Seasonal succession of four Acartia copepods (Copepoda, Calanoida) in Okkirai Bay, Sanriku, northern Japan

Abstract

Seasonal changes in abundance of four neritic Acartia species (A. hudsonica, A. omorii, A. longiremis and A. steueri), including identifiable copepodid stages, were investigated in the inner reaches of Okkirai Bay, Sanriku, northern Japan, to elucidate their seasonal succession patterns. Samples were collected monthly at intervals from August 2007 to July 2009 by vertical hauls of a NORPAC net of 100 μm mesh aperture. For identification of morphologically allied A. omorii and A. hudsonica, the dimensional differences between them were statistically analyzed for the stages of C4 to C6. The dominant species were A. longiremis in the colder season and A. steueri in the warmer season. A. longiremis and A. omorii appeared from early spring (February or March) to summer with numerical peaks in April. These April peaks were considered to result from immigration from outside the bay with intrusions of Oyashio Current water. A. steueri increased during the summer with a peak in September, then decreased until December or January, and disappeared for two or three months from April, when they were probably only present as diapausing eggs. A. hudsonica occurred from early spring to mid-summer as in A. omorii but with higher abundances in summer than in spring, though the seasonal abundances varied somewhat between years. These results suggest that temperature, determining reproductive activity (and production of diapausing eggs in A. steueri), and intrusions of Oyashio water are important environmental factors determining seasonal succession of acartiid copepods in the bay.