Mass appearance of the jellyfish Nemopilema nomurai Kishinouye and its influences on fisheries in Japan

In 2002 and 2003 consecutively, large numbers of giant jellyfish Nemopilema nomurai appeared along coastlines of the Japan Sea and Pacific Ocean, causing severe damage to local fisheries. In 2003, Agriculture, Forestry and Fisheries Research Council funded the Fisheries Research Agency to conduct a brief investigation on influences of the jellyfish mass-bloom on fisheries, and to assess possible countermeasure techniques. The survey of places and periods of appearance illustrated possible routes on which jellyfish were carried. The jellyfish seemed to be carried on Tsushima Current from the west of Kyushu through the western channel of the Korean Strait. Causes of the jellyfish mass-bloom are unknown. Together with elucidation of causes of mass-blooms, a numerical model of currents in East China Sea and Japan Sea should be developed to forecast the places and times of coastae jellyfish mass-bloom appearances. Local fisheries suffered severe damages. Fewer fish entered trap nets due to jellyfish densities jellyfish, and the fish caught in nets were pressed and stung by nematocysts, resulting in lower market price. In some cases, fishermen removed fixed shore nets from the sea in the middle of fishing season. Because fishermen had to separate fish from jellyfish, the work on board became very hard, and sorting continued on laud. However, fishermen tried various countermeasures, such as putting an extra net at the mouth of trap nets, which prevented jellyfish to enter but allowed fish to enter into the net. For a trawl net, a net was placed in the middle of the net to separate jellyfish and target fish. Studies on behavior of jellyfish around fishing gear should be conducted to develop efficient strategies and net operations. In the brief investigation, the author also assessed possibilities of utilization of jellyfish for food and other functional materials. Techniques for efficient removal of large amount of water and salts are necessary to be developed.