FISHERY BULLETIN: VOL. 70. NO. I 



diel migration. The tows were made to deter- 

 mine what portion of different food organisms 

 contributed to the salmon's ration during the 

 day and night, as well as during the period of 

 their diel migration. Data on depth and time 

 of each tow, the number, size range, and mean 

 length of sockeye caught, together with those 

 sampled for the number and species of food 

 organisms, and the weight of the food as a per- 

 centage of the body weight are given in Table 2. 



The degree of freshness of food organisms 

 was arbitrarily determined as fresh, fragmented, 

 or largely unidentifiable. Fresh food was des- 

 ignated when no indication of digestion had oc- 

 curred. The percentage of fresh zooplankton in 

 the stomachs is given in Table 2. Depth of the 

 densest portion of the layer of juvenile sockeye 

 at different times of the day and night is indi- 

 cated in Figure 10a by a broken line. The depth 

 and time of each trawl tow relative to the depth 

 of the fish is also indicated in Figure 10a. 



The 24-hr data collection shows that in the 

 day the densest poi'tion of the layer of juvenile 

 sockeye was formed at 75 m where the temper- 

 ature of the water was 4° C; of the fish exam- 

 ined for stomach contents (Table 2) from tow 

 No. 1, few food organisms per fish (Figure 10b) 

 were noted and only 5Vf of the species were in 

 fresh condition (Bosmina) . The remaining spe- 

 cies, Epischura, Cyclops, and Daphnia, were 

 digested. Tow No. 2, through a less dense sec- 



ondary layer at 105 m, indicated the same feed- 

 ing pattern. Young sockeye commenced to mi- 

 grate upward from 75 m between 1700 and 1800 

 hr. No differentiation in migration between 

 underyearling or yearling sockeye could be de- 

 tected at any level in the layer, either by net 

 sampling or from high frequency echograms. 



A tow just after sunset at a depth of 35 m 

 revealed that the fish were eating Bosmina and 

 Cyclops (Figure 10c) as they moved upward 

 and 22% of the contents were in fresh condition. 

 At 2200 hr the sockeye had passed 25 m 

 where the heaviest concentration of Cyclops and 

 Daphnia was located (LeBrasseur and Kennedy, 

 1972) ; in passing they had eaten Cyclops (Fig- 

 urelOc) . It should be recognized, however, that 

 there is a natural time lag between feeding at 

 any depth and the time the fish was captured by 

 the trawl at a shallower depth, as they migrated 

 toward the surface. 



At nautical twilight, most of the fish had com- 

 pleted their upward migration and were distrib- 

 uted in a layer between 10 and 20 m where tem- 

 peratures ranged from 6° to 12°C (Figure 10a), 

 Echograms indicated many of the juvenile sock- 

 eye salmon appeared to spend brief periods be- 

 tween and 10 m at temperatures ranging from 

 14° to 23° C, during which time the young fish 

 fed heavily upon Epischura (Figure 10c). In 

 the 4 hr between the beginning and end of nau- 

 tical twilight no feeding occurred (Table 2). 



1 Number in parentheses is number of items In sample which contained no food in stomachs. 



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