Phenology in large grazing copepods in the Oyashio region, western subarctic Pacific

Seasonal sequence of population structure (=copepodid stage composition) of large grazing copepods (Metridia pacifica, Eucalanus bungii and Neocalanus spp.) was analyzed based on seasonal samples collected with 100 μm mesh nets from 0-500 m stratum at Site H in the Oyashio region, western subarctic Pacific, during 1996-1997 and 2002-2007. On the premise that there are little year-to-year differences, the composite data were arranged to the date of samplings of each year to yield seasonal developmental patterns of each copepod. Seasonal developmental pattern estimated by tracing the sequence of mean copepodid stages of the population at each sampling date revealed that the recruitment season of the population was January for N. cristatus, March for N. flemingeri and May for N. plumchrus and E. bungii. In contrast to these copepods with single recruitment seasons in the year, M. pacifica exhibited two recruitment seasons (mid-May and August) in a year. Phenology in reproduction and development of these copepods reflects their species-specific differences in energy utilization pattern ; M. pacifica and E. bungii spawn in phytoplankton-rich surface layer in spring (females need to feed for spawning) while Neocalanus spp. spawn in deep layer in winter (females do not feed). Development from C1 to C5 of N. cristatus, N. flemingeri and N. plumchrus was in January to June, March to June and May to August, respectively, thus the three sympatric Neocalanus spp. showed a clear temporal separation in the developmental timing in the western subarctic Pacific. This temporal separation in utilizing the surface layer is considered to be a mechanism to reduce inter-specific food competition. Regional comparison of phenology in copepods within the entire subarctic Pacific and its adjacent waters revealed that reproduction timing of the surface spawning M. pacifica and E. bungii was highly variable, while this was not the case for deep spawning Neocalanus spp.