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Research in Action: Giant Jellyfish Jam the Sea of Japan
A diver is dwarfed by a giant jellyfish.
Credit: Yomiuri Shimbun

This Research in Action article was provided to LiveScience in partnership with the National Science Foundation.

China, which is currently producing some of the world's biggest buildings, dams, ports and casinos, is probably also producing the world's biggest jellyfish.  Almost every summer since 2002, billions of giant jellyfish — each weighing a maximum of 450 pounds (204 kilograms), bearing a bell reaching almost 7 feet (2 meters) in diameter, and trailed by a tangle of long, stinging tentacles — have floated daily into the Sea of Japan, wreaking havoc on the Japanese fishing industry. 

Source of the blooms

Where do the Sea of Japan's flotillas of giant jellyfish (Nemopilemanomurai) originate?  Probably in the coastal waters of China, according to Dr. Shin-ichi Uye, an expert in giant jellyfish from Hiroshima University in Higashi-Hiroshima, Japan. Dr. Uye recently identified Chinese coastal waters as the probable source of the giant jellyfish by surveying jellyfish populations as he rode on ferries travelling from China to Japan. During these ferry trips, Dr. Uye 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.

Scientists such as Dr. Uye are increasingly concerned about the ecology of giant jellyfish, because their blooms in Asian waters are becoming more frequent. 

Long tentacles of environmental change

No one knows for sure why blooms of giant jellyfish in Asian waters are becoming more common. But Dr. Uye says that current theories center on the various ways that Chinese coastal waters are currently being degraded. This degradation is associated with environmental pressures generated by the 800 million people who currently live along the coasts of the East Asian Marginal Seas (EAMS), which include the Bohai, Yellow, East China and Japan Seas. Many of these environmental pressures on the EAMS favor the proliferation of giant jellyfish. 

For example:

  • Dr. Uye has conducted laboratory studies of the larva of giant jellyfish, which live as tiny polyps that can reproduce asexually before developing into free-swimming jellyfish.  His results suggest that rising water temperatures may increase the rate of polyp reproduction, and speed their development into free-swimming jellyfish. During the last 25 years, temperatures in the Sea of Japan and Chinese coastal waters have increased by more than 1 degree Celsius.
  • Populations of predators and competitors of jellyfish, which usually provide important controls on jellyfish populations, are being removed by overfishing.
  • The construction of artificial structures, such as ports, along the Chinese coast increases habitat for jellyfish polyps, which must attach to hard surfaces to survive. 
  • The flow of runoff containing fertilizers and sewage into China’s coastal waters, promotes the growth of tiny phytoplankton and then zooplankton, microscopic animals that drift in seawater. These phytoplankton provide food for giant jellyfish.
  • The Three Gorges Dam on the Yangtze River, which is the world's largest dam, may be changing the chemistry of China's coastal waters in ways that support jellyfish blooms. (The area near the confluence of the polluted Yangtze River with the East China Sea is believed to provide a breeding ground for giant jellyfish.)

Nevertheless, year-to-year differences in the sizes and locations of recent blooms of giant jellyfish suggest that other factors, in addition to environmental stresses, influence the occurrence of jellyfish blooms. These factors may include biological factors, such as the potential dormancy of podocysts — young jellyfish that live as tiny, dome-shaped organisms encapsulated in a hard cuticle cover — for up to five years. Scientists suspect that when conditions favor the survival of jellyfish blooms, dormant podocysts may move into other stages of development and emerge en masse to ultimately form jellyfish blooms.

The damage done

Blooms of giant jellyfish are damaging the Japanese fishing industry in varied ways.  For example, when these hefty creatures are accidentally caught, they: 1) reduce fish catches by clogging and busting nets; 2) sting fish that are netted with them, and thereby spoil or kill the catch; and 3) increase labor needed to remove jellyfish from nets. In 2009, giant jellyfish capsized a Japanese trawl boat that was carrying three fishermen, who were fortunately rescued.

Losses from marauding mobs of giant jellyfish in the Sea of Japan typically total hundreds of millions of dollars during a single bloom season.

Reducing the damage

To reduce such damage in the future, Dr. Uye has developed an early warning system for the arrival of giant jellyfish in the Sea of Japan. This system involves monitoring populations of giant jellyfish from ferries traveling from Chinese coastal waters to the Sea of Japan. Resulting data are then fed into mathematical models that can predict the drifting pathways of the giant jellyfish into the Sea of Japan and their approximate arrival dates up to about three months ahead of time.

These types of warnings give fishermen time to adjust their nets in ways that help bar giant jellyfish from their nets. However, such adjustments are too costly for many fishermen to make. What’s more, such methods are, of course, only a short-term fix — not a long-term solution.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. See the Research in Action archive.