DIEL VARIATIONS IN THE FEEDING HABITS OF PACIFIC SALMON 



CAUGHT IN GILL NETS DURING A 24-HOUR PERIOD 



IN THE GULF OF ALASKA 



W. Pearcy, 1 T. Nishiyama, 2 T. Fujii, 3 and K. Masuda 3 



ABSTRACT 



Changes in prey composition and stomach fullness indicate diel variations in feeding behavior of 

 sockeye, pink, and coho salmon caught in surface gill nets set for 2 hours each over a 24-hour period at a 

 station in the Gulf of Alaska. All of these species of salmon switched from feeding primarily on squids, 

 fishes, and amphipods during the day to euphausiids at night. Apparently dense concentrations of 

 euphausiids can be exploited by salmon in surface waters at very low light intensities, even during an 

 overcast night. Day-night changes were less obvious in the food of chum salmon, which fed largely on 

 salps. Total catches of salmon and catches in the near-surface portion of the gill nets were highest 

 between sunset and sunrise, suggesting that diel vertical movements contribute to the higher night 

 than day catches of surface gill nets. 



Although many studies have been published on 

 the feeding habits of salmonids in oceanic waters 

 of the North Pacific Ocean (Andrievskaya 1957; 

 Allen and Aron 1958; LeBrasseur 1966; Ito 1964; 

 Manzer 1968; Takeuchi 1972 ), most studies of daily 

 feeding patterns have been conducted on juvenile 

 salmon in fresh water or in coastal waters. These 

 have generally shown that juvenile pink, sockeye, 

 and chum salmon are diurnal or crepuscular 

 (dawn and dusk) feeders (see Godin 1981 for re- 

 view). The few studies conducted on diel feeding 

 variations of adult or maturing Pacific salmon in 

 oceanic waters of the northwestern Pacific Ocean 

 have not revealed a consistent pattern (Machidori 

 1968; Shimazaki and Mishima 1969; Ueno et al. 

 1969). 



To further elucidate the diel feeding patterns of 

 these fishes, we collected and examined stomach 

 contents of four species of Pacific salmon caught in 

 the Gulf of Alaska during one 24-h period. 



METHODS 



Two gill nets, each 800 m long and 6 m deep, 

 with 300 m of 115 mm, 250 m of 121 mm, and 250 m 

 of 130 mm (stretch) mesh, were alternately fished 



'School of Oceanography, Oregon State University, Corvallis, 

 OR 97331. 



institute of Marine Science, University of Alaska, Fairbanks, 

 AK 99701. 



3 Faculty of Fisheries, Hokkaido University, Hakodate, Hok- 

 kaido, Japan. 



for about 2-h periods over a 24-h period in the Gulf 

 of Alaska from the Oshoro Maru, training ship of 

 the Faculty of Fisheries, Hokkaido University, 

 Hakodate, Hokkaido. The first net was set at 1200 

 h local time ( GMT - 9 h ) on 13 July; the last set was 

 hauled at 1206 h on 14 July 1981 (Table 1). The time 

 that the gill nets were fishing varied in the first 11 

 sets from 140 to 152 min (from start of set to start of 



TABLE 1. — Summary of gill net sets and catches for salmon. 

 13-14 July 1981. 



Manuscript accepted November 1983. 

 FISHERY BULLETIN: VOL. 82, NO. 2, 1984. 



391 



