Jellyfish populations are likely to be increasing globally, particularly in waters under significant human impact such as the East Asian Marginal Seas (EAMS), where the moon jellyfish Aurelia aurita s.l. and the giant jellyfish Nemopilema nomurai have regularly caused problematic blooms. Blooms of jellyfish impose considerable socio-economic hardship to fisheries, aquaculture, power plant operations and tourism. Blooms appear to be difficult to eradicate, but these industries are striving to develop management strategies that will enable them to successfully co-exist with more jellyfish. In A. aurita, medusae tend to bloom in coastal waters with strong human-perturbation (e.g. warming, eutrophication, marine construction) to cause serious problems in local fisheries and power plant operations. Whilst N. nomurai is distributed widely over the entire EAMS and bloomed intermittently (once per ca. 40 years) for most of the 20th century, but became almost annual between 2002 and 2009. Recent frequent blooms of this species might be attributed to environmental deterioration in Chinese coastal waters, a seeding and nursery ground. In 2005, the bloom of N. nomurai caused severe nuisance in fisheries in Japan, with >100,000 complaints and reports of damage in fishery and estimated monetary loss of ca. 30 billion JPY (=ca. 230 million Euro). Early forecast of the bloom based on deck-on-ferries survey of young medusae in Chinese waters has greatly contributed toward reduction of damage by this jellyfish. The actual countermeasure taken by set-net fishermen is to modify their nets to exclude jellyfish, for which they have to invest extra money.