Physical forces determine the annual bloom intensity of the giant jellyfish Nemopilema nomurai off the coast of Korea

During the summer, giant jellyfish (Nemopilema nomurai) are transported from their main seeding and nursery ground, Jiangsu Province and the coast of China, to Korean coastal waters by the currents of the Yellow Sea (YS) and the East China Sea, causing problems for swimmers and hampering fisheries. In this study, we derive new velocity fields based on satellite measurements and develop particle-tracking experiments to simulate and analyze the interannual change of N. nomurai abundance off the coast of Korea. The velocity fields are calculated by combining Ekman currents and geostrophic currents based on an analytical solution of an approximated momentum equation. For validation, we compare the particle-tracking experiment results with in situ observations obtained from the Korean National Institute of Fisheries Science. The combined velocity fields during the summer show that geostrophic currents are of the same order as Ekman currents over the YS, implying that the interannual changes in N. nomurai abundance off the coast of Korea are determined by combined Ekman and geostrophic currents. In addition, N. nomurai distribution off the coast of China in April is considered an important factor driving blooms around Korean coasts. To test this supposition, we conducted a sensitivity experiment that adjusted the position of particles off the coast of China in April. Results demonstrate that the appearance of N. nomurai can increase by a factor of three depending on the initial distribution of the jellyfish off the coast of China before eastward transport in April.