FOOD HABITS OF JUVENILE SALMON IN 

 THE OREGON COASTAL ZONE, JUNE 1979 



William T. Peterson, 1 Richard D. Brodeur, 2 and William G. Pearcy 2 



ABSTRACT 



Euphausiids, hyperiid amphipods, crab larvae, and fishes were the important prey identified from 

 stomachs of 408 juvenile salmon collected in a purse seine along the Oregon coast in J une 1979. Food 

 habits of juvenile salmon differed among species. About 95% of the weight and numbers of prey of 

 chum salmon consisted of euphausiids and hyperiids. Euphausiids and hyperiids were numerically 

 the most abundant prey items of juvenile coho and chinook salmon, but, on a weight basis, over half 

 the stomach contents consisted of fishes. 



Variability in food habits was high for both juvenile coho and chinook salmon. Fishes from only 

 2 of 45 station pairs (coho) and 3 of 28 (chinook) had diet similarities >75%. The statistical relation- 

 ship between weight of euphausiids and weight of fishes in stomachs for coho and chinook juveniles 

 showed a strong tendency for both species to contain large amounts of either fishes or euphausiids, 

 but not both simultaneously. 



Diet overlap between coho and chinook juveniles was high overall, but low between the same 20 

 mm size classes of these same species. Euphausiids were eaten in equal numbers throughout the 

 100-200 mm coho size range; euphausiids were not eaten by chinook <180 mm fork length. 

 Hyperiids were mainly eaten by 180-220 mm coho and by 140-180 mm chinook. Fishes were con- 

 sumed mainly by juveniles of both species >160 mm. 



Based on estimated zooplankton standing stocks, an average (160 mm) coho salmon would have to 

 search and consume all prey in a minimum volume of about 2-8 m 3 per day to fill its stomach. The 

 average abundance of juvenile coho, as determined from purse seining, was 1 smoltper 11,500 m 3 ,or 

 about 1,440-5,760 times the minimum search volume. These data are related to the question of 

 whether food limitation exists for juvenile salmonids in the sea. 



Our knowledge of the ecology of salmon in the 

 ocean, especially during early juvenile life, is 

 scant compared with our understanding of the 

 freshwater phase of salmon life. The first few 

 months that juvenile salmon spend at sea have 

 been identified as a critical period when year- 

 class success may be affected (Gunsolus 1978 3 ; 

 Walters et al. 1978; Healey 1980). Basic studies of 

 abundance and distribution, growth, mortality, 

 and feeding habits of young salmon during their 

 first few months at sea are needed to evaluate 

 how the ocean environment and the density of 

 juvenile salmon affect the production of adult 

 salmon. 

 This paper contributes new information on 



■School of Oceanography, Oregon State University, Corval- 

 lis, Oreg.; present address: Marine Sciences Research Center, 

 State University of New York-Stony Brook, Stony Brook, NY 

 11794. 



2 School of Oceanography, Oregon State University, Corval- 

 lis, OR 97331. 



3 Gunsolus, R. T. 1978. The status of Oregon coho and 

 recommendations for managing the production, harvest, and 

 escapement of wild and hatchery-reared stocks. Intern, rep., 

 59 p. Oregon Department of Fish and Wildlife, Clackamas 

 Laboratory, 17330 S.E. Evelyn Street, Clackamas, OR 97015. 



Manuscript accepted April 1982. 



FISHERY BULLETIN: VOL. 80. NO. 4. 1982. 



feeding habits of juveniles of three species of 

 salmon off the Oregon coast: coho, Oncorhynchus 

 kisutch; chinook, O. tshawytscha; and chum, O. 

 keta, salmon. The authors describe the food 

 habits of each species, variability in food habits 

 among fishes collected at different stations, diet 

 overlap between coho and chinook, and speculate 

 on the impact of foraging juvenile coho on zoo- 

 plankton populations in coastal waters. 



METHODS 



Fish were collected in a purse seine 457 m long 

 X 30 m deep, constructed of 32 mm stretch mesh 

 with 30 meshes of 127 mm mesh along the bottom 

 of the net. The maximum volume of water en- 

 compassed by a round haul set that fished to 10 m 

 depth was calculated to be no more than 1.5 X 

 10 5 m 3 . A total of 56 purse seine sets were made 

 between 18 and 29 June 1979 in three regions of 

 the Oregon coastal zone: Off the Columbia River 

 (northern Oregon), off Newport (central Ore- 

 gon), and in the vicinity of Coos Bay (southern 

 Oregon) (Fig. 1). A total of 509 salmonids <35 cm 

 FL (fork length) (henceforth called juveniles) 



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