PETERSON ET AL.: FOOD HABITS OF JUVENILE SALMON 



Table 1.— Average number of prey in individual stomachs of 

 juvenile chum, coho, and chinook salmon. A more detailed tax- 

 onomic breakdown is given in Table 3. 



Table 2.— Average wet weight (in grams) of major prey 

 groups found in juvenile chum, coho, and chinook salmon stom- 

 achs. Numbers in parentheses are percentages of salmon stom- 

 achs containing the specific prey item. Average weight of 

 stomach contents is for fish with food in their stomachs. 

 Weights of salmon are wet weights calculated from mean 

 lengths using the length-weight equations of Healey (1980). 



Total weight of stomach contents of each of the 

 three species of juvenile salmon reflects the size 

 of the fishes sampled (Table 2). Weights of stom- 

 ach contents expressed as percent of total body 

 weight are similar, however, averaging about 

 1.3%. 



Frequencies of occurrence and average abun- 

 dances of specific prey taxa for each of the three 

 juvenile salmon are shown in Table 3 and are re- 

 ferred to in the following discussion of the diets 

 for each of the species. 



Chum Salmon 



The diet of chum salmon consisted mainly of 

 the euphausiid Thysano'essa spinifera and the 

 hyperiid amphipod Hyperoche medusarum. 

 Mean numerical abundances of T. spinifera per 

 chum stomach collected from northern, central, 

 and southern Oregon were 30.1, 149.7, and 3.7, 

 respectively; abundances of hyperiids were 2.9, 



104.8, and 17.5, respectively. Both these prey 

 were most common in chum salmon stomachs 

 collected off central Oregon, but sample sizes 

 were so small that it is difficult to attach any real 

 significance to these differences. 



Coho Salmon 



A total of 19 invertebrate and 13 fish taxa were 

 identified from coho stomachs (Table 3). Major 

 prey items were juvenile euphausiids (T. spini- 

 fera, average length about 9.0 mm), unidentified 

 hyperiid amphipods (average length about 4.5 

 mm), and various fishes (most between 25 and 30 

 mm long). The most frequently occurring fish 

 identified from the juvenile coho stomachs were 

 Pacific sand lance, Ammodytes hexapterus; juve- 

 nile rockfishes, Sebastes spp.; and larval or juve- 

 nile stages of several species of flatfishes, clupe- 

 ids, and osmerids. 



Average length of the prey euphausiid, T. 

 spinifera, was directly related to length of the 

 juvenile coho predator. The slope of the regres- 

 sion line (Fig. 2) was significantly different from 

 zero (r = 0.46, 28 df, P-0.01), indicating that 

 coho between 100 and 210 mm long eat progres- 

 sively larger euphausiids. Juvenile coho fed on a 

 broad spectrum of fish prey sizes, but, again, 

 larger fish often consumed larger prey. Coho 

 141-180 mm long fed mainly on fish that were 

 11-30 mm long, whereas 181-200 mm coho con- 

 sumed mostly larger fishes, ranging from 21 to 

 40 mm long (Table 4). However, the regression of 

 lengths of whole prey fishes on lengths of juvenile 

 coho, 94-220 mm, was not significantly different 

 from zero. 



Relationships between size of coho and num- 

 bers and sizes of prey were studied for 87 juve- 



100 



1 50 200 



COHO LENGTH (mm) 



Figure 2.— The relationship between coho length and mean 

 euphausiid prey length. 



843 



