Fishfarmers off the British Islands could only watch helplessly as their whole stock was killed in the matter of minutes by the non-native marauding stingers and in Japan massive Nomura’s jellyfish the size of sumo wrestlers, more commonly found in Chinese and Korean waters, were proliferating off Japan’s coast where they have grown a hundredfold in some areas turning into a pest for Japanese fishermen. During the recent years the massive sea creatures, which can grow two meters wide and weigh up to 220 kilograms, were clogging and ripping fishing nets, causing havoc for fishermen who have to spend hours hacking them out of their nets. The fishermen’s catch were also being poisoned by the invertebrates’ toxic stingers. At one point, the crisis prompted fishermen to come up with cooking recipes, although the jellyfish are rarely eaten in Japan.

Japanese scientists speculated that the jellyfish grow big along the coast of China and have been drifting from China’s Yangtze River Delta, where unusually heavy rains may be pushing the jellyfish to Japan.

Then suddenly this year—to the great relief and puzzlement of fishermen and researchers alike—the Sea of Japan saw a drastic decline in swarms of the huge jellyfish. According to the Japan Fisheries Information Service Center, fishermen had reported about 6,300 sightings of Nomura’s jellyfish as of November 20 last year, compared with only 128 this year. “This is a dramatic fall in numbers,” said Katsuya Saito, an official at the Tokyo-based nonprofit research center.

“Up to last year, 3,000 to 5,000 of the jellyfish would get tangled up in a single fixed net in some cases. But this year, only one or two were reported to have been caught,” Saito said.

Saito said that until 2001, a heavy presence of the jellyfish occurred only once every several decades. But from 2002 to 2007, thousands were seen in fall and winter in the Sea of Japan and parts of the East China Sea, Saito said.

Hitoshi Iizumi, an official at Japan Sea National Fisheries Research Institute, a government-affiliated agency, said scientists have not determined the cause of the sudden disappearance. Researchers believed three factors near China conspired to create the surge in jellyfish in the recent years: Eutrophic water coming to the sea from modern Chinese cities, global warming that has increased the sea temperatures, and increased fish catches resulting in more zooplankton. But researchers are not sure if those factors changed this year, Iizumi said. Yet another is that China has over-fished their waters and reduced the populations of the jellyfish’s natural predators, which fed on the larvae while they are still zooplankton. Yet another cause may be China’s new dam, the Three Gorges Dam. On the Yangtze River, the Three Gorges Dam, the world’s largest hydroelectric project, has increased the amount of phosphorus and nitrogen in the waters off China, creating an ideal breeding ground for Nomura’s jellyfish. A final possibility is global warming, which would cause the heating up of the seawater and encourage jellyfish breeding. Jellyfish also have the ability to take in oxygen directly from their skin allowing the jellyfish to thrive in the oceans growing dead zones.