Ontogenetic diel vertical migration of the planktonic copepod Calanus sinicus in the Inland Sea of Japan

The ontogenetic diel vertical migration of the planktonic copepod Calanus sinicus was investigated in the Inland Sea of Japan in June 1989, when the water column was thermally weakly stratified. Because of fewer eggs and less variation in their abundance, nocturnal spawning was not apparent. A pronounced upward migration occurred in NIII. NIII to CIII resided in the upper 20 m layer throughout the day, and from CIV on their median depths descended. CV and adult females underwent significant diel vertical migration, whereas adult males did not migrate. By integrating the results from the present study and those from our previous investigations (in August–September 1988, November 1988 and March 1989), we review seasonal variation in the ontogenetic diel vertical migration of C. sinicus. Spawning was largely nocturnal, reaching its maximum level around dawn, but spawning depth and fecundity changed seasonally. The distribution of pre-feeding stages, NI and NII, was similar to that of eggs. A pronounced upward migration always occurred in the first feeding stage, NIII, and late nauplii and early copepodites always resided in the food-rich upper layer, indicating that upward migration by NIII is feeding migration. As the stages progressed, they extended their vertical distribution range, and CV and adult females usually underwent diel vertical migration. However, the pattern and strength of this migration differed seasonally. Their day depths increased with the increase of relative biomass of planktivorous fish, indicating that predator avoidance induces their diurnal downward migration. High chlorophyll a concentrations in the upper layer (<15 m deep) relative to the lower layer (>20 m deep) amplified their diel vertical migrations. Diel vertical migration of C. sinicus is a phenotypic behavior.