been acute. We had no direct way of detecting 

 such competition, but despite the superabund- 

 ance of insects in the area, no quantities of 

 floating dead insects were observed on the 

 lake. If a surplus of insects were present, 

 such concentrations would have been visible 

 on the surface in calm weather. Most species 

 of fish were easily caught with an artificial 

 dry fly anywhere on the periphery of the lake, 

 and trout were often observed feeding on 

 insects along the lake shore near the field 

 station, suggesting high utilization of floating 

 insects. 



There was no indication of serious predation 

 by other fish on sockeye salmon, although 

 lake trout, which are voracious fish-eaters, 

 could be a serious threat if present in suffi- 

 cient numbers. Lake trout, however, are not 

 numerous and do not grow as large in Brooks 

 Lake as in adjacent lakes. Rainbow trout were 

 mainly insectivorous in Brooks Lake in 1957, 

 corresponding with findings in British Colum- 

 bia lakes by Idyll (1942), who cites six papers 

 documenting the preference of rainbow trout 

 for insects, even when fish are readily avail- 

 able. 



Many of the statements in the foregoing 

 section on juvenile sockeye salmon in their 

 lacustrine environment are based on limited 

 data, and some of the conclusions are in the 



realm of conjecture. Further, food relation- 

 ships may be entirely different during the 

 winter (which has not been studied) or during 

 other years. Nevertheless, factual published 

 data on this subject are scant, and findings 

 presented will be useful in future comparative 

 studies at Brooks Lake and elsewhere. 



PARASITES 



A record was kept of external parasites 

 whenever they were seen and of internal 

 parasites encountered during the analysis of 

 stomach contents of fish caught in Brooks 

 Lake. Table 15 lists the parasites and their 

 hosts. The occurrence of the cestode 

 Triaenophorus indicates that sockeye salmon 

 are occasionally piscivorous because the adult 

 Triaenophorus requires an intermediate fish 

 host for maturation. Young sockeye salmon 

 could only have become infected by eating an 

 infected fish. Lake trout contained both larval 

 and mature forms of Triaenophorus, as might 

 be expected since they occasionally feed on 

 sculpins, sockeye salmon, and sticklebacks. 



The parasitic copepod Salmincola eduardsii 

 was commonly found around the mouths and 

 gills of adult sockeye salmon in the lake but 

 was never found on adults examined as they 

 entered the lake attheweir trap. This indicates 



TABLE 13. — Plaakrters and insects contained in stomachs of sockeye salmon captured in Brooks Lake, 



June 27 to September 12, 1957 



39 



