BEACHAM: FOOD OF PACIFIC SALMON OFF BRITISH COLUMBIA 



Methodology for the first, percent occurrence of each 

 of the prey types, has been outlined by Hynes (1950). 

 All chi-square tests in the analysis for frequency of 

 occurrence of prey types have one degree of freedom. 

 The second technique involved determining percent- 

 age by volume of total stomach contents for fish, 

 crustaceans, miscellaneous organisms, and also for 

 the individual prey classifications. Fish, crustaceans, 

 and miscellaneous organisms were recorded by 

 volume, and thus determining percentage of total 

 stomach volume for each classification was direct. 

 For individual prey types, it was necessary to con- 

 vert numbers of individual organisms to volumes by 

 calculating the volume displaced by a single 

 organism of each prey type This was done by selec- 

 ting individual salmon of each species with only one 

 fish and/or one crustacean prey type in the stomach. 

 The unit volumes for each prey type were then 

 calculated as the sum of the fish or crustacean 

 volumes for the selected fish divided by the number 

 of the prey type under consideration. If there was 

 only one unknown in the stomach contents with prey 

 of known (calculated) volumes (the number of prey 

 types multiplied by their unit volumes), the total 

 volume of known prey was subtracted from the total 

 fish or crustacean volume until only one unknown 

 prey class remained. Then the volume of the prey 

 class in question was obtained and its unit volume 

 calculated. Comparisons of prey size among the 

 species were analyzed by analysis of variance 



For an individual salmon with more than one fish 

 or one crustacean prey class in its stomach, volume 

 of each prey class was determined by multiplying the 

 number of organisms by their unit volume This total 

 volume obtained was scaled proportionately so that 

 individual components when summed equalled the 

 total known fish or crustacean volume 



RESULTS 



Volume and Frequency of 

 Food Items 



For each species, over 30% of the individuals had 

 empty stomachs (Table 1). In comparing fish with 

 non-empty stomachs, sockeye salmon was the least 

 piscivorous, with a mean 7% fish component in the 

 diet (Fig. 2). In sockeye salmon <55 cm fork length 

 (FL), only 2% of the stomach volume was comprised 

 of fish. At 17% of total food volume, fish was a 

 greater dietary component of pink salmon than of 

 sockeye (Fig. 2). However, the fish component of the 

 diet of sockeye and pink salmon was considerably 

 less than that of coho (46%) and chinook (56%) 



salmon. Fish comprised 30% of the stomach content 

 volume of coho <40 cm FL, but almost 50% of the 

 stomach content volume of larger coho. Chinook 

 salmon was the most piscivorous of the four species, 

 and the 56% fish component of the diet was constant 

 for the three size classes of chinook salmon inves- 

 tigated, although the species composition of the fish 

 prey changed. 



The relative importance of individual prey types 

 was investigated for the four salmon species. Sand 

 lance, Ammodytes hexapterus, was virtually the sole 

 fish component of the diet of sockeye salmon, oc- 

 curring in 4% of the 81 non-empty sockeye salmon 

 stomachs sampled (Fig. 3). Euphausiids were the 

 most important prey for sockeye, occurring in 58% 

 of non-empty stomachs and comprising 71% of the 

 total volume of food eaten. The hyperiid amphipod 

 Parathemisto comprised over 11% of the volume of 

 food eaten. Of the fish prey species, sand lance was 

 again the most important for pink salmon, occurring 

 in 9% of 562 non-empty stomachs and comprising 

 10% of total stomach contents (Fig. 4). There was 

 no significant difference between sockeye and pink 

 salmon in the frequency of occurrence of sand lance 

 in their diets (x 2 = 2.65, P > 0.05). Fish species 

 other than sand lance (herring, Clupea harengus, and 

 rockfish, Sebastes sp.) comprised less than 1% of 

 stomach contents of pink salmon. As in sockeye 

 salmon, the dominant invertebrate prey types were 

 euphausiids at 62% of stomach content volume and 

 Parathemisto at 14%. Frequency of occurrence of 

 euphausiids (x 2 = 1.63, P > 0.05) and Parathemisto 

 (x 2 = 3.54, P > 0.05) were similar for sockeye and 

 pink salmon. 



Fish species were a significant food for coho and 

 chinook salmon. For example, sand lance occurred 

 in 27% of 1,364 non-empty stomachs of coho salmon, 

 and also comprised 27% of total stomach volume 

 (Fig. 5). Herring comprised <1% of the stomach con- 

 tent volume of coho <40 cm FL, but 25% of the 

 volume for coho >60 cm FL. The dominant inverte- 

 brate prey type was euphausiids, comprising 51% of 

 total stomach contents, while all invertebrate prey 

 types combined comprised only 54%. The relative 

 importance of fish as a prey type was greatest in 

 chinook salmon, with sand lance again the dominant 

 prey species, occurring in 34% of 914 non-empty 

 stomachs, and comprising 35% of total volume of 

 contents (Fig. 6). Sand lance occurred in the diet of 

 chinook and coho salmon at similar frequencies (x 2 

 = 0.80, P > 0.05), as did herring (x 2 = 0.08, P > 

 0.05). Herring comprised 9% of the stomach contents 

 for chinook salmon <40 cm FL, but 33% of the 

 stomach contents for chinook salmon >60 cm FL. 



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