Trophic relations in the subArctic North Pacific ecosystem: Possible feeding effect from pink salmon

Year-to-year variations in biomass of phytoplankton (surface chlorophyll a concentration) and macrozooplankton (wet weight obtained by NORPAC net operation above 150 m), and abundance of pink salmon (catch per unit effort of pink salmon) from 1985 to 1994 in the subarctic North Pacific in summer were studied. After 1989, phytoplankton biomass and pink salmon abundance showed corresponding yearly patterns, whereas the pattern shown by macrozooplankton biomass was always the inverse of that shown by phytoplankton and salmon. We suggest that macrozooplankton biomass remained low when pink salmon was abundant due to the intense feeding impact of pink salmon, which in turn allowed phytoplankton biomass to remain high as a result of the lesser grazing effect of macrozooplankton. The opposite phenomenon probably occurred when pink salmon was not abundant. Prior to 1989, macrozooplankton biomass was at a rather high level while phytoplankton biomass and pink salmon abundance were low. We suggest that macrozooplankton biomass remained high due to a lesser feeding impact of the pink salmon, and phytoplankton biomass remained low due to the intense grazing effect of macrozooplankton. Our study therefore shows the possibility that the feeding effect from the pink salmon controls summer macrozoo- and phytoplankton biomass in the subarctic North Pacific.