TABLE 12. — Frequency of occurrence of insects and plankton in sockeye salmon 

 captured in Brooks Lake, June 27 to September 12, 1957 



, . . Total Total Plaiikters Insects 



containing — 



T— — — 7— plankters insects per fish per fish 



Period 



Plankton 



June 27-July 31 

 August 1-31 

 September 1-12 



Total 



Number 



49 

 6A 

 89 



202 



Number Number Number Number Number 



83 45,042 1,850 919 22 



120 36,576 5,202 572 43 



27 101,289 314 1,U8 11 



230 182,907 7,366 



905 



32 



0-IO% r~l II -50% ^^ 51-100%! 



JUNE 27- JULY 31 

 AUGUST 1-31 

 SEPTEMBER 1-12 





I r i '' i ' i rVi' i i i ; ■ • ■ ••' . • ■ ' ■ ■ •• ■ •i' i i' 



25 50 75 



PERCENT 



100 



Figure 25,-- Percent fullness of stomachs from gill 

 net-caught juvenile sockeye salmon. Brooks Lake, 

 June 27 to September 12, 1957, 



The species composition of food items was 

 also compared through the season (table 13). 

 Four species of zooplankton ( Daphnia longispina, 

 Bosmina longirostrls, Bosmina coregoni, and 

 Cyclops sp,) made up over 96 percent of the 

 plankton. The relative percentages of each 

 species were almost identical in July and 

 September, whereas in August, Cyclops doubled 

 in frequency of occurrence and Daphnia oc- 

 curred only one-third as often. During this time 

 no major fluctuations in actual abundance of 

 plankton occurred in the lake, as shown in 

 part IV, the section on plankton distribution 

 and abundance. 



More than 76 percent of the insects eaten 

 by juvenile salmon were midges. This high 

 ratio of midges to other insects in stomachs 

 was quite constant through the summer in 

 contrast to marked differences In the species 

 composition of plankton in the diet from 



month to month. The abundance and avail- 

 ability of midges is apparently an important 

 factor in sockeye salmon ecology at Brooks 

 Lake, 



The absolute number of each kind of or- 

 ganism in stomachs probably is of little 

 significance, except as a relative measure 

 of the number of separate ingestions or food 

 selections made. Ninety-six percent of in- 

 dividual organisms were plankters, and only 

 4 percent were insects (table 13). 



Sockeye salmon juveniles fed upon a greater 

 variety of food than any other fish species 

 present in Brooks Lake (table 14). This food 

 included 9 different planktonic forms (10 

 including copepod nauplii), 19 insects, and 3 

 identified fish (sockeye salmon, sculpins, and 

 pygmy whitefish). Two other fish species in 

 Brooks Lake (whitefish and sticklebacks) are 

 competitors for plankton food. This competi- 

 tion was probably of little consequence in 

 1957, considering the high density of zoo- 

 plankton and low density of sockeye salmon, 

 whitefish, and sticklebacks. In years of greater 

 densities of juvenile sockeye salmon, however, 

 competition from sticklebacks might be a 

 serious problem. Krogius and Krokhin (1956a, 

 1956b) and Krogius (1951) found in Lake 

 Dalnee, Kamchatka, that survival of sockeye 

 salmon is directly related to the abundance 

 of threespine sticklebacks. 



All fish except blackfish were insectivorous, 

 and competition for winged insects may have 



38 



