Seasonal occurrence of Calanus sinicus in the northern South China Sea: A case study in Daya Bay

•Calanus sinicus disappears from the bay due to high temperatures.•Reproduction of Calanus siniucus occurs at a low rate.•The spatial distribution of Calanus sinicus may be influenced by tidal currents.•Summer coastal upwelling may provide an over-summer refuge for Calanus sinicus.

Calanus sinicus is a calanoid copepod species that is distributed broadly in the continental shelf waters of the Northwest Pacific Ocean. This study aims to understand the seasonal variations of the distribution and abundance of C. sinicus in Daya Bay from the northern South China Sea (nSCS) and to explore its possible seasonal occurrence based on current and historical data. Monthly surveys of the species were conducted in Daya Bay, a subtropical bay in the nSCS, during the period between May 2013 and April 2014. C. sinicus was present from January to May, and disappeared after June. The spatial pattern of C. sinicus in the bay was characterized by its distribution into the southwestern part of the bay in January, bay-wide spread in February, patchiness in March and virtual retreat from the bay mouth in April. Reproduction occurred from January to April at a low rate. Adults were mostly abundant in January and declined to a minimum in April. The percentage of early developmental stages increased from 54.1% in January to 90.1% in April, as collected by a fine mesh size. Based on historical data from Daya Bay and from the coastal waters of the nSCS, C. sinicus was carried into the nSCS from the East China Sea by the China Coastal Current during the northeastern monsoon period and survived from December to October of the next year. The summer coastal upwelling may provide suitable refuges for the species in the nSCS by limiting the adverse effects caused by high temperatures. Our results confirm the viewpoint that C. sinicus could exist in the nSCS in summer and fall.