Giant jelly fish are taking over parts of the world's oceans due to overfishing and other human activities, say researchers.
Dr Anthony Richardson of CSIRO Marine & Atmospheric Research and colleagues, report their findings in the journal Trends in Ecology and Evolution.
"We need to take management action to avert the marine systems of the world flipping over to being jellyfish dominated," says Richardson, who is also a marine biologist at the University of Queensland.Richardson says jellyfish numbers are increasing, particularly in Southeast Asia, the Black Sea, the Gulf of Mexico and the North Sea.
He says the Japanese have a real problem with giant jellyfish that burst through fishing nets.
"[They're] a jelly fish called Nomura, which is the biggest jellyfish in the world. It can weigh 200 kilograms, as big as a sumo wrestler and is 2 metres in diameter," says Richardson.
Overfishing and eutrophication
Richardson and colleagues reviewed literature linking jellyfish blooms with overfishing and eutrophication - high levels of nutrients.
Jellyfish are normally kept in check by fish, which eat small jellyfish and compete for jellyfish food such as zooplankton, he says.
But, with overfishing, jellyfish numbers are increasing. Jellyfish feed on fish eggs and larvae, further impacting on fish numbers.
To add insult to injury, nitrogen and phosphorous in run-off cause red phytoplankton blooms, which create low-oxygen dead zones where jellyfish survive, but fish can't.
"You can think of them like a protected area for jellyfish," says Richardson.
Richardson and colleagues say climate change may also encourage more jellyfish.
They have postulated for the first time that these conditions can lead to what they call a "jellyfish stable state", in which jellyfish rule the oceans.
Action
Richardson and colleagues recommend a number of actions in their paper, to coincide with World Oceans Day.
They say it's important to reduce overfishing, especially of small pelagic fish, like sardines, and to reduce run-off.
They also say it's important to control the transport of jellyfish around the world in ballast water and aquariums.
Richardson says researchers are experimenting with different ways of controlling jellyfish.
Some methods involve sound waves to explode jellyfish, while others use special nets to try and cut them up.
Simple animals
Jellyfish are considered simple jelly-like sea animals, which are related to the microscopic animals that form coral.
They generally start their life as a plant-like polyp on the sea bed before budding off into the well-known bell-shaped medusa.
Jellyfish have tentacles containing pneumatocyst cells, which act like little harpoons that lodge in prey to sting and kill them.
The location and number of pneumatocysts dictate whether jellyfish are processed for human consumption.
While dried jellyfish with soya sauce is a delicacy served in Chinese weddings and banquets, not all kinds of jellyfish can be eaten, says Richardson.
According to Richardson, the species increasing in number aren't generally eaten.
Jellyfish boom threatens ocean
University of Queensland, ScienceAlert 9 Jun 09;
Early action could be crucial to addressing the problem of major increases in jellyfish numbers, which appears to be the result of human activities.
New research led by University of Queensland and CSIRO Climate Adaptation Flagship scientist, Dr Anthony Richardson presents convincing evidence that this "jellyfish joyride" is associated with over-fishing and excess nutrients from fertilisers and sewage.
"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," Dr Richardson, of UQ's School of Mathematics and Physics, said.
"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 2m in diameter and weigh 200kg."
The new research, by Dr Richardson and colleagues at the University of Miami, Swansea University and the University of the Western Cape, was published in the international journal: Trends in Ecology and Evolution, in time for World Oceans Day on June 8.
"Fish normally keep jellyfish in check through competition and predation but overfishing can destroy that balance," Dr Richardson said.
"For example, off Namibia intense fishing has decimated sardine stocks and jellyfish have replaced them as the dominant species."
Climate change may favour 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.
"Mounting evidence suggests that open-ocean ecosystems can flip from being dominated by fish, to being dominated by jellyfish," Dr Richardson said.
"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'."