BEACHAM: FOOD OF PACIFIC SALMON OFF BRITISH COLUMBIA 



a larger proportion of the diet of larger chinook 

 salmon than of smaller ones (Milne 1955; Reid 1961). 

 In my study, the fish component of the diet was 

 similar for all size classes of chinook salmon. This 

 may be due to differences in availability of inverte- 

 brate prey to the smaller chinook salmon among the 

 studies. For example, Ito (1964) found that squid 

 were the largest dietary component of chinook and 

 coho salmon caught in drift nets in high seas 

 fisheries. Variability in diets of the different species 

 may be due in part to prey abundance, selection by 

 the predator, and possible selectivity by the sampling 

 gear used. Hook and line sampling may select fish 

 of different diets than would perhaps gill nets. 

 Salmon caught by trolling may have a higher com- 

 ponent of fish in the diet than those caught by gill 

 nets. In my study, fish did constitute a larger pro- 

 portion of the diet in larger coho salmon than in 

 smaller ones, as noted for chinook salmon. My study 

 has examined the distribution of prey types and sizes 

 for salmon caught from June to October only. 

 Although the relative proportions of fish and inverte- 

 brate prey could change seasonally for the salmon 

 species examined, the relative ranking of the species 

 in terms of proportion of fish in their diet should re- 

 main constant. 



Availability of prey types can alter markedly the 

 proportions in a predator's diet. Herring comprised 

 over 70% of the stomach contents of troll-caught 

 chinook and coho salmon caught off the east and 

 west coasts of Vancouver Island in 1957 (Prakash 

 1962). My study showed that during 1967-68, her- 

 ring comprised <20% of the stomach contents of 

 chinook and coho salmon in the same area. Stock 

 abundances of herring declined rapidly in the late 

 1960's in British Columbia (Hourston 1978), in- 

 dicating that during a period of low herring abun- 

 dance, sand lance became an important dietary com- 

 ponent of chinook and coho salmon in this area. 



Pink salmon in southern British Columbia and 

 Washington State show 2-yr cycles of abundance, 

 with returns absent in even-numbered years. This 

 pattern of abundance has been suggested to be a 

 result of predation by returning adults of the domi- 

 nant brood year on fry of the alternate brood year 

 (Ricker 1962). In my study, fish other than sand 

 lance, herring, or rockfish comprised <1% of the 

 stomach contents of pink salmon sampled in 1967. 

 These results suggest that predation by the domi- 

 nant broodline on the alternate broodline may be 

 neither necessary nor sufficient to account for cycles 

 in pink salmon abundance. 



The effect of prey size on selection by planktiv- 

 orous fish has been examined by Werner and Hall 



(1974), O'Brien et al. (1976), O'Brien (1979), Gibson 

 (1980), and Eggers (1982). Eggers found that 

 juvenile sockeye salmon prefer large nonevasive prey, 

 but will eat small and/or evasive prey when the 

 former is not available I found that as predator size 

 increased, prey size increased also, both in terms of 

 size of individuals within a prey type, and a shifting 

 from smaller to larger prey types. The predators 

 presumably decrease the amount of time and energy 

 needed to ingest a given amount of food by switch- 

 ing from smaller to larger prey, given that the large 

 prey types are sufficiently abundant. Werner and 

 Hall (1974) attributed a preference by predators for 

 only a part of the prey types available as a method 

 for increasing foraging efficiency. These results sug- 

 gest that the salmon species examined do select prey 

 both for size and availability, presumably to increase 

 foraging efficiency. 



Morphological differences and diet partitioning 

 have been previously noted for many fish species 

 (Keast and Webb 1966; Hyatt 1979). As outlined by 

 Hyatt (1979), many planktivorous feeding fish tend 

 to have numerous, well-developed, close-set gill- 

 rakers. My study indicated that the more piscivorous 

 chinook and coho salmon have fewer gillrakers than 

 the more planktivorous sockeye and pink salmon. 

 Lake trout, Salvelinus namaycush, populations that 

 are more planktivorous also have more and longer 

 gillrakers than less planktivorous ones (Martin and 

 Sandercock 1967). Oncorhynchus masou (masou or 

 cherry salmon), found in the western Pacific Ocean, 

 has fewer gillrakers than either chinook or coho 

 salmon (Hikita 1962) and, as an adult, feeds largely 

 on fish (Tanaka 1965). Chum, 0. keta, salmon have 

 an average of 2-3 more gillrakers than chinook and 

 coho (Hikita 1962), and the diet of chum salmon sam- 

 pled in the spring and summer during 1956-63 in 

 the North Pacific comprised between 10 and 35% 

 fish (Neave et al. 1976). In the genus Oncorhynchus, 

 as gillraker number declines, the proportion of fish 

 in the diet increases. Morphological differences 

 among the species account for a greater partition- 

 ing of the diet than do differences in water depths 

 in which the individual species are located. 



Pacific salmon are adaptable in their diet, shift- 

 ing their preferred prey species in relation to prey 

 size and abundance It seems unlikely that salmon 

 abundance is affected by the abundance of any one 

 type of prey. For example, the decline in abundance 

 of British Columbia herring stocks was not followed 

 immediately by declines in salmon abundance 

 Growth rates of salmon may be affected by changes 

 in diet and this could have an impact on stock popula- 

 tion dynamics. 



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