A new threat to fisheries around the world has developed over the last decade---a surge in the number of jellyfish in coastal waters. The most dramatic of these outbreaks is in Japanese waters, where the giant Nomura’s jellyfish has increased significantly, wreaking havoc with fisheries in the Sea of Japan.
Jellyfish populations are normally held in check by fish, mostly because these two groups compete for the same food sources. However, overfishing in many parts of the ocean has led to increasing jellyfish populations. Jellyfish may also be aided by warming ocean temperatures, which favors their development, and by the destruction of habitats of other natural predators such as turtles.
The massive Nomura’s jellyfish is a great threat to Chinese, Japanese and Korean fisheries. These creatures can grow to two meters diameter (the size of large refrigerators) with a weight of 200 kg.
Examples of Giant Jellyfish
Because of their size, they consume massive amounts of zooplankton, depleting this vital part of the food chain for other organisms. The key threat of the Nomura’s derives from the fact that this jellyfish reproduces extremely rapidly. A mature jellyfish has the ability to produce billions of eggs at a time, and they can do this when they are attacked. Once fertilized, these eggs develop into a resting polyp stage that also has the ability to multiply rapidly, effectively carpeting areas of the seafloor. When conditions are suitable, the polyp reproduces asexually, developing into the medusa stage, which grows into the mature jellyfish.
Video: Jellyfish Life Cycle (1:41)
Once ideal conditions develop, either by increasing nutrients or warming of the surface ocean, Nomura’s jellyfish populations literally explode and render fishing virtually impossible because nets become filled with jellyfish. These jellyfish can also continue to reduce the number of fish in the oceans by feeding on their eggs. Moreover, there is evidence that jellyfish can tolerate conditions, like hypoxia, that fish cannot.
The following video summarizes the impact of giant jellyfish on Japanese fisheries.
Video: Monster Jellyfish (2:35)
Blooms of other jellyfish species are being reported in many other parts of the ocean. In the Gulf of Mexico, for example, the last thirty years populations of two species of jellyfish, the sea nettle, and the moon jellyfish, have exploded especially in dead zones as these are one of the few organisms that can tolerate hypoxia. Jellyfish in the Gulf now swarm over hundreds and perhaps even thousands of square miles each summer.
Here also, invasive species of jellyfish, including the Australian jellyfish, have been reported. Several other factors besides hypoxia have caused the increase in Gulf of Mexico jellyfish. As in the Sea of Japan, overfishing has reduced one of the main jellyfish competitors. In addition, drilling platforms have provided habitats in which jellyfish polyps can multiply. As in the Sea of Japan, jellyfish in the Gulf of Mexico are impacting the fishing industry.
Jellyfish swarms, too, have plagued other regions; they include northern Australia where the highly venomous box jellyfish has expanded its range, the Black Sea, and the Bering Sea off Alaska. Worldwide, jellyfish are one of the few organisms that can thrive in dead zones. With the spread of such dead zones in the oceans as a consequence of marine pollution and climate change, we could be entering the age of the jellyfish.