Increasing Jellyfish Populations

Scientists have had a tough time trying to discover whether recent increases in jellyfish populations are really worth worrying about. On the one hand, jellyfish are known to proliferate rapidly in response to positive changes in prey abundance or environmental conditions such as water temperature and sunlight. The size of these “blooms” can vary from year to year. On the other hand, these population explosions are occurring in many places on a scale now widely viewed as unprecedented. In the Sea of Japan, for instance, Nomura’s jellyfish are known to have drifted in from the south in large numbers three times during the twentieth century: in 1920 1958 and 1995. Beginning in 2002, however, they have turned up every summer but one, and in astonishingly high numbers. In 2005, one of the worst years, up to 500 million Nomura’s jellyfish were reported to be drifting into the sea each day

Several factors have now been identified as possible contributions to the increased success of jellyfish worldwide. One of the leading suspects is the human exploitation of fish and other marine resources. something that has intensified in recent decades partly because of advances in large-scale seafood harvesting and processing techniques. Only a handful of species are thought to prey directly on jellyfish, and most of these predators-including giant sea turtles-are becoming increasingly rare. The main impact of overfishing, though, may stem from the reduction of filter-feeding fish such as sardines and anchovies, which eat the same food as jellyfish. In the southern Atlantic waters off Namibia. where overharvesting has resulted in the complete collapse of a once-thriving sardine fishery, unusually large numbers of jellyfish are now a permanent feature of the near-shore marine ecosystem.

At the same time, jellyfish seem to thrive under conditions that are becoming increasingly widespread because of human-associated activities. Although the theory is highly speculative and still under debate, global warming and acidification of the oceans a result of more carbon dioxide dissolving in the water-may be two such factors. Jellyfish love warm water, for one thing. And at least one study, from the North Sea, has reported finding a connection between greater jellyfish abundance and higher acid levels.

Another environmental change, and one that is more firmly linked to expanding jellyfish populations, is eutrophication, the process by which water becomes enriched with dissolved nutrients, resulting in more aquatic plant life and, subsequently, reduced levels of oxygen. Eutrophication occurs in near-shore ocean waters close to large human population centers and the mouths of large rivers. A strong correlation exists between blooms of algae and other plankton, caused by excessive nutrients from sewage and fertilizer runoff, and jellyfish eruptions. One example is the waters off the southern United States coast. Nutrient-enriched waters from the Mississippi River have created in the Gulf of Mexico a massive dead zone (ocean area with low oxygen levels).

While fish and other aquatic life-forms are finding survival increasingly challenging, jellyfish such as moon jellyfish and sea nettles are becoming increasingly numerous. This is not really surprising. A checklist of jellyfish traits-good survival rates during periods of starvation, rapid reproduction, diverse diet, ability to feed in murky water, capacity for surviving under low oxygen conditions typical of dead zones-reveals that these seemingly fragile organisms are actually tough life-forms ideally suited to survive in disturbed environments. According to the National Science Foundation, the main science-funding agency in the United States there are currently 400 known ocean dead zones where almost nothing lives except jellyfish.

Finally, like many other marine invasive species, jellyfish have benefited from the globalization of human trade. Many species are hardy enough to survive in the ballast water of ships, and jellyfish polyps can attach themselves to their hulls. Because of this, jellyfish species are being inadvertently introduced into new aquatic habitats. One of the best examples involves the warty comb jelly (Mnemiopsis leidyi ), a species that likely made its way into the Black Sea for the first time in 1982. Since then this highly adaptable organism has been thriving; its original nonnative range has expanded to include not just the Black Sea but also the Caspian, Baltic and North seas

1.Scientists have had a tough time trying to discover whether recent increases in jellyfish populations are really worth worrying about. On the one hand,?jellyfish are known to proliferate rapidly in response to positive changes in prey abundance or environmental conditions such as water temperature and sunlight. The size of these “blooms” can vary from year to year. On the other hand, these population explosions are occurring in many places on a scale now widely viewed as unprecedented. In the Sea of Japan, for instance, Nomura’s jellyfish are known to have drifted in from the south in large numbers three times during the twentieth century: in 1920 1958 and 1995. Beginning in 2002, however, they have turned up every summer but one, and in astonishingly high numbers. In 2005, one of the worst years, up to 500 million Nomura’s jellyfish were reported to be drifting into the sea each day