Anatomical specialization in the gut of Pacific salmon (Oncorhynchus): Evidence for oceanic limits to salmon production?

The stomach (but not the intestine) of chum salmon (Oncorhynchus keta) is greatly enlarged relative to that of other species of Pacific salmon. This permits the exploitation of gelatinous zooplankton (jellyfish, ctenophores, and salps), an abundant but low-energy prey unused by other species of salmon, as a major food source. The unique gut structure of chum salmon therefore allows efficient feeding on a little-exploited branch of the food web and reduces interspecific trophic competition. The development of this remarkable anatomical specialization suggests that salmon abundances were previously high enough that the resulting trophic competition led to evolutionary selection to reduce trophic competition. As total salmon abundances in the North Pacific are now probably the highest of this century, the carrying capacity of the ocean rather than that of fresh water could limit overall salmon production if abundances are once again approaching pre-exploitation levels.