Neocalanus species are large grazing copepods, occurring abundantly from subarctic Pacific Ocean and its marginal seas. These species are important prey components of pelagic fishes, whales and sea birds, thereby considered as a vital link between primary production and production of animals at higher trophic levels in pelagic ecosystems of these regions. However, little is known about their ecology because of taxonomic confusions in the past and logistic difficulties to obtain time-series samples in oceanic waters. The aim of this study is to establish the life cycles of Neocalanus species (N. cristatus, N. plumchrus and N. flemingeri) in the subarctic Pacific, and analyze interannual variabilities of these species in the central subarctic Pacific using samples collected during the summer of 1979-1997 (19 years).
All three Neocalanus copepods spawned below 250 m depth in autumn-winter. Early copepodite stages of the species developed in the surface layer during phytoplankton bloom season (March-June) and their copepodite stage 5 (C5) migrated down to deeper layers in summer where they molted to adults. N. cristatus/plumchrus entered the diapause at C5, but N. flemingeri at C4 and C6 female. The life cycle of was estimated as annual for N. cristatus/plumchrus but as annual/biennial for N. flemingeri in the western subarctic Pacific and its marginal seas.
Interannual variabilities in population size of the three Neocalanus copepods were revealed to have biennial cycle in the Subarctic Current System (SA) but were at random in the Northern-Southern Transition Domain (TN-TS). These variations were significantly correlated with biological environment parameters (chlorophyll a concentration, total zooplankton wet weight, salmon abundance) in the SA and physical environment parameters (water temperature) in the TS (not in the TN). These results suggested that the interannual variabilities were controlled by top-down effect in the northern regions and by bottom-up effect in the southern subarctic Pacific. Interannual changes in body size were biennial cycle in the SA-TS, but no significant environment parameters causing the changes were detected.