Ecological Studies on the Alaska Pollack in the Adjacent Waters of the Funka Bay, Hokkaido. IV, Seasonal Variations of Zooplankton Distribution in Food for the Alaska Pollack

摘要

Euphausids, Amphipods and Copepods are important food organisms for the Alaska pollack in the adjacent waters of the Funka Bay. In this paper, the seasonal variations of distribution of these three crustacean groups were examined on the date obtained from the vertical hauls with a NORPAC net at six definite stations once each month from February 1974 to February 1975. (1) Euphausia pacifica of calyptopis and furcilia stages appeared at the inshore station of 50m depth with the inflow of the surface layer water of the Oyashio current in February and March. From July to September, the larger one of this species were predominant at the offshore stations (300-350m depth) where the tip of the bottom layer water of the Oyashio current existed, and became most abundant in the same stations with the access of the mixed water mass of the Tsugaru warm current and the Oyashio mid water current in December and January. (2) Amphipods, mainly Parathemisto japonica, were abundant in the offshore stations in March-April and July-August. Their seasonal distribution showed approximately a similar trend to that of E. pacifica. (3) Pseudocalanus minutus of small size in the Copepods group appeared at the inshore station from March to May. Calanus cristatus, C. plumchrus and Eucalanus bungii bungii were chiefly distributed at the offshore stations (300-400m depth). Their abundance increased from February to May and decreased from August to September. (4) There is a large variation in the abundance of distribution of these crustacean groups during some years in the abundance of the Alaska pollack, suggesting that the feeding ecology of the Alaska pollack also varies widely with the availability of these food organisms.