FISHERY BULLETIN: VOL. 74, NO. 4 



stream than in the control were due to the high 

 densities of filamentous algae in the riffles and the 

 considerable accumulation of organic detritus in 

 the pools. Increased primary production associated 

 with elevated temperature in laboratory streams 

 has been measured by Kevern and Ball (1965) and 

 Phinney and Mclntire (1965). The dominant algal 

 species in our model streams, Cladophora glome- 

 rata, grows rapidly at high temperatures 

 (Whitton 1971; Adams and Stone 1973). 



Heavy growths of algae on the riffles apparently 

 accelerated sedimentation rates in the heated 

 stream (Table 4) by acting as filters to trap and 

 consolidate fine particles introduced with ex- 

 change water. In the pools, where filamentous 

 algae did not grow, fine sediment levels in both 

 streams were similar. By indirectly enhancing 

 sediment accumulation, elevated temperature 

 probably had an important effect on the numbers 

 of food organisms available to salmon in the 

 heated stream. Hynes (1960) described how silta- 

 tion alters the habitat of many invertebrates, with 

 the result usually being a reduction in benthic 

 biomass (Cordone and Kelly 1961). Greatly 

 reduced mean annual biomasses of Ephemerop- 

 tera, Plecoptera, and Trichoptera in the heated 

 stream (Table 3) compared with the control sug- 

 gest that these groups were influenced by the 

 amount of fine sediments in the substrate, and 

 these insects were often preferred food items of 

 the fish (Table 2). 



Table 4. -Levels of fine sediments, expressed as grams dry 

 weight per square meter, in the model streams during May 1974. 

 The figures in parentheses refer to the amount of time that had 

 elapsed since a major disturbance to the riffles. 



Our study was designed to examine the effects 

 of elevated temperature on the production of 

 juvenile chinook salmon. The constantly elevated 

 temperature was not meant to simulate a par- 

 ticular type of thermal increase, but was within 

 the range of temperature elevations caused by 

 heated discharges into running waters (Wilber 



1969, Parker and Krenkel 1970), irrigation runoff 

 (Eldridge 1963), and removal of streamside vege- 

 tation (Brown and Krygier 1970). It was also 

 within the limits of temperature increase legally 

 allowed by some regulations (Burd 1969). 



Both direct and indirect temperature effects 

 influenced chinook salmon production, but the 

 magnitude of these effects varied seasonally. 

 Production was high in spring because tempera- 

 ture was in a range that was favorable to growth, 

 parasitism had not yet become an important 

 factor, and the small fish were able to efficiently 

 exploit available food. Summer was generally a 

 period of declining production because high tem- 

 peratures resulted in an increase in maintenance 

 requirements and, for the control stream, because 

 parasites had attacked the majority of the 

 population. Low production during late summer 

 and fall was associated with high levels of infes- 

 tation and the ineffectiveness of large fish in 

 exploiting small organisms that were abundant in 

 the drift. 



The lack of correlation that existed between 

 growth rates (Table 1) and food availability 

 (Figure 6) may have been related to the species 

 composition of drifting invertebrates. A high 

 percentage of summer and fall drift was composed 

 of very small forms such as oligochaetes (Nais 

 communis) and chironomids (Table 2). During 

 those seasons, tiny organisms were not preferred 

 food items of the young salmon, which were larger 

 and less numerous than during the spring. High 

 growth rates exhibited by fish during winter and 

 spring when drift rates were comparatively low 

 suggest that smaller, more abundant fish were 

 able to utilize the entire range of sizes of inverte- 

 brate species that left the substrate. It was im- 

 possible to determine whether food size prefer- 

 ence affected fish in the two streams identically, 

 but based on overall invertebrate composition 

 (Table 3), taxa containing species of large size 

 (Ephemeroptera, Plecoptera, Trichoptera) were 

 more abundant in the control than in the heated 

 stream. This was reflected in higher growth rates 

 of salmon in the control than in the heated stream 

 during summer and fall. Clearly, more intensive 

 examination of the relationship between prey size 

 and prey selection by salmonids is needed. 



Low benthic invertebrate biomasses in the 

 heated stream were associated with increased 

 sedimentation rates and reduced numbers of taxa. 

 Iverson (1972) suggested that the poor success of 

 certain invertebrates in the heated stream was 



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