The species range of Calanus sinicus along the Chinese coast extends from the Bohai Sea and the Yellow Sea in the north to the northern part of the South China Sea in the south. The subtropical seas along the southern coast of China mark the southern edge of the range of C. sinicus. In coastal seas off eastern Hong Kong, C. sinicus appears first in December, but densities comparable to those in the Yellow Sea and the East China Sea are reached only in January and February when temperature is <15°C. Density decreases in March as temperature increases. No individuals remain after May when temperature is >25°C. The average prosome length of females collected in February is comparable to that of females from the Yellow Sea, but females collected after mid-March are smaller than the smallest females from the Yellow Sea. Reproduction occurred mainly between January and March. Rapid decline of the population in April and the absence of a summer population suggest that the local population is derived from individuals advected from the north by ocean currents. Eggs produced locally probably did not hatch or develop into adults.