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.
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