Jellyfish Numbers Exploding In British Waters - Particular Threat To Remaining Cod - Guardian

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British fish stocks are threatened by an unusual consequence of global warming: a dramatic rise in jellyfish numbers, scientists warned yesterday. Warmer ocean temperatures have seen jellyfish populations surge in the North Sea and scientists fear they may soon dominate at the expense of other marine life. Many of the jellyfish species feed directly on fish larvae or the plankton and tiny crustaceans that make up the larvae's staple diet, before they are big enough to hunt more substantial prey. Research by marine biologists shows that as ocean temperatures rise over the next century jellyfish populations will continue to grow, putting renewed pressure on fish stocks already devastated by overfishing.

The predictions bode particularly badly for cod stocks, which have plummeted through over-exploitation by fisheries. Cod larvae hatch in waters that are rich in plankton but these are also feeding grounds for the jellyfish.

Martin Attrill, a marine ecologist at Plymouth University's marine institute, analysed more than 40 years of records on marine organisms. They showed that jellyfish numbers have risen in line with a warming of the oceans caused by a meteorological process called the North Atlantic oscillation (NAO). When the NAO is positive high air pressures build up over the Azores and bring warmer water to the seas around Europe.

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http://www.guardian.co.uk/uk_news/story/0,,1992835,00.html