FISHERY BULLETIN: VOL. 82, NO. 2 



TABLE 2. — Frequency of occurrence and mean percent composition of major prey 

 taxa in the gastric portions of salmon stomachs containing more than trace amounts 

 of food. 



sockeye, euphausiids in pink and chum salmon, 

 and squids in coho salmon stomachs. 



The same taxa that ranked first and second on a 

 frequency of occurrence basis usually ranked first 

 and second on the basis of mean percent volume. 

 Euphausiids (mainly Euphausia pacifica and 

 Thysanoessa longipes) were most important for 

 sockeye, pink, and coho salmon; "other taxa" were 

 most important for chum salmon. Amphipods 

 (mainly Parathemisto pacifica and Hyperia 

 medusarum ) ranked second in sockeye and pink 

 salmon. Gonatid squids ranked second in coho 

 stomachs and euphausiids ranked second in chum 

 salmon stomachs. Thus sockeye fed primarily on 

 euphausiids and secondarily on amphipods and 

 myctophid fishes. Pink salmon fed mostly on 

 euphausiids and amphipods. Coho fed mainly on 

 euphausiids and squids, and chum on salps and 

 euphausiids (see Table 2). Squids comprised only 

 2% of the volume of the stomach contents of chum 

 salmon, and fishes comprised only 4% of the vol- 

 ume for chum and coho salmon. Copepods were not 

 important (<1% of volume) for any species of 

 salmon captured during the study. 



Dietary overlap, based on the sum of minimum 

 percentage volumes (percent similarity index, 

 PSI, Sanders 1960) of the four main prey taxa, was 

 78% between sockeye and pink, 69% between 

 sockeye and coho, and 53% between pink and coho. 

 Because chum salmon had the most unique diet of 

 the four species consuming mainly salps and 

 gelatinous zooplankton, they had overlap values of 

 only 45% with sockeye and pink and 38% with 

 coho. 



Although all species of salmon fed on a variety of 

 taxa, individual fish usually contained only a few 

 prey taxa. Only two major prey taxa were found in 

 85% , 89% , 93% , and 89% of the cardiac portions of 

 sockeye, pink, coho, and chum salmon stomachs, 

 respectively, containing more than trace amounts 

 of food. Most sockeye and pink salmon had only 

 one taxon of food in their stomachs. When salmon 

 had only one food type in their stomachs, it was 

 euphausiids in 65%, 52%, 85%, and 28%- of the 



individual sockeye, pink, coho, and chum salmon, 

 respectively. Euphausiids were obviously the most 

 important prey for sockeye, pink, and coho salmon 

 during this study. They were often the exclusive 

 prey. 



Sometimes the contents of the cardiac portion of 

 sockeye and pink salmon stomachs were clearly 

 divided with one type of prey in the anterior and 

 one in posterior portion of the stomach. Generally 

 this "stratification" involved euphausiids and am- 

 phipods, or euphausiids and squid. Usually, how- 

 ever, the cardiac and pyloric portions of the 

 stomach had similar percentage compositions of 

 major taxa (excluding empty stomachs and 

 stomachs with trace amounts). Cardiac and 

 pyloric contents were similar for 70% of the sock- 

 eye, 72% of the pink, and 60% of the coho and chum 

 salmon. When sockeye and coho had the same prey 

 composition in cardiac and pyloric stomachs, both 

 portions usually contained only euphausiids. 

 When pink salmon had the same prey composi- 

 tion, amphipods or euphausiids were found. 



The relative composition of major prey taxa in 

 the stomachs of each species caught in the 12 gill 

 net sets is illustrated in Figure 4 and is discussed 

 below. Open circles in Figure 4 indicate when fresh 

 prey were common, except for amphipods which 

 usually showed little evidence of being digested. 



Sockeye Salmon 



Prey composition of sockeye salmon had a dis- 

 tinctive diel pattern. Sockeye caught at night 

 (2158-0430 h) contained a high percentage of 

 euphausiids compared with the afternoon and 

 morning sets (Fig. 4). In these night sets, 

 euphausiids averaged over 80% of the volume of 

 the stomach contents, and about 90% of the sock- 

 eye contained only euphausiids. Fish caught dur- 

 ing and after sunset (1957-0224 h) also contained 

 large numbers of freshly ingested euphausiids. 

 Some fish in set 5 (1957-2227 h) had a clear divi- 

 sion between euphausiids in the fore portion and 

 amphipods in the posterior portion of the cardiac 



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