Physical oceanographer Wei Hao first encountered her nemesis in the summer of 2001, during a research cruise through a Yellow Sea brimming with slimy, gelatinous masses. “Jellyfish were everywhere,” recalls Wei, dean of marine science and engineering at Tianjin University of Science and Technology. Since then, three more massive blooms have beset the region. The outbreaks have Wei and other scientists worried that the Yellow Sea is on the brink of regime change, in which jellyfish supplant fish as the dominant open-ocean species. “For so long we were focused on tracking fish,” says marine ecologist Sun Song, director of the Institute of Oceanology of the Chinese Academy of Sciences here. “But now we realize jellyfish may be more important … as a major indicator of environmental change.”
Last month, researchers met in this port city to plot strategy on a 5-year, $4 million mission to understand the disturbing ascendancy of jellyfish in Asian waters, one that parallels the apparent rise of the animals in the Mediterranean and Caspian seas and other bodies of water (Science, 16 September 2005, p. 1805
). “It's an amazing amount of money directed at a neglected problem,” says Anthony Richardson, a marine ecologist at CSIRO Marine and Atmospheric Research in Cleveland, Australia. One objective of Sun's team of 70 researchers from six institutions is to probe the creature's life cycle and interactions with other organisms. Another aim is to pin down the cause of blooms; in Asia, chief suspects are overfishing, climate change, and eutrophication, or excess nutrients in coastal waters. Such knowledge may prove critical to thwarting an ecological hijacking. “Once the ecosystem shifts to jellyfish, it may be impossible to go back,” warns Yu Zhigang, a marine chemist here at Ocean University of China.
Jellyfish already take a heavy toll on society. Stings kill dozens of people a year—far more than shark attacks. “It's the most dangerous marine creature in the world,” says Sun. The box jellyfish is especially lethal: A victim can die within 3 minutes of being stung. The jellyfish's long rap sheet also includes fouling fishing nets, killing farmed fish, closing down popular beaches, and clogging cooling intakes at coastal power plants.
Although data are sparse, anecdotal reports indicate that jellyfish blooms are becoming more frequent worldwide. East Asia has been particularly hard hit. In the Sea of Japan, outbreaks of Nomura's jellyfish (Nemopilema nomurai), which can reach up to 2 meters in diameter and weigh 200 kilograms, once were a rare phenomenon. But massive blooms have occurred almost every summer since 2002, inflicting billions of dollars' worth of damage to fisheries, says Shin-ichi Uye, a marine ecologist at Hiroshima University. He estimates that in 2005, up to 20 billion of the refrigerator-sized jellyfish choked the Sea of Japan. That breathtaking abundance may sound like a bonanza for researchers, but studying jellyfish isn't easy. Even titans like Nomura's are fragile and break easily, and fishers are loath to handle them. Satellite imaging is useless: During their medusa stage, jellyfish mostly hover meters below the surface. “We're dealing with something that's very hard to measure,” says Richardson.
Much about jellyfish consequently remains an enigma. One aim of the new initiative, called the China Jellyfish Project, is to shed light on the animal's “mysterious and complicated life history,” Sun says. The vast majority of jellyfish belong to the phylum Cnidaria, which includes the Portuguese man-of-war and the three species—Nomura's, Aurelia aurita, and Cyanea nozakii—plaguing Chinese waters. Cnidaria species can spend years on the sea bottom as polyps. These reproduce asexually, popping off medusae that drift up the water column toward the surface. In the Chinese project, scientists will search for the elusive polyps in the Yellow Sea and East China Sea. The team also plans to integrate sonar fish finders with cameras to hunt for medusae. That approach will be “much better” than using plankton nets or trawl nets, says Uye.
The most pressing task may be to unravel the cause of blooms. Overfishing of certain species “opens up ecological space for jelly fish,” says Richardson. Decimated sardine stocks may be to blame for the rise of Chrysaora jellyfish off Namibia. Fish prey on jellyfish, so overfishing removes a check on jellyfish population growth. And jellyfish prey on fish eggs and larvae, making it harder for battered fish stocks to mount a comeback.
Another concern is eutrophication. A surfeit of nutrients from agricultural runoff and sewage spurs phytoplankton blooms in coastal waters that can trigger jellyfish outbreaks. Such conditions also create low-oxygen dead zones. Compared with fish, jellyfish—both polyps and medusae—are much more tolerant of hypoxic conditions. Because the number of marine dead zones worldwide has doubled each decade since the 1960s, there's more habitat better suited to jellyfish, says Richardson.
Recent climate change also appears to have given jellyfish a helping hand. Warmer ocean temperatures are correlated with jelly fish outbreaks, “but we don't know the mechanism,” says Sun. He and others fear that warming, overfishing, and eutrophication may work in concert to create jellyfish-dominated ecosystems reminiscent of those during the Precambrian world, more than 550 million years ago.
The waters off China may be nearing the tipping point beyond which fish predators are unable to hold jellyfish in check. In the past 5 years, says Sun, anchovy catches in the Yellow Sea have decreased 20-fold. During a Yellow Sea research cruise in the summer of 2009, jellyfish constituted 95% of hauls. In coordination with colleagues in Japan, Korea, and Russia, Sun says, field surveys and modeling may allow researchers to calculate “how many jellyfish it will take to change the ecosystem.” That, in turn, could lay the groundwork for interventions that forestall doomsday for fish.
(Source: Science Magazine)