METHODS
Daya Bay is a shallow embayment with depths ranging from 5-20 m and an
area of 650 km2, located in the northern South China
Sea. On June 12, 2020, a huge mass of C. acicula was spotted in
the waters close to the southwest shore of Daya Bay. Afterward,
biological and environmental surveys in the entire Daya Bay were
conducted from July 4 to 18, 2020. C. acicula was collected using
vertical hauls of a plankton net (mouth diameter of 0.5 m and mesh size
of 505 μm) from 1 m above the bottom to the surface. The filtered water
volume was determined by the rope length multiplied by the net mouth
size. Collected C. acicula was immediately taken back to the
laboratory for subsequent abundance counting and biological parameter
measurements, which included shell length, diameter at the aperture, and
body weight. Environmental factors of surface water temperature,
salinity, pH, DO were measured in situ using a YSI 6600
multi-parameter water quality monitor (Yellow Springs Instrument Co.,
Ohio, USA). Daily mean surface water temperature, salinity, and
chlorophyll a (Chl a ) in May-July were obtained from a
maritime buoyage system located in southwestern Daya Bay (see the buoy’s
location in Fig. 3). The buoyage system record these data at an interval
of half-hour. Historical data of C. acicula abundance in Daya Bay
is from Xu (1985) and from an ocean observatory program of South China
Sea Fisheries Research Institute (SCSFRI), which has been carried out
every year since 2011.