Temporal fluctuations in abundance and size of the giant jellyfish Nemopilema nomurai medusae in the northern East China Sea, 2006-2017

The abundance and size of the giant jellyfish (Nemopilema nomurai) medusae collected by midwater trawl from 2006 to 2017 in the northern East China Sea (nECS) were investigated. While absent in 2008 and 2011, in other years, medusae were prevalent in June and July, rare in October, and not found from January to May. This seasonality in medusa abundance might be related to the life cycle characteristics of this species, with strobilation occurring near Chinese coast of the nECS from late March to early June but medusae dispersing in oceanic currents or dying by early autumn. Peak abundances in June and July varied from 0 to 62.1 x 10(6) medusa m(-2) in the repeatedly observed area (30 degrees 30 ' N-31 degrees N, 124 degrees 15 ' E-126 degrees E). These annual fluctuations in medusae abundance in the nECS roughly coincide with patterns in the Sea of Japan (JS) in the autumn, which suggests that monitoring in nECS would be effective in predicting the occurrence of this species in the JS which undergoes massive occurrences of this species. The medusae sizes were comparable in June, but smaller in 2006, 2007, 2009, and 2010 than that in 2012-2017 in July. Hence, the existence of a large annual variation in the size of N. nomurai even in the same area and period was indicated.