Nemopilema nomurai grows to a bell roughly two metres across and a mass reported around 200 kilograms, which makes it one of the heaviest jellyfish known. In bloom years it appears in the Sea of Japan in enormous numbers, entering set nets and trawls in quantities that damage gear, crush the catch and cost fishing communities substantial sums.
On 1 November 2009 the trawler Diasan Shinsho-maru capsized off Chiba Prefecture while its crew attempted to haul a net loaded with the jellyfish. All three crew were recovered by another vessel. The incident is widely cited as a marker of how heavy a bloom can become.
The cause of major blooms is not settled. Warming water, eutrophication in the Yellow Sea and East China Sea where the polyps are thought to develop, removal of predators and competitors through fishing, and added hard substrate from coastal construction have all been proposed as contributors. Blooms have occurred at irregular intervals rather than annually, and it is worth resisting the frame that jellyfish are uniformly increasing worldwide; long-term data support regional oscillation better than a global trend.