FISHERY BULLETIN: VOL. 70, NO. I 



reaching a peak in September-October (Figure 

 5). In August, 1970, 3 months after lake 

 fertilization began, the underyearling sockeye 

 had twice the number of organisms per stomach 

 as in August, 1969 and contained about 60% 

 more food by weight. The high consumption in 

 September-October, 1970 represents an increase 

 of about 45^ r in number of organisms, and 40% 

 by weight, compared to the stomach contents per 

 fish in the same period in 1969. A slight decline 

 in number and a significant decline in weight 

 of food organisms per fish was shown from No- 

 vember, 1970 to February, 1971; an abrupt in- 

 crease occurred to a second high in March, which 

 represented an increase manyfold over March, 



1970. This increased food consumption occurred 



I to 2 months prior to their emigration from 

 the lake as yearling smolts. 



Five species of food organisms contributed 

 chiefly to the diet of underyearling sockeye in 

 Great Central Lake in 1970. In April and May 

 Bosviina contributed about 50 '^r of both the total 

 number (Figure 6) and total weight (Figure 7) 

 of all organisms found in their stomachs. The 

 numbers of Bosmhia consumed were insignificant 

 throughout the rest of 1970 and the first 3 

 months of 1971. Epischura was the most im- 

 portant food organism from May to July (Fig- 

 ures 6 and 7) and was probably the principal 

 source of energy for the rajiid growth of the 

 underyearlings during this period (see Figures 



II and 12). There was a transition in late July 

 and August when Cyclops, Holopedium, and 

 Daphnia gradually became more abundant in the 

 stomach samples. In the 3 months which 

 followed, September to November, Cyclops 

 and Holopedium were the predominant genera. 

 Cyclops continued in importance and formed 

 about 50% of the number of food organisms to 

 the end of January, 1971. However, during this 

 period of 6 months Cyclops formed only 15 to 

 30% of the food by weight whereas Holopedium 

 constituted 30 to 80% by weight. Diaptomus 

 was first observed in the stomachs in the latter 

 part of October, increased markedly in Decem- 

 ber and January, and was the predominant food 

 organism by number in February and March, 



1971. Thus Diaptomus was the most numerous 

 food organism in the stomach samples just before 



SOOr 



200 



I APR I MAY I JUN I JUL I AUG I SEP IQCT I NOV I DEC I JAN 1 FEB I MAR I 

 1970 19 7 1 



Figure 5. — Number and weight of all food organisms 

 per fish, for underyearling sockeye salmon from April, 

 1970 to March, 1971. 



Figure 6. — Food of underyearling sockeye salmon ex- 

 pressed as a percentage of the total number of organisms 

 from April, 1970 to March, 1971. 



'APR 'MAY 'JUN ' JUL ' AUG ' SEP ' OCT ' NOV 'DEC ' JAN ' FEB ' mar' 

 1970 1971 



Figure 7. — Food of underyearling sockeye salmon ex- 

 pressed as a percentage of the total weight, from April, 

 1969 to March, 1971. 



42 



