Giant jellyfish are taking over parts of the world's oceans due to overfishing and other human activities, researchers say.

Nomura jellyfish are the biggest in the world and can grow as big as a sumo wrestler. They weigh up to 200 kilograms and can reach 2 metres in diameter.

Dr Anthony Richardson and his colleagues from CSIRO Marine and Atmospheric Research says jellyfish numbers are increasing, particularly in South East Asia, the Black Sea, the Gulf of Mexico and the North Sea.

"We need to take management action to avert the marine systems of the world flipping over to being jellyfish dominated," says Dr Richardson, who is also a marine biologist at the University of Queensland.

He says the Japanese have a real problem with giant jellyfish that burst through fishing nets.

He says other researchers are experimenting with different ways of controlling jellyfish, including using sound waves to explode jellyfish and using special nets to try and cut them up.

Overfishing

Dr Richardson and his 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 cannot.

"You can think of them like a protected area for jellyfish," Dr Richardson says.

The researchers say climate change may also encourage more jellyfish and 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.

Taking action

The team recommends a number of actions in its paper, published in the journal Trends in Ecology and Evolution and released to coincide with World Oceans Day.

They say it is important to reduce overfishing, especially of small pelagic fish like sardines, and to reduce run-off.

They also say it is important to control the transport of jellyfish around the world in ballast water and aquariums.

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, Dr Richardson says.

According to Dr Richardson, the species increasing in number are not generally eaten.