Recurrent jellyfish blooms in the East Asian seas: how to adapt and manage for fisheries sustainability

Aurelia aurita and Nemopilema nomurai are bloom-forming scyphozoans in the East Asian seas, which cover <1% of the total marine area but sustain 11% of the world fish catch. Their recurring blooms exert severe nuisance primarily in net-based fisheries. In the face of increasing jellyfish populations, the fisheries have to develop adaptive and management strategies, such as identifying causes for the blooms, forecasting outbreaks, and developing countermeasures. The causes may be multiple, but closely associated with increased anthropogenic impacts to the coastal environment and ecosystem. The forecast of year-to-year bloom intensity is possible for N. nomurai by monitoring juvenile medusae from ships of opportunity in Chinese waters, the seeding ground of this species, and Japanese fishermen can prepare countermeasures 1-3 months in advance of jellyfish outbursts. Bloom forecasting is, however, more challenging in A. aurita, since all life cycle stages coexist within a relatively small geographical area such as a bay. The basic parameters for the forecast may be 1) population size of polyps, 2) population size of ephyrae, and 3) mortality of ephyrae before recruitment to the medusa stage. As countermeasures, Japanese fishermen have introduced various types of jellyfish excluding devices in their fishing nets.