Physical processes influencing the occurrence of the giant jellyfish Nemopilema nomurai (Scyphozoa : Rhizostomeae) around Jeju Island, Korea

Abstract

The physical and biological processes affecting the spatial distribution of the giant jellyfish Nemopilema nomurai were investigated in the northern East China Sea and southern Yellow Sea in July 2006, when large abundances of this jellyfish occurred in Korean waters. Well-characterized water masses were detected during the study period. These were the Tsushima Current water, Yellow Sea Bottom Cold Water and Changjiang Diluted Water (CDW). The spatial distribution of Chl a concentrations estimated by ocean colour imagery showed that the CDW flows to the northeast. The CDW was accompanied by high abundances of small and medium-sized zooplankton (0.2-0.5 mm and 0.5-1.0 mm, respectively) and N. nomurai, in comparison with the other water masses. Large (1.0-2.0 mm) and macro-sized zooplankton (2.0-5.0 mm) did not show a relationship with the CDW. The distribution of N. nomurai was very restricted and intimately related with the CDW. In fact, large abundances only occurred in the western boundary region of the CDW. The results were supported by Pearson's correlation analysis, which showed a positive relationship between the CDW, small- and medium-sized zooplankton and the giant jellyfish. We conclude that the CDW had entrained N. nomurai into the waters off the southwest coast of the Korean peninsula and it was then transported into the Yellow and East/Japan seas.