BARRACLOUGH and ROBINSON: LAKE FERTILIZATION. III. 



500,- 



-i50 



AUG 



SEPT  OCT 

 1969 



NOV 



DEC JAN 



FEB MAR 

 1970 



APR 



Figure 2. — Average number and weight of all food or- 

 ganisms (all species combined) per fish for underyearling 

 sockeye salmon in Great Central Lake from August, 1969 

 to April, 1970. 



lOO 



AUG I SEP I OCT I NOV DEC JAN FEB 'MAR APR 

 1969 1970 



Figure 3. — Food of underyearling sockeye salmon ex- 

 pressed as a percentage of the total number of organisms 

 from August, 1969 to April, 1970. 



Table 1. — List of organisms found in juvenile sockeye 



stomachs. 



Size range 



Organism 



mm 



0.3-0.6 Bosmina, usually B. coregoni 



0.6-1.1 Cyclops, usually C. biscuipidatus thomasi and C. vernalis 



0.9-1.2 Holopedium gibberum 



0.9-1.5 Daphnia, usually D. longiremis 



0.8-1.3 Diaptomus, usually D. oregonensis 



2.0-2.5 D. kenai 



1.1-1.9 Epischura nevaiemii 



3 Insects of the order Diptera (other than Chironomidae) 



3-5 Insects of the family Chironomidae— larvae 



8-11 Insects of the family Chironomidae— larvae 



6-11 Larvae of the sculpin, Cottus asper 



Bosmina and Cyclops increased gradually from 

 less than 5 Sf in August to a peak of 30 to 50 % in 

 January-February, 1970. Chironomid larvae 

 were the only organisms eaten from February 

 to early March and in turn were replaced by Bos- 

 mvna (SO^r) in late March and April. The per- 

 centages of Epischura and Holopedium in the 

 stomachs by number (Figure 3) and by weight 

 (Figure 4) were similar from August to De- 

 cember. There was a pronounced difference 

 between the percentages by number and by 

 weight of Cyclops and Bosmina per fish. Al- 

 though the percentage by numbers of both 

 organisms per fish increased markedly between 

 December, 1969 and February, 1970, the per- 

 centage weight per fish remained less than 7% 

 for Bosmina and never exceeded 20% for Cy- 

 clops. The importance of the chironomid larvae 

 in the diet of underyearling sockeye from Feb- 

 ruary through March to early April, 1970 is 

 more indicative when expressed as a percentage 

 by weight (Figure 4) than by number of or- 

 ganisms (Figure 3). 



AUG I SEP ' OCT I NOV ' DEC ' JAN ' FEB ' MAR 

 1969 197 



Figure 4. — Food of underyearling sockeye salmon ex- 

 pressed as a percentage of the total weight of organisms 

 from August, 1969 to April, 1970. 



The fry which emerged in late March and 

 April, 1970 commenced to feed actively by mid- 

 April and, as juvenile sockeye, they continued 

 to increase their intake in number and weight 

 of all food organisms throughout the summer, 



41 



