Spatiotemporal distribution of jellyfish in the Yellow and Bohai Seas: 2018's monitoring results using ship of opportunity Yoon W.1, H. Jeon2, K. Hahn1, C. Yu2, Y. Kim2, K. Nam2 and J. Chae2 1 Human and Marine Ecosystem Research Laboratory, Gunpo, 15850 Korea 2 Marine Environmental Research and Information Laboratory, Gunpo, 15850 Korea * Correspondence. E-mail: [email protected] Keywords: East China Sea, Nemopilema nomurai, Cyanea nozakii, Rhopilema esculentum, distribution, Yellow Sea, Bohai Sea Jellyfishes were monitored by sighting method in the Bohai and northern Yellow seas every 3 weeks from July to October, 2018 using ships of opportunity. Monitoring areas were divided into 10 subareas taking into account current pattern, and temporal abundance was followed and compared. Monitoring has revealed appearance and disappearance of 4 species of jellyfish: Nemopilema nomurai, Cyanea nozakii, Aurelia coerulea, Rhopilema esculentum. N. nomurai appeared in the 2 seas from July to September, whereas C. nozakii in the inner Liaodong Bay from July to October, A. coerulea in the inner and outer Liaodong Bay from July to September, and R. esculentum in the northeastern coastal area of Yellow Sea in August-September. Abundance of N. nomurai varied spatiotemporally: they were most abundant in July in the inner Liaodong Bay and continuously decreased afterward; in the Bohai Strait they were first in the central area and extended into the whole strait and since the end of August, their abundance decreased and appearance area reduced; N. nomurai in the area north of Shandong Peninsula (northwestern North Yellow Sea) followed similar variation of abundance and spatial distribution with that of Bohai Strait; in the northeastern Yellow Sea, they were most abundant in the end of August; in the western Yellow Sea, they were only observed in areas off Qingdao and their abundance was always lower than other areas. Size of N. nomurai was checked; N. nomurai grew is its abundance increased. Meanwhile size structure differed along the area and required more detailed field investigation. Dead N. nomurai began to appear since the beginning of August to the end, and peaked in the end of August. Variation of spatiotemporal distribution and abundance are discussed along with areas. .