Millions of the creatures, each weighing almost 450 pounds with a bell reaching almost 7 feet across and trailing a tangle of long, stinging tentacles, have moved into the Sea of Japan every summer since 2002, LiveScience.com reported Wednesday.
Armadas of giant jellyfish, Nemopilemanomurai, probably originate in the coastal waters of China, Shin-ichi Uye of Hiroshima University said. Uye said he has documented the movement of young giant jellyfish from Chinese waters into the Sea of Japan on currents, and the growth of these jellyfish along the way.
No one knows for sure what is causing the exploding numbers of giant jellyfish in Asia but Uye suggests rising ocean temperatures are increasing the production of the tiny polyps that can reproduce asexually before developing in free-swimming jellyfish. Temperatures in the Sea of Japan and Chinese coastal waters have increased by more than 1.89 degree Fahrenheit, Uye said.
He also points to predators and competitors of jellyfish being removed by overfishing.
Giant jellyfish reduce fish catches by clogging and bursting nets, stinging netted fish and thereby spoiling or killing the catch and requiring increased labor to remove jellyfish from nets.
Giant jellyfish in 2009 capsized a Japanese trawler carrying three fishermen, who were all rescued, LiveScience.com reported.