DISCUSSION
C. acicula is a common species widely distributed in the world’s ocean. Its abundance is usually very low in Daya Bay. According to results from Xu (1985) and our annual routine monitoring in Daya Bay since 2011, the average abundance of C. acicula in the bay ranged from 0.04 to 1.9 ind. m-3 (Fig. 4). The highest abundance ever recorded was 5.0 ind. m-3 at a single station in September 2012. However, during this bloom, a maximum abundance of 5595 ind. m‑3 was recorded, which is 1000 times of the highest value in historical documents of Daya Bay. Also, this maximum value in Daya Bay is much higher those reported in Indian seas, for example, 372 ind. m-3 in Cochin in 1973 (Sakthivel and Haridas, 1974) and 474.8 ind. m-3 in Bengal Bay in 1977 (Peter and Paulinose, 1978).
Taken together, this bloom in Daya Bay in 2020 is an extreme event. What could have caused such a severe outbreak of C. acicula ? Generally, the outbreak of a marine animal population needs to meet at least several conditions at the same time, including suitable environmental conditions (e.g., temperature and salinity), a reduction of predators and/or competitors, adequate food, and a high self-reproduction capacity.