Changes in the Trophic Structure of an Epipelagic Community in the Western Bering Sea and Western North Pacific Ocean with an Emphasis on Pacific Salmon (Oncorhynchus spp.)

A comparative analysis of the trophic structure and interactions between Pacific salmon (Oncorhynchus spp.) and epipelagic communities in the western Bering Sea and Pacific waters off the Kuril Islands was conducted using the Ecopath modeling approach. In recent decades, the nekton communities in the Bering Sea and western North Pacific Ocean have changed greatly. For each region, we built two models describing the trophic structure of communities (1) in a period of relatively low salmon biomass and high biomass of other nekton species (walleye pollock, Theragra chalcogramma, and/or Pacific sardine, Sardinops melanostictus) characteristic of the 1980s and early 1990s, and (2) in a period of high salmon biomass and greatly decreased biomass of walleye pollock and/or sardine characteristic of the 2000s. To evaluate possible changes in trophic flows, we also examined hypothetical scenarios in which Pacific salmon biomass was multiplied by 1.5 relative to their highest level in the 2000s. Despite drastic changes in the biomass of several abundant species, the overall trophic structure of epipelagic nekton communities in both the western Bering Sea and Pacific waters off the Kuril Islands has not changed appreciably during the last 30 years. Between the 1980s and 2000s, Pacific salmon biomass increased greatly in the western Bering Sea and Pacific waters off the Kuril Islands resulting in increased food consumption. The increase in total food consumption appears to be associated with decreases in their diet composition from groups occupying relatively high trophic levels (e.g., amphipods and squids) and a rise in prey groups occupying relatively low trophic levels (e.g., euphausiids, copepods, and pteropods). As a result of this diet shift, the estimated trophic level of Pacific salmon in the food web declined between the 1980s and 2000s. In the simulation with salmon biomass expanded 1.5 times relative to the 2000s estimate, the abundance of forage species was sufficient to maintain higher salmon consumption. The ability of Pacific salmon to access a variety of prey species at a variety of trophic levels appears to give them the capacity to satisfy their food requirements even during periods of extremely high biomass.