Jelly invasion: Why jellyfish are taking over our oceans.

Image by Olly Coffey |

Image by Olly Coffey |

Jellyfish are taking charge of our oceans - and it might be too late to stop them.

In 2000, a jellyfish bloom off the coast of Australia could be seen from space.  Since then, many other notable jellyfish-based incidents have occurred, including the severe damage of a desalination plant in Oman in 2013, caused by 300 tonnes of jellyfish, and the capsizing of a 10-tonne fishing boat near Japan in 2009, when a net full of Nomura jellyfish grew to the size of a refrigerator. In fact, Japan now experiences an annual Nomura jellyfish bloom. In 2013, a moon jellyfish invasion clogged the piping of a Swedish nuclear power plant reactor, causing it to shut down.  (An American nuclear plant suffered the same fate in 2006).  Also in 2013, one million jellyfish piled up along 300 kilometers of Mediterranean coastline, shortening swimming season and causing the tourism industry to take a major hit.

When you think about jellyfish, it’s quite likely that your mind might jump to that scene in Finding Nemo where Dory gets stung. It’s important to know that some jellyfish stings are harmless, but others are not - some are even deadly. Box jellyfish can insert venom from their 10 foot tentacles and cause death within four minutes. Meanwhile, Irukandij jellyfish (though rarely spotted because they are typically the size of a raspberry) cause pain, vomiting and blocked airways in humans within 10 minutes of delivering a sting, which can also lead to brain hemorrhaging and heart failure. While these jellyfish used to be found mostly only off the coasts of Australia and south east Asia, they are now found in many coastal areas around the world, including Florida, Hawaii, Cape Town, Wales, and India.

Recent mass blooms of jellyfish are considered a sign of deteriorating ocean health, and indicate an imbalance within ocean ecosystems.  Increasing numbers of jellyfish have been consisered the result of overfishing, pollution, and climate change (which is strongly linked to ocean acidification). While rising ocean temperatures are detrimental to marine ecosystems, it allows jellyfish to thrive.

How do these events actually cause jellyfish blooms?

With regards to overfishing, the answer is relatively straightforward. Overfishing removes large numbers of jellyfish predators from the food web, allowing jellyfish to survive (by eating fish eggs and plankton, which fish require for food), and also to breed at far higher rates than ever before.  This shift harms ocean biodiversity, and also depletes fish stocks for humans.

The effects of pollution and climate change on oceans are more complex. Pollution from farming pesticides and sewage ends up in rivers and in the ocean. The nitrogen and phosphorus in this waste is then consumed by phytoplankton, leading to large algal blooms. This change creates food for jellyfish, but also decreases oxygen in the water. Many ocean organisms need a certain level of oxygen to survive, but jellyfish need very little. As a result, many organisms die and are outlived by thriving jellyfish.

Climate change causes the ocean to warm, which jellyfish actually prefer. However, ocean warming also leads to ocean acidification, which disintegrates calcifying organisms such as shellfish and allows for space for non-calcifying species like jellyfish to thrive.  Acidification also reduces oxygen just as pollution does, killing off many marine species, but allowing jellyfish to survive.

It gets worse, though. Massive jellyfish blooms actually release high-carbon content mucus and feces into the oceans. This massive amount of carbon is then added to the atmosphere, contributing to further warming of the atmosphere and causing a climate positive feedback loop. This allows more warming, more ocean acidification and more future jellyfish blooms, which contribute to more climate warming. 

Industrial development has also helped jellyfish population growth. In order to clone themselves, jellyfish need to attach their clone sacks to hard surfaces. This used to be limited to rocky ocean coasts and floors, but now that we have built piers, drilling platforms, wind turbines and boats, and have added plastic debris to the ocean, there are many hard surfaces for these clone sacks, leading to increasing jellyfish populations.   Ballast water from ships can carry jellyfish from one part of the world to another, introducing exotic jellyfish to new ecosystems, where they are considered an invasive species. 

There is uncertainty about whether we can reduce jellyfish populations to normal levels. However, if we do not want to experience the end of fish as food and increased swimmer deaths, we need to find a way to at minimum curb the recent exponential growth of jellyfish.

One way to help is to report sightings to, which helps researchers in their efforts to better understand and find solutions to the jellyfish bloom problem.