TYPE, QUANTITY, AND SIZE OF FOOD OF 



PACIFIC SALMON (ONCORHYNCHUS) IN 



THE STRAIT OF JUAN DE FUCA, BRITISH COLUMBIA 



Terry D. Beacham 1 



ABSTRACT 



The volume, numbers, and size of prey of sockeye, Oncorhynchus nerka; pink, 0. gorbuscha; coho, 0. kisutch; 

 and chinook, 0. tshawytscha, salmon were investigated for troll-caught salmon in the Strait of Juan de 

 Fuca off southwestern Vancouver Island during 1967-68. Sockeye salmon was the least piscivorous species 

 with only 7% of the stomach volume comprised of fish, while chinook salmon was the most piscivorous 

 species at 56%. Sand lance, Ammodytes hexapterus, and euphausiids were the most important fish and 

 invertebrate prey, respectively. As predator size increased, mean size of fish prey increased, and predators 

 shifted to species of larger mean size Similar results were found for the invertebrate prey, with mean 

 number of prey consumed per predator increasing for the larger invertebrate species as predator size 

 increased. Rate of increase in mean length of fish prey was proportional to increasing predator length. 

 The observed increase in invertebrate size with increasing predator length was not statistically signifi- 

 cant. Although chinook and coho salmon had similar diets, they were caught at significantly different 

 water depths. Oncorhynchus species with fewer, shorter, and more widely spaced gillrakers have higher 

 proportions of fish in their diet than species with numerous, long, and narrow set gillrakers. 



The life history of Pacific salmon is quite variable 

 among species, with fry of pink salmon, Oncorhyn- 

 chus gorbuscha, and chum salmon, 0. keta, migrating 

 to sea soon after emergence from the gravel, while 

 those of sockeye salmon, 0. nerka, coho salmon, 0. 

 kisutch, and chinook salmon, 0. tshawytscha, may 

 spend up to 2 yr in freshwater. Once in the ocean 

 they can migrate a considerable distance from their 

 natal streams and feed on a variety of organisms 

 (Godfrey et al. 1975; French et al. 1976; Major et al. 

 1978; Takagi et al. 1981). Salmon thus move through 

 a number of habitats during their life cycle and con- 

 sume a diverse array of prey. 



Food preferences of salmon in the range of 

 habitats that they occupy have been an area of con- 

 tinuing investigation (Allen and Aron 1958; Prakash 

 1962; LeBrasseur 1966; Parker 1971; Eggers 1982). 

 Relative amounts of different prey types eaten in 

 varying environments have been examined, as well 

 as preferences by different sizes of predators in rela- 

 tion to the size and abundance of prey. Oncorhyn- 

 chus species differ considerably in their size, mor- 

 phology, and ocean distribution (Hikita 1962; Neave 

 et al. 1976; Takagi et al. 1981; Beacham and Mur- 

 ray 1983). Morphological differences and diet parti- 

 tioning have been reported for many fish species 



(Keast and Webb 1966; Hyatt 1979), and diet parti- 

 tioning may thus be expected among Oncorhynchus 

 species. Prey size is related to predator size (O'Brien 

 1979; Gibson 1980), and differential prey selection 

 among Oncorhynchus species may also be apparent. 

 Stomach contents of sockeye, pink, coho, and 

 chinook salmon were investigated in a research troll- 

 ing program conducted off southern Vancouver 

 Island in the Strait of Juan de Fuca during 1967-68. 

 The relative importance of different prey types, in- 

 cluding fish and invertebrates, in the diet of the four 

 species was studied with respect to prey size, preda- 

 tor size, predator morphology, and diet partitioning 

 in relation to salmonid habitat and morphology. 



MATERIALS AND METHODS 



The salmon were obtained by test trolling in the 

 Strait of Juan de Fuca during 19 June-11 October 

 1967 and 1 May-12 July 1968 (Fig. 1). Detailed 

 methodology of the program has been outlined by 

 Graham and Argue (1972). For each salmon sampled, 

 date, fork length (mm), round weight, and sex were 

 recorded. Stomachs were removed, placed in num- 

 bered cloth sample bags along with any food 

 organisms in the mouth cavity, and preserved in 10% 

 Formalin 2 solution. 



'Department of Fisheries and Oceans, Fisheries Research 

 Branch, Pacific Biological Station, Nanaimo, British Columbia V9R 

 5K6, Canada. 



2 Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Manuscript accepted March 1985. 



FISHERY BULLETIN: VOL. 84, No. 1, 1986. 



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