Regional Diversity of Juvenile Pink Salmon Diet in Autumn in the Bering, Okhotsk and Japan Seas

摘要

Regional variability in the diet of juvenile pink salmon was studied in the Bering, Okhotsk and Japan seas during the autumn of 2002-2004. During this time young pink salmon preyed mainly upon ichthyo- and mero-plankton in the eastern Bering Sea. In the Okhotsk, western Bering and northwestern Japan seas the most frequently occurring prey items in the juvenile pink salmon diet were planktonic crustaceans: hyperiids (Themisto pacifica, T. libellula and Primno macropa), euphausiids (Thysanoessa longipes), copepods (Neocalanus plum-chrus) and pteropods (Limacina helicina). Other food organisms (irrespective of their high biomass in the pelagic plankton community) were of secondary importance or would only occur occasionally in fish stomachs. We suspect that food habits of juvenile pink salmon were associated with the accessibility of forage groups in the habitat strata of juvenile pink salmon. In the upper epipelagic layer (0-50 m), the biomass of zooplankton (particularly copepods and euphausiids) increased at night due to vertical migrations from deeper layers, while hyperiids and pteropods (small- and medium-size L. helicina) were present in dense aggregations in the surface layer during day and night hours. However, juvenile pink salmon consumed prey mainly during daylight hours. Thus, juvenile pink salmon preyed upon plankton groups and species that were more abundant in habitat strata of juvenile pink salmon during daylight hours.