道東および常磐沿岸域におけるツノナシオキアミEuphausia pacificaの胃内容物の特徴

抄録

The feeding habits of adult Euphausia pacifica, widely distributed from the subarctic zone to the transition area in the North Pacific, were examined in the two different food conditions, southeastern Hokkaido (Oyashio area; about 42°40’N) and Joban waters (transiton zone; about 36°50’N) off northeastern Japan bimonthly from April 1997 to February 1998. In the southeastern Hokkaido waters, E. pacifica most abundantly consumed diatoms, but no copepods were found in their guts in April, when the chlorophyll a concentrations peaked in the study period. On the contrary, the mean numbers of copepods in the gut of E. pacifica in August and February, when chlorophyll a concentrations were lowest, were higher than other months in the same waters. In Joban waters, where chlorophyll a concentrations were lower than those in the southeastern Hokkaido waters almost all year round, the mean copepod abundance in the gut of an adult E. pacifica was higher. We hypothesized that E. pacifica switched the feeding behavior according to ambient phytoplankton abundance, and the switching may have contributed the wide distribution of this species.