BISSON and DAVIS: PRODUCTION OF JUVENILE CHINOOK SALMON 



during most of the year, the experimentally 

 elevated temperature contributed directly to the 

 reduced growth and production of the fish. 



Disease 



An unexpected indirect effect of elevated tem- 

 perature was apparent protection from infesta- 

 tion by an intermediate stage of the trematode 

 Nanophyetus salmincola, which was present in the 

 streams from late spring through fall. Infective 

 cercaria emerged from the snail Oxytrema silicula 

 to encyst in the skin and tissues of juvenile 

 Chinook as metacercaria. The distinction between 

 heavy vs. light infestation was made visually and 

 was somewhat arbitrary (Figure 5): conspicuous 

 bumps at the base of the caudal peduncle, darken- 

 ing of fins, and papules on the body surface were 

 considered symptoms of heavy infestation. While 

 the parasite was obviously present in 1972, it was 

 not until after its appearance in 1973 that at- 

 tempts were made to quantify its effects. 



Infestation rates in the heated stream remained 

 low through summer and early fall and increased 

 until termination of the experiment. Heavy in- 

 festations were present in most of the control fish 



HEATED STREAM 



CONTROL STREAM 



..°-^. 



a— a 



V 



J J A S O N D 



-D— L.9M 



a. ^ 



v^^ 



V 



J J A S O N D 



19 73 



Figure 5.-Infestation rates and weight differences of juvenile 

 Chinook salmon infested by metacercaria of Nanophyetus 

 salmincola. 



soon after cercaria had begun emerging from the 

 snails. In addition, a greater difference existed 

 between the mean weights of heavily and lightly 

 infested individuals in the control stream than in 

 the heated stream. The impact of this parasite 

 thus appeared to be more severe in the control 

 than in the heated stream. 



Food Availability 



An understanding of changes in food availabili- 

 ty required: 1) that preferred food items be 

 identified, 2) that it be determined when they were 

 available for consumption, and 3) that their rela- 

 tive abundance was estimated under comparable 

 circumstances. In this study, the second require- 

 ment was met through observation; food organ- 

 isms became available only when they entered the 

 drift and then mainly during daylight. Unlike 

 many other salmonids, juvenile spring chinook 

 salmon placed in the model streams were never 

 seen feeding on invertebrates in the benthos. The 

 extent of feeding during darkness was not deter- 

 mined, but was believed to be small. Identical 

 sampling procedures were assumed to fulfill the 

 third requirement, although differential con- 

 sumption of food before it entered the drift nets 

 could have caused some error. 



Oligochaetes were almost completely excluded 

 from the diet of large fish even though they 

 composed an important fraction of the drift (Table 

 2). Mollusca (exclusively Gyraulus sp.) and Tri- 

 choptera were comparatively large food items and 

 were consumed more readily by large fish than by 

 small fish. Ostracod Herpetocypris chevreuxi was 

 taken throughout the year in proportion to its 

 relative abundance, while Ephemeroptera and 

 Chironomidae— generally small organisms that 

 were usually numerous in the drift— were 

 preferred by smaller fish although these groups 

 were always major components of the diet. In 

 general, differences in food habits between 

 populations in the streams were related to 

 differences in the relative abundance of various 

 food groups. One exception was the greater con- 

 sumption of terrestrial forms (primarily aphids 

 and spiders) by fish in the heated stream, despite 

 approximately equal input of these invertebrates 

 into both streams. 



Measurements of food organisms drifting dur- 

 ing daylight hours (Figure 6) were not well cor- 

 related with measurements of the biomass of 

 those organisms in the riffle benthos (Figure 7). 



769 



