TOKYO -- Seas around the world are turning into jellyfish soup, as swarms of the creatures hit coastal areas, paralyzing power plants and undermining fisheries.
These massive outbreaks are being caused by coastal development, overfishing and other man-made factors. Giant jellyfish have been swarming into the Sea of Japan in recent years.
The phenomenon in Japan is believed to be closely related to China's economic development.
Power plant damage
The situation at Sweden's Oskarshamn nuclear power plant in September 2013 was tense. The plant, situated on the Baltic Sea coast, could not get water from the sea to cool its reactors. Its water intake was clogged with jellyfish. OKG, the plant operator, temporarily halted operations at the facility as a precautionary measure.
A similar thing happened at Kansai Electric Power's Ako thermal power plant in Japan's Hyogo Prefecture in August 2012. Seas surrounding the plant were swamped with jellyfish.
Because the thermal power plant could not get cooling water from the sea, it had to be kept offline for a while. The baskets used to pluck jellyfish from the sea reportedly broke under the weight of so many of the blobby sea creatures.
The French Riviera, one of the world's most popular luxury seaside resorts, has been flooded with armies of jellyfish for the past several years.
Vacationers staying there are said to pay more attention to jellyfish forecasts than the weather. A local oceanographic institute has started issuing jellyfish alerts.
The jellyfish problem was one of the main items on the agenda at an international marine conservation conference held in Barcelona, Spain, last November.
Shinichi Ue, a professor at Japan's Hiroshima University, delivered a lecture at the conference. He warned the world's seas "will be in big trouble" if the international community "fails to get serious about countermeasures against jellyfish."
Japan has a head start in this area, as it has been grappling with species such as the moon jellyfish and Nomura's jellyfish before the problem became a global one.
Swarms of Nomura's jellyfish -- a behemoth that grows to more than 2 meters in diameter and weighs upward of 150kg -- used to arrive on Japan's coasts about every 40 years. Since 2002, however, these visits have become an almost annual occurrence.
A fishing boat off Inubosaki in Chiba Prefecture capsized in 2009 after its fishing net was clogged with Nomura's jellyfish.
One year, outbreaks of Nomura's jellyfish reportedly caused 30 billion yen ($251 million) in damage to Japan's fisheries.
Cost of growth
So where are the swarms of Nomura's jellyfish overwhelming Japan coming from?
A study by professor Ue, the Fisheries Research Agency and others found the jellyfish are born in northern parts of the East China Sea.
China's coastal areas have been significantly developed, and its seashores are rapidly being overrun by concrete embankments. These embankments provide ideal surfaces for polyps -- baby jellyfish -- to stick to and grow.
Fast-developing Chinese coastal areas are also pouring huge amounts of sewage and agricultural wastewater into the sea. This contains nitrogen and phosphorus, which boost the populations of plankton on which jellyfish thrive.
"China's coastal region contains the perfect culturing fluid for jellyfish," Ue said.
In spring, polyps clinging to embankments release juvenile jellyfish measuring only 2-3mm. These young are pushed by currents into the Genkainada area off the coast of Japan's southern island of Kyushu in early summer. By this time, the jellyfish weigh several kilograms, growing to more than 100kg in autumn, when they reach the northern part of Honshu, the biggest of Japan's four main islands.
Jellyfish feed on zooplankton drifting in the seas. A particularly large Nomura's jellyfish takes in enough seawater each day to fill a swimming pool, gobbling up any plankton it catches in the process.
Plankton are also the prey of juvenile sardines and horse mackerel. The overfishing of such species is believed to be one cause for the proliferation of jellyfish.
There have been no confirmed swarms of Nomura's jellyfish reaching Japan in the past year or two, but it is too early for the country to let down its guard. Juvenile jellyfish can remain in a form of suspended development covered by stiff shells for eight years or so. China's embankments are home to an astronomical number of these giants-to-be, and any stimulus could cause them to awake en masse.
Resolving Japan's jellyfish problem will require trilateral cooperation among Japan, China and South Korea.
Explosive jellyfish population growth in recent years has been confirmed in various parts of the world, including the Baltic Sea, the Mediterranean, the Black Sea and the Gulf of Mexico.
As in Japanese waters, the causes of outbreaks in those seas are believed to be coastal development, nutrient enrichment, overfishing of commercial fish species, global warming and so on.
Enormous blooms of jellyfish around the world may be the wake-up call humanity needs if it is to stop destroying marine ecosystems.