Anna Salleh
ABC
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.
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.
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.
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.
