PETERSON ET AL.: FOOD HABITS OF JUVENILE SALMON 



Based on the percent by number, euphausiids 

 and fishes were more important in the diet of 

 chinook collected off the Columbia River than off 

 the central Oregon coast (Table 8). As with coho, 

 between-station variability was high. Only three 

 station pairs had high similarities in diet (PSI 

 >90%: 11-12, 11-14, and 12-14) mainly due to the 

 high proportions of T. spinifera consumed at 

 these stations. 



salmonids of both species increased in length, 

 they consumed larger fish, but coho between 140 

 and 330 mm consumed larger fish on the average 

 than chinook of the same size. Juvenile chinook 

 also consumed more pleuronectid larvae and 

 fewer scorpaenids than coho. Chinook ate very 

 few pteropods and no barnacle cyprids while 

 these taxa occurred in about 10% of the coho 

 stomachs (Table 3). 



Diet Overlap 



Similarity (PSI) was calculated as before to 

 study diet overlap among species of juvenile 

 salmon at four stations where at least eight indi- 

 viduals of two or more salmon species occurred. 

 Diets were similar (PSI >66%) at three of these 

 stations. At station 12, chum, coho, and chinook 

 juveniles ate 94.7, 93.4, and 90.9% euphausiids, 

 respectively, by number; at station 27, coho and 

 chinook ate nearly equal proportions of hyperiids 

 and fishes; and at station 39, coho and chinook ate 

 94.0 and 91.3% hyperiids, respectively. Diets 

 were dissimilar at station 1. 



This dietary overlap among cooccurring spe- 

 cies of juvenile salmonids suggests that a poten- 

 tial exists for competition, should food be limit- 

 ing. This potential was highest among different 

 size classes of juvenile coho and chinook salmon 

 but was reduced among similar-sized fishes (Fig. 

 3). Euphausiids were eaten most often by coho 

 100-200 mm long, but not by chinook <180 mm. 

 The opposite pattern is seen with hyperiid am- 

 phipods: Small chinook (<180 mm) ate more 

 hyperiids than similar-sized coho. As juvenile 



DISCUSSION 



Fishes, euphausiids, hyperiid amphipods, and 

 crab larvae were the most important prey for 

 juvenile salmon off Oregon. Other published 

 studies dealing with the diet of juvenile salmo- 

 nids in the ocean show basically the same result, 

 although there are notable differences. Manzer 

 (1969) concluded that juvenile chum salmon 

 from Chatham Sound, British Columbia, were 

 planktivorous, feeding mostly on larvaceans 

 (Oikopleura spp.) and unidentified copepods, and 

 that coho were piscivorous, feeding mostly on 

 Pacific herring and sand lance. Healey (1980) 

 found that juvenile chum salmon from Saanich 

 Inlet also fed predominantly on larvaceans and 

 copepods, but individuals caught in more open 

 waters of Georgia Strait ate euphausiids, amphi- 

 pods, and fishes, as found off Oregon in this study. 

 Juvenile coho studied by Healey contained 34% 

 fishes (by volume) in Georgia Strait and 3% in 

 Saanich Inlet, appreciably less than the 70% re- 

 ported by Manzer in Chatham Sound. Healey 

 concluded that chinook and coho from Georgia 

 Strait had very similar food habits. Fresh et al. 



Table 8.— Sampling data and percent of total numbers of major prey items found in juvenile chinook stomachs off the Oregon coast 

 (only at those stations where six or more individual chinook were taken). Includes only those prey taxa comprising at least 2% of 

 the total number of prey items. 



Northern Oregon 



Central Oregon 



Station 

 Time of day 

 Day in June 

 Water depth (m) 

 Distance from shore (km) 

 Number of chinook examined 

 Average length of chinook (mm) 

 Percent of chinook with empty 

 or nearly empty stomachs 



Thysanoessa spinifera (juveniles) 



Hyperiid amphipods 



Fishes 



Crab megalops 



Pteropods 



Euphausia pacilica 



T. spinifera (adults) 



At y I us tridens 



Calanus cristatus 



Unidentified items 



Total number of prey items 



847 



