Forage base of Pacific salmon in the western Bering Sea and adjacent Pacific waters in 2002-2006

The objective of this study was to estimate the total relative biomass of the forage base (zooplankton + nekton) of Pacific salmon (Oncorhynchus spp.) in the upper epipelagic zone of the western Bering Sea and adjacent Pacific waters in summer and fall 2002–2006. Zooplankton biomass was estimated from plankton survey data, and nekton biomass was estimated from salmon diet data using a mathematical model of selective feeding. In 2002–2006, estimated total relative biomass of the salmon forage base varied from 690–1590 mg/m3. Biomass was lowest in fall 2004, and was highest in fall 2002. Copepods and chaetognaths dominated the potential forage base. Squids and fishes were 15–22% (average 19%) of the overall biomass. The biomass of fish was highest in the continental shelf area (Anadyr Bay). Squids were more abundant in deep-water regions of the Bering Sea and adjacent Pacific waters. Walleye pollock Theragra chalcogramma, capelin Mallotus villosus, and Pacific sand lance Ammodytes hexapterus were the dominant nekton species in the northwestern shelf region. Shortarm gonate squid Gonatus kamtschaticus, boreopacific gonate squid Gonatopsis borealis, Atka mackerel Pleurogram-mus monopterygius, and myctophids were prevalent items in the salmon forage base in deep-water areas. In general, the results indicated that immature salmon in both summer and fall were concentrated within deep-water regions of the western Bering Sea, where their forage (overall and preferred prey items) was also concentrated, and were much less numerous in the Pacific waters off the Commander Islands and in the western Bering Sea shelf zone, where their forage was less concentrated.