PEARCY ET AL: DIEL FEEDING HABITS OF PACIFIC SALMON 



100 



50 



O 



SET NUMBER 



I 2 3 4 5 6 7 8 9 10 II 12 

 n 1 1 1 1 p 1 1 1 1 1 <~ 



SOCKEYE 



-fe ^y" , " v ;w^ 



6 I 6 15 9 9 15 5 10 4 10 (NO. OF FISH) 



100- 



50- 



'-■■^■■' V 



CHUM 



3 4 1 14 2 5 1 3 4 



100 



50 



° SO 



COHO 





2 4 



1800 | 0000 



SS SR 



TlME(hrs) 



0600 



1200 



sunset (2158-0430 h), when they comprised over 

 65% of the food and were often in fresh condition. 

 All three fish with over trace amounts of food in 

 the set that fished from 2359 to 0224 h contained 

 100% euphausiids. Squids and amphipods are 

 most important in the afternoon (1600-2020 h), 

 and fresh squids were found in stomachs of pink 

 salmon caught from 1800 to 2020 h, just before 

 sunset. Fishes and amphipods were the most 

 important prey during the morning daylight 

 period. 



Chum Salmon 



A diel trend for this species, which fed on a 

 variety of prey taxa, was less obvious than for 

 other species of salmon (Fig. 4). Salps composed 

 over 75% of the stomach contents during the after- 

 noon (1200-2020 h). Euphausiids were the most 

 common prey taxa from sunset to the last set at 

 midday, with the exception of a single chum salm- 

 on caught at 2158-0025 h whose stomach con- 

 tained many salps and a salmon caught at 0358- 

 0627 h whose stomach contained 95% fish. Most 

 euphausiids in the stomachs of fish caught about 

 the time of sunset (1957-2227 h) appeared to be 

 recently ingested. Squids, which were only a 

 minor part of the stomach contents, are not indi- 

 cated in Figure 4. 



FIGURE 4. — Diel variations in the percent composition of major 

 prey taxa in the stomachs of the four species of salmon contain- 

 ing more than trace amounts of food. E = euphausiids, A = 

 amphipods, F = fishes, SQ = squids, S = salps. Open symbols 

 show when fresh prey was common. The number under each 

 figure indicate the number of fish with more than trace amounts 

 of food in their stomachs. SS = sunset; SR = sunrise. 



stomach, indicating a switch from amphipods to 

 euphausiids during dusk. Euphausiids comprised 

 <30% of the food in sets before sunset and after 

 sunrise, and no fresh euphausiids were noted dur- 

 ing these daytime periods. Amphipods and fishes 

 formed the highest percentage of the food during 

 daytime. Squids were also eaten by sockeye salm- 

 on and were most important during late afternoon 

 and sunset (1800-2227 h) and during sunrise 

 (0350-0627 h). Fresh squids were noted in 

 stomachs of fish caught in sets that fished from 

 1957 to 0025 h. 



Pink Salmon 



As with sockeye salmon, euphausiids attained 

 peak importance as prey for pink salmon after 



Coho Salmon 



Coho salmon fed mainly on euphausiids during 

 the night and on squids during the day. 

 Euphausiids were not observed in stomachs of 

 coho salmon during the afternoon but increased in 

 importance from to 100% of the stomach volumes 

 between 1800 and 0240 h (Fig. 4). Most of the 

 euphausiids during this period were in fresh con- 

 dition. Euphausiids also comprised most (>60% > 

 of the stomach contents during the morning hours 

 (0159-0832 h) but were never fresh. Squids were 

 the most important prey of coho salmon caught 

 during the afternoon-daylight period and in the 

 last set in late morning. Amphipods and fishes 

 were of minor importance. 



DISCUSSION 



The larger catches of salmon in surface gill nets 

 during twilight-night periods than in daytime 

 periods have three possible explanations: Avoid- 

 ance of nets during the daytime when visual 

 acuity of salmon is highest, increased swimming 



395 



