This study was carried out to elucidate the causes of massive outbreaks of Aurelia coerulea in Geoje Bay, Korea, from November 2022 to October 2023. Adult medusae consistently spawn with planulae, and the populations of A. coerulea in Geoje Bay could be categorized into current-year and overwintering populations. The current-year population began with the emergence of ephyrae in February and grew until October, while the overwintering population comprised a mixture of surviving current-year population and additional individuals that joined during the warm season. The size of the planulae are significantly larger than the annual average during the cold season. These results appear to be the energy accumulation of planulae for polyp formation under low water temperatures. Planulae form polyps within a temperature range of 5–25 °C, suggesting the possibility of year-round polyp recruitment. In Geoje Bay, the highest appearance rate of A. coerulea was in April (8.71 ± 12.5 ind. m⁻³), with ephyrae experiencing higher growth rates up to the young medusa stage. However, from April, a decline in zooplankton biomass resulted in reduced growth rates in adults, indicating that jellyfish growth was primarily regulated by food availability. Additionally, submersed oyster shells in oyster farms served as the main habitat for jellyfish polyps. A. coerulea populations were also characterized by the continuous spawning of planulae throughout the year. In conclusion, this study suggests that stable polyp habitats, abundant food supply during the initial developmental period of the population, and suitable ranges of water temperature were significant factors inducing the massive outbreak of A. coerulea in Geoje Bay, Korea.

We estimated the total production of Calanus sinicus over four seasons by measuring somatic and egg production rates in the Yellow Sea. In the process of measuring somatic production, the length–weight relationship and growth rates of C. sinicus in this region were also derived. The population abundance and biomass of C. sinicus were especially high in spring. All copepodite stages had similar proportions in spring, whereas the late stages and adults were most common in summer. The mean total production of the C. sinicus population was 107.6–259.7 μg C m⁻³ day⁻¹, ranging from higher values in spring to lower values in summer. The annual total production of C. sinicus in the Yellow Sea was 64.3 mg C m⁻³ year⁻¹ (4032 mg C m⁻² year⁻¹). The production of C. sinicus was positively correlated with chlorophyll a concentration, rather than water temperature or salinity. This study is the first to quantify the secondary production of C. sinicus in the Yellow Sea. These findings will broaden our understanding of the ecology of C. sinicus and its contribution to the Yellow Sea food web.

Fisheries and Aquatic Sciences