FISHERY BULLETIN: VOL. 74, NO. 4 



heated stream was about twice as high in 1973 as 

 in 1972, while average biomasses were slightly 

 reduced following repairs in the control stream. 

 Peak production in both streams occurred from 

 April to June (Table 1, Figure 3), this being related 

 to the high growth rates that took place during 

 spring. Differences in production between the 

 streams, however, were related primarily to 

 higher population biomasses maintained in the 

 control stream than in the heated stream, rather 

 than to differences in growth rate. 



Production of salmon in the heated stream 

 during the spring, 1973, was higher than in the 

 spring 1972 (Figure 3). The fish were stocked as fry 

 in 1973, whereas in 1972 they were introduced as 

 eyed eggs. The low average growth rate and 

 survival (Table 1) of fish reared in the heated 

 stream from the egg stage suggest that produc- 

 tion was influenced by conditions during early 

 development. Some individuals grew very rapidly 

 during their first few weeks of residence; others 

 apparently did not make the transition to feeding 

 in the heated stream and died from the effects of 

 starvation. Negative production occurred during 

 fall months, when many fish had stopped growing 

 and some were losing weight. 



3Sr 



30- 



25 



.2 20 



S 15 



10- 



A - Meoied 

 a - Coo'fol 





// 



J F M AM JJASONDJ FMAMJ JASOND 

 1972 1973 



Figure 3.-Cumulative production of juvenile chinook salmon 

 during 1972 and 1973. 



Direct Temperature Effects on Growth 



Relationships between average relative growth 

 rate and food consumption rate of juvenile chinook 

 salmon held in water from the model stream 

 (Figure 4) showed that differences between fish 

 held in heated and unheated water were greatest 

 at low rations and least at high rations. At low 

 rations, control individuals were most efficient; at 

 high levels, there was no appreciable difference 



768 



except during spring when the elevated tempera- 

 ture facilitated increased food consumption and 

 growth efficiency. The highest rations were close 

 to the maximum amount of food that the young 

 salmon would eat at one feeding in a day, and the 

 graphs for summer and fall indicate that max- 

 imum consumption declined as individuals' size 

 increased. 



The relationships observed in the experiments 

 between temperature, ration level, and fish size 

 were consistent with the results of laboratory 

 studies of sockeye salmon, 0. nerka (Brett et al. 

 1969; Brett and Shelbourn 1975); coho salmon 

 (Averett 1969); and steelhead trout, Salmo gaird- 

 neri, (Wurtsbaugh 1973). At low levels of food 

 availability, increased metabolic requirements 

 associated with elevated temperature resulted in 

 reduced growth rates; at high levels of food 

 availability, growth rates were not appreciably 

 altered by thermal increases. If responses of 

 juvenile chinook to the range of ration levels in the 

 aquarium growth experiments approximated 

 growth of fish in the model streams at differing 

 consumption rates (Carline and Hall 1973), the 

 growth rate data of Table 1 suggest that during 

 most of the year the fish were feeding well below 

 their maximum possible consumption. Only during 

 certain periods in late winter and spring did 

 growth rates approximate the maximum rates 

 shown in Figure 4. From this we concluded that, 



25 



.. ,15 



J — i — 6 — t~is — rt — <r 



25 3 



30 2- 



15 1- 

 lO 



^^ 



k 



»r 



Food Consumption Role ( X / day I 



Figure 4. -Seasonal changes in the growth rates of juvenile 

 chinook salmon. E.xperiments continued for 10 days and were 

 preceded by 10 days of acclimation to temperature and ration 

 size. Plotted values of growth rate at each feeding level were 

 based upon the following numbers of fish: winter - 20; spring - 20; 

 summer - 12; and fall - 10. Mean caloric contents (kcal) of the fish 

 at the beginning of each experiment were; winter - 0.59; spring - 

 1.26; and summer - 7.05; and fall - 8.37. 



