BARRACLOUGH and ROBINSON: LAKE FERTILIZATION. III. 



smelt migration in 1971. Bosmina was the most 

 numerous in the previous year. The few chiro- 

 nomid larvae (Figure 6) in the diet of juvenile 

 soekeye from December, 1970 to March, 1971, 

 formed 30 to 70 7^- by weight of all the food or- 

 ganisms (Figure 7). The importance of chiro- 

 nomid larvae during the winter months was ob- 

 served also in the previous winter (Figures 3 

 and 4). 



FOOD OF YEARLING (AGE 1) SOCKEYE 



Those underyearling soekeye in Great Central 

 Lake in 1969 which did not migrate to sea as 

 yearling smolts in 1970, but remained in the 

 lake for a second year, attained a mean length 

 of only 51 mm and weight of 1.1 g between the 

 latter part of April and May, 1970, whereas 

 the migrating smolts had a mean length of 70 mm 

 and weighed 3.5 g. The yearling soekeye which 

 remained in the lake were collected from samples 

 taken at all six stations and not from the end 

 of the lake where smolts were schooling and 

 heading seaward. Reference will be made later 

 to the fact that the smallest size smolt, caught 

 in the Robertson Creek weir (Figure 1) or in 

 the nets set to capture smolts in the Stamp River, 

 measured 55 mm and weighed 1.5 g. 



Food organisms found in the stomachs of the 

 yearling soekeye were similar to those eaten by 

 the underyearlings during most of the year in 

 1970, but the yearling soekeye were more se- 

 lective in cropping the larger forms of zooplank- 

 ton (Figure 9). Both the underyearling and 

 yearling soekeye fed heavily upon Epischura 

 from May to July (Figures 6 and 9) , but it was 

 evident from the large numbers and weight of 

 food organisms per fish (Figure 8) that the year- 

 ling soekeye elected to feed or were able to prey 

 more heavily upon Epischura (Figure 9) than 

 the underyearlings during September and Oc- 

 tober. Few yearling soekeye were caught in the 

 trawls during the winter months of 1970-1971 

 prior to their migration as 2-year-old smolts. 

 Diaptomus, Holopedium, and Cyclops were in 

 the stomachs of these fish. 



° 500- 



400 



100 



TBO 



-70 



60 



A-A, 



50 



30 ^ 



20 * 



-10 



°l APR : MAY I JUN I JUL UUG I SEP I OCT I NOV I DEC I JAN I FEB I MAR P 

 1970 1971 



Figure 8. — Number and weight of food organisms of 

 yearling soekeye salmon from April, 1970 to February, 

 1971. 



APR MAY JUN JUL I AUG I SEP I OCT I NOV I DEC I JAN I FEB 



1970 



1971 



Figure 9. — Food of yearling soekeye salmon expressed 

 as a percentage of the total number of organisms from 

 April, 1970 to February, 1971. 



DIEL FEEDING OF JUVENILE SOCKEYE 



From midafternoon on June 17 to midday on 

 June 18 a series of 11 tows, each of 15 min dur- 

 ation, were made with an Isaacs-Kidd midwater 

 trawl. The trawl was towed through the middle 

 of the densest portion of the stock during their 



43 



