Seasonal changes in abundance and development of Calanus pacificus (Crustacea: Copepoda) in the Oyashio-Kuroshio Mixed Region

Seasonal changes in abundance and development of Calanus pacificus Brodsky were investigated by analyzing samples of different depth strata (0-150 m and 0-1000 m) collected monthly in the Oyashio-Kuroshio Mixed Region. Copepodite stage 5 (C5) emerged from dormancy and matured in early summer. A new generation appeared in July and developed into C5 during summer to autumn. Some of the summer generation arrested development at C5 and persisted below 150 m depth until the following early summer. Although the remainder matured and reproduced in October, a new generation was not observed at the surface during winter. These results suggest that C. pacificus shows two different life-cycle patterns, i.e. one generation annually, with overwintering C5 in deep waters, and two generations annually, with surface development during autumn to winter. The complex life-cycle patterns may be an adaptation to the highly fluctuating surface environment in the Oyashio-Kuroshio Mixed Region.