 the model streams. Each point is the mean of two triweekly 

 samples. 



771 



