Ontogenetic vertical migration and life cycle of Neocalanus plumchrus (Crustacea: Copepoda) in the Oyashio region, with notes on regional variations in body sizes

The ontogenetic vertical migration and life cycle of Neocalanus plumchrus were investigated by analyzing monthly population structure at Site H in the Oyashio region from September 1996 through October 1997. Additional sampling was also done at several stations covering the entire subarctic Pacific, Okhotsk Sea and Japan Sea as a basis for regional comparison of life cycles and body sizes. At Site H, N. plumchrus spawned October to April below 250 m depth. Young copepodite stages (C1-C5) occurred during June late in the phytoplankton bloom. The C5 migrated to the deeper layers in July-August where they molted to adults. Development time of C5 to C6 was highly variable. The ontogenetic vertical migration of N. plumchrus ranged from the surface to 1000-2000 m depth, and the life cycle was annual. Temporal data on population structure and vertical distribution suggested the annual life cycle was generally synchronized throughout the subarctic Pacific and its marginal seas. Geographical comparison of C5 prosome length indicated the occurrence of significantly large specimens in the Oyashio region and Okhotsk Sea but small specimens in the Japan Sea. Possible causes for regional variability in body sizes are discussed.