Jellyfish blooms are up almost everywhere and it's going to become a menace.  The reason?   Over-fishing and excess nutrients from fertilizers and sewage, say researchers from The Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia's national science agency.

"Dense jellyfish aggregations can be a natural feature of healthy ocean ecosystems, but a clear picture is now emerging of more severe and frequent jellyfish outbreaks worldwide," says CSIRO Climate Adaptation Flagship and University of Queensland scientist Dr Anthony Richardson.

The new research, by Dr Richardson and colleagues at the University of Miami, Swansea University and the University of the Western Cape, has been published in the international journal Trends in Ecology and Evolution.

"In recent years, jellyfish blooms have been recorded in the Mediterranean, the Gulf of Mexico, the Black and Caspian Seas, the Northeast US coast, and particularly in Far East coastal waters.   The most dramatic have been the outbreaks in the Sea of Japan involving the gargantuan Nomura jellyfish which can grow up to 2 m in diameter and weigh 200 kg."

Climate change may favor some jellyfish species by increasing the availability of flagellates in surface waters – a key jellyfish food source. Warmer oceans could also extend the distribution of many jellyfish species.

"Fish normally keep jellyfish in check through competition and predation but overfishing can destroy that balance," Dr Richardson says. "For example, off Namibia intense fishing has decimated sardine stocks and jellyfish have replaced them as the dominant species."

"Mounting evidence suggests that open-ocean ecosystems can flip from being dominated by fish, to being dominated by jellyfish," Dr Richardson says "This would have lasting ecological, economic and social consequences.  We need to start managing the marine environment in a holistic and precautionary way to prevent more examples of what could be termed a 'jellyfish joyride'."