ONTOGENIC VARIATIONS IN THE TYPE AND SIZE OF PREY CONSUMED BY JUVENILE COHO, ONCORHYNCHUS-KISUTCH, AND CHINOOK, O TSHAWYTSCHA, SALMON

Stomach contents of juvenile coho, Oncorhynchus kisutch, and chinook, O.tshawytscha, salmon collected in purse seines off the coast of Washington and Oregon were examined for variations related to predator size. There was a general trend toward increasing consumption of fish with increasing body size, due mainly to the increase in northern anchovy biomass consumed by the larger salmon. Most of the major prey taxa showed significant differences among the size classes examined for both salmon species. There was a direct relationship between predator and prey size for both coho and chinook, but considerable variation was found in prey length consumed within each size class. Prey width did not provide as good a fit as prey length for either species. In general, coho consumed larger fish prey in relation to their body length than chinook but there were substantial differences by month or year of collection.