BEACHAM: FOOD OF PACIFIC SALMON OFF BRITISH COLUMBIA 



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Figure 5.— Percentage frequency of occurrence and percentage stomach volume 

 of prey types for coho salmon. 



vestigated for the two most frequently occurring fish 

 prey (sand lance, herring) and crustacean prey 

 (euphausiids, Parathemisto). Size classes for sock- 

 eye and pink salmon were below and above 55 cm 

 FL, and those for chinook and coho salmon below 

 and above 60 cm FL. I assume that the value of the 

 cube root of the volume ratio of the prey is propor- 

 tional to the prey length ratio, and thus changes in 

 prey size can be compared with changes in predator 

 size 



Mean size of the fish component of the prey in- 

 creased as predator size increased (Table 3). As the 

 size of pink, coho, and chinook salmon increased by 

 13%, 65%, and 69%, respectively, the size of the sand 

 lance consumed increased by 16%, 83%, and 83%, 



respectively. The size of herring eaten also increased 

 as predator size increased, and for pink and chinook 

 salmon it was about equal to the increase in size of 

 the predator species. When the predator responses 

 to increase in size of both prey species are pooled, 

 there is a weak correlation between increasing 

 predator length and increasing prey length (r = 0.69, 

 n = 6, P > 0.05); but if the coho salmon response 

 to increasing herring size is deleted, the relationship 

 is much stronger between increasing predator and 

 prey size (r = 0.98, n = 5, P < 0.01). 



Apparent trends of invertebrate prey size with 

 predator size were not statistically significant. For 

 sockeye and pink salmon, mean size of individuals 

 in the two invertebrate prey classes decreased as 



83 



