ONTOGENIC DIEL VERTICAL MIGRATION OF THE PLANKTONIC COPEPOD CALANUS SINICUS IN THE INLAND-SEA OF JAPAN .3. EARLY SUMMER AND OVERALL SEASONAL PATTERN

The ontogenetic diel vertical migration of the planktonic copepod Cala nus sinicus was investigated in the Inland Sea of Japan in June 1989, when the water column was thermally weakly stratified. Because of fewe r eggs and less variation in their abundance, nocturnal spawning was n ot apparent. A pronounced upward migration occurred in NIII. NIII to C III resided in the upper 20 m layer throughout the day, and from CIV o n their median depths descended. CV and adult females underwent signif icant diel vertical migration, whereas adult males did not migrate. By integrating the results from the present study and those from our pre vious investigations (in August-September 1988, November 1988 and Marc h 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 s easonally. The distribution of pre-feeding stages, NI and NII, was sim ilar 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 m igration by NIII is feeding migration. As the stages progressed, they extended their vertical distribution range, and CV and adult females u sually underwent diel vertical migration. However, the pattern and str ength of this migration differed seasonally. Their day depths increase d with the increase of relative biomass of planktivorous fish, indicat ing that predator avoidance induces their diurnal downward migration. High chlorophyll a concentrations in the upper layer (< 15 m deep) rel ative to the lower layer (> 20 m deep) amplified their diel vertical m igrations. Diel vertical migration of C. sinicus is a phenotypic behav ior.