TROPHIC RELATIONS IN THE SUB-ARCTIC NORTH PACIFIC ECOSYSTEM - POSSIBLE FEEDING EFFECT FROM PINK-SALMON

Year-to-year variations in biomass of phytoplankton (surface chlorophy ll a concentration) and macrozooplankton (wet weight obtained by NORPA C 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 P acific in summer were studied. After 1989, phytoplankton biomass and p ink salmon abundance showed corresponding yearly patterns, whereas the pattern shown by macrozooplankton biomass was always the inverse of t hat shown by phytoplankton and salmon. We suggest that macrozooplankto n biomass remained low when pink salmon was abundant due to the intens e feeding impact of pink salmon, which in turn allowed phytoplankton b iomass to remain high as a result of the lesser grazing effect of macr ozooplankton. The opposite phenomenon probably occurred when pink salm on was not abundant. Prior to 1989, macrozooplankton biomass was at a rather high level while phytoplankton biomass and pink salmon abundanc e were low. We suggest that macrozooplankton biomass remained high due to a lesser feeding impact of the pink salmon, and phytoplankton biom ass remained low due to the intense grazing effect of macrozooplankton . Our study therefore shows the possibility that the feeding effect fr om the pink salmon controls summer macrozoo- and phytoplankton biomass in the subarctic North Pacific.