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The life cycle of Neocalanus flemingeri was investigated by analyzing population struc- ture data collected monthly at Site H in the Oyashio region, Japan, from September 1996 through October 1997. Additional non-time-series sampling was also done at several stations covering the entire subarctic Pacific, Japan Sea and Okhotsk Sea, as a basis for regional comparison of the differ- ing life cycles and body sizes of this species. At Site H, N. flemingeri spawned between January and February below 250 m depth. Copepodite Stage 1 (C1) occurred in March, and most of these had developed into C5 by early June, i.e. by the end of the phytoplankton bloom. A portion of the C4 pop- ulation ceased development and remained at the thermocline to 500 m layer throughout the year. The remainder developed to C5, then migrated to 250-2000 m depth in June, and matured immediately. C6 males were present in May to July and died immediately thereafter. C6 females were observed in June to December with immature gonads and in January to February with fully mature gonads. The life cycle of N. flemingeri was estimated as annual for most of the population, but the small popula- tion overwintering as C4 may have a biennial life cycle. While the prosome length distributions of the C2, C3, C6 males and C6 females were unimodal, C4 and C5 exhibited bimodality. We believe this bimodality reflects sexual dimorphism for C4 and C5. Possible development sequences of each of the 2 size groups are proposed in relation to annual and biennial life cycles of N. flemingeri. Temporal population-structure data in other regions suggested an annual life cycle for the populations in the central-eastern subarctic Pacific, and a possible mixture of various degrees of annual/biennial life cycles for the populations in the western subarctic Pacific, Japan Sea and Okhotsk Sea. Geographi- cal comparison of the prosome length of the C6 females between the Oyashio (Site H) and other regions indicated significantly larger specimens in the Okhotsk Sea, with smaller individuals in the eastern and part of the western subarctic Pacific. Possible causes for regionl variability in life cycle and body size are discussed.

Life cycle of Neocalanus flemingeri (Crustacea: Copepoda) in the Oyashio region, western subarctic Pacific, with notes on its regional variations