It’s hard to tell what may be causing jellyfish to proliferate. The fishing industry has depleted populations of big predators such as red tuna, swordfish and sea turtles that feed on jellyfish. And when small, plankton-eating fish such as anchovies are overharvested, jellies flourish, gorging on plankton and reproducing to their hearts’ content (if they had hearts, that is).
In 1982, when the Black Sea ecosystem was already weakened by anchovy overfishing, the warty comb jelly (Mnemiopsis leidyi) arrived; a species native to the East Coast of the United States, it was most likely carried across the Atlantic in a ship’s ballast water. By 1990, there were some 900 million tons of them in the Black Sea.
Pollution, too, may be fueling the jelly frenzy. Jellyfish succeed in all sorts of fouled conditions, including “dead zones,” where rivers have pumped fertilizer runoff and other materials into the ocean. The fertilizer fuels phytoplankton blooms; after the phytoplankton die, bacteria decompose them, hogging oxygen; the oxygen-depleted water then kills or forces out other marine creatures. The number of coastal dead zones has doubled every decade since the 1960s; there are now roughly 500. (Oil can kill jellyfish, but no one knows how jellyfish populations in the Gulf of Mexico will fare in the long run after the BP oil spill.)
Carbon-based air pollution may be another factor. Since the Industrial Revolution, the amount of carbon in the atmosphere from burning fossil fuels and wood as well as from other enterprises has risen by some 36 percent. That contributes to global warming, which, some researchers speculate, may benefit jellyfish at the expense of other marine animals. Moreover, carbon dioxide dissolves in seawater to form carbonic acid—a major threat to marine life. As the seas become more acidic, scientists say, ocean water will begin to dissolve animal shells, stunt coral reefs and disorient larval fish by skewing their sense of smell. Jellies, meanwhile, may not even be inconvenienced, according to recent studies by Jennifer Purcell of Western Washington University.