Brodeur and Pearcy Trophic relations of juvenile salmon off Oregon and Washington 



621 



Many invertebrate prey also occurred frequently or 

 were important numerically in the diet of juvenile coho 

 salmon, but were much less important than fish by 

 weight (Appendix Table 1). Principal invertebrate prey 

 were Cancer crab megalopae, the hyperiid amphipods 

 Hyperoche medusarum and Themisto pacifica, the 

 euphausiids Thysanoessa spinifera and Euphausia 

 pacifica, and the pteropod LimaciHa helicina. Numer- 

 ous other species of decapod larvae and amphipods 

 were eaten, as well as copepods and insects, but were 

 of lesser importance. Juvenile Loligo squid were also 

 important prey by weight. 



Temporal variations Some between-cruise variabil- 

 ity in the weight composition of the major food cate- 

 gories was evident (Table 2). During June 1980, coho 

 salmon consumed mainly fishes, although cephalopods 

 were relatively important compared with other cruises. 

 Except during July, fishes dominated the diet during 

 1981. Pteropods, mainly L. helicina, comprised over 

 one-half the total weight of prey in July 1981. Fishes 

 (mostly A. hexapterus and E. mordax) and euphausiids 

 (mainly T. spinifera) dominated the diet during 1982. 

 During 1983 and 1984, coho salmon juveniles fed more 

 upon decapod larvae than during most other years 

 (Table 2). Several anomalous prey items of more 

 southerly origin appeared in the diet in 1983 and 1984, 

 including the euphausiid Nyctiphanes simplex, the 

 pteropod Evclio pyrimidnta, and the hyi.ieriid am- 



phipod Vihilia spp. The greatest number of major prey 

 categories were found during July 1984 when terres- 

 trial insects made up over a third of the biomass found 

 in the coho salmon stomachs. One large lepidopteran, 

 Choristoneura occidevtalis, comprised greater than 

 37% of the total prey IRI. The diet during June 1985 

 resembled that of June 1982 at both the specific and 

 general taxonomic levels. 



Based on the percent of IRI for the major prey cate- 

 gories, juvenile coho salmon diets changed somewhat 

 as the summer progressed (Fig. 3). Fishes were the 

 main prey during May, although decapod larvae and 

 pteropods were important numerically. During June, 

 fishes and decapod larvae were the primary prey, with 

 amphipods and euphausiids of relatively minor impor- 

 tance. By July and August, fish consumption decreased 

 substantially, and pteropods and euphausiids were the 

 major taxa consumed. Fishes were again the dominant 

 food in September, but their importance was based 

 mainly on weight because individual fish in the stom- 

 achs were comparatively larger. In September, ptero- 

 pods, euphausiids, and amphipods were important prey 

 numerically. 



Geographic variations Diets of juvenile coho salmon 

 in the three geographic areas were similar in that the 

 same major prey categories were represented, despite 

 differences in mean size of salmon among the regions. 

 Fishes, decapod larvae, euphausiids, pteropods, and 



