Sumo jellyfish invade Japan

Overfishing is cause of giant creatures

Jellyfish the size of Sumo wrestlers are taking over parts of the world's oceans as a result of overfishing and other human activities according to a recent study conducted by the CSIRO Marine Atmospheric Research Group.

The study reports there has been a significant rise in the number of Nomura jellyfish, which can reach up to two metres in diameter and six feet in length, in parts of the South East Asia, the Black Sea and the Gulf of Mexico, and warns that killer jellyfish could end up along the coastline of popular tourist beaches if action is not taken to address the problem.

Dr Anthony Richardson, who led the research behind the study, said: "We need to start managing the marine environment in a holistic and precautionary way to reduce the likelihood of venomous tropical species, such as the box jellyfish and irukandji, from moving into subtropical and temperate areas. Dense jellyfish aggregations can be a normal feature of a healthy ecosystem, but a clear picture is emerging of more severe and frequent jellyfish outbreaks worldwide."

Overfishing and the dumping of fertilizers and other waste products into the sea were identified by the team as two of the main reasons for the jellyfish boom, along with climate change.

Small fish such as anchovies, sardines and herrings help to keep the numbers of jellyfish in check by providing competition for zooplankton - the foodstuff upon which jellyfish are dependent for survival. They also stop the population from getting out of control by eating the jellyfish in their early stages. 

The dumping of fertilizers and other waste products into the sea - a process known as 'run-off' - increases the ocean's nitrogen and phosphorous levels, creating an algae boom. This results in a reduction in the amount of oxygen in the ocean creating 'dead zones' in which the jellyfish are able to thrive.

Dr Richardson explained: "Fish avoid lower-oxygen water but jellyfish, having lower oxygen demands, not only survive but can thrive in these conditions as there is less predation and competition from fish."

Once jellyfish have taken hold of a particular stretch of ocean it is almost impossible for the balance to be restored as the jellyfish eat the larvae and fish eggs of the competing fish. As the numbers of jellyfish continue to rise in an area, the numbers of fish, therefore, continue to fall, along with the mammals and birds that eat them.

One possible solution could be to eat jellyfish.

"The best types are slightly crunchy," said Dr Richardson. "Not a strong taste and usually had with a sauce. It is excellent diet food, as it has virtually no calories!"

Not the most appetising of prospects, then, but one we may have to get used to if something else isn't done to address the problem.

For more on the CSIRO Marine and Atmospheric Research group's findings and the affects of climate change, read the journal Trends in Ecology and Evolution