State of the Planet

News from the Columbia Climate School

Giant Jellyfish Swarms – Are Humans the Cause?

It’s now summer in Australia where scientists recently discovered a mysterious new species of jellyfish in New South Wales. In Auckland, New Zealand swimmers are encountering a profusion of tiny stinging jellyfish the size of pepper grains. Many scientists believe that jellyfish, particularly jellyfish swarms or blooms, are on the increase worldwide, turning up in regions where they never existed before.

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Jellyfish are not without their benefits. They provide food for us, create shelter for juvenile fish in their tentacles, and the green fluorescent protein responsible for their bioluminescence, their toxins, and their tissues are being studied for medical uses.

But jellyfish blooms, which sometimes extend for miles and miles, can create big problems. In the thickest spots there may be more jellyfish than there is water. Jellyfish blooms disrupt the fishing industry by tearing nets and harming fish. Smaller jellyfish or jellyfish tentacles can enter aquaculture pens and suffocate the fish. Jellyfish sting swimmers, discouraging tourism. They clog cooling water intakes at power plants, causing power reductions or shutdowns. And jellyfish reduce the population of the fish that humans like to eat, by consuming the zooplankton they need and eating their eggs, larvae and juveniles.

Since the 1990s, there have been jellyfish problems in the Sea of Japan, the East China and Yellow Seas, the Northern Benguela Current off Namibia, the Black Sea, the Baltic Sea, coastal Middle Eastern waters, and off the coasts of Spain and France.

Nomura’s jellyfish. Photo credit: Shin-ichi Uye, Hiroshima University

The most dramatic of all jellyfish blooms are Nomura’s jellyfish that have regularly swarmed into the Sea of Japan over the last decade. These giant creatures, with thousands of stinging tentacles, can be as large as seven feet in diameter and weigh over 600 pounds under normal conditions.  When there are blooms, they wreak havoc on the Japanese fishing industry by breaking fishermen’s nets, and crushing the fish or poisoning them with their stingers.  In 2009, a fishing trawler capsized as the crew tried to pull in nets filled with jellyfish.

Photo credit: Shin-ichi Uye, Hiroshima University

Many jellyfish have natural cycles of abundance. Before 2000, Nomura’s jellyfish blooms occurred in 1920, 1958, and 1995 in Asian waters.  But since 2002, the blooms have occurred almost every year, with particularly large ones in 2005 and 2009. There was no bloom in 2010, though scientists don’t know exactly why.

Dr. Jenny Purcell, a marine scientist at the Shannon Point Marine Center at Western Washington University, said, “I think there are much more frequent blooms of giant jellyfish than there used to be, and the region where they occur most is one of the most environmentally damaged on earth. It’s entirely possible that humans have helped the jellyfish, changing things so they can thrive.”

While scientists are finding almost no direct evidence of human effects on jellyfish, Purcell’s research shows that many human activities are strongly correlated to jellyfish blooms.

Climate change and resulting warmer sea temperatures favor most jellyfish species, and water temperatures of the seas around Japan have definitely increased in recent decades. Higher water temperatures both speed jellyfish reproduction and extend the reproductive season. Swimming jellyfish reproduce sexually, giving birth to larvae that settle on the ocean bottom to become polyps from which many more jellyfish “bud off.” Warming temperatures result in more polyps.  Scientists who don’t believe that jellyfish are increasing worldwide cite the case of the Bering Sea near Alaska where the jellyfish population grew rapidly during the 1990s when temperatures were moderate, peaked in 2000 at 40 times the 1982 level, then began to decline even as sea temperatures rose. But a 2008 study suggested that food sources influenced the Bering Sea jellyfish decline.

Overfishing favors jellyfish because it eliminates their predators (they have very few anyway) and competitors. Jellyfish also benefit when forage fish, such as herring and sardines, are harvested for aquaculture fishmeal, because forage fish normally compete with them for zooplankton.

Agricultural, animal waste and sewage runoff loads coastal waters with nutrients (nitrogen, phosphorus and potassium), which cause eutrophication where algae proliferate, then die, robbing the water of oxygen. Eutrophication provides more food for jellyfish polyps and favors jellyfish and polyps over fish because they can tolerate low oxygen levels. In addition, eutrophication clouds the water, which makes it harder for fish, who are visual feeders, to feed, while jellyfish don’t rely on sight to feed.

Human coastal development has also helped jellyfish thrive. The structures and construction that we have placed in the water, such as piers, marinas, oil platforms, artificial reefs, refuse, rubble, aquaculture pens and structures, etc. provide an abundance of habitats for polyps to settle on.

Aquaculture in Fuzhou, China. Photo credit: Jack Parkinson

Jellyfish flourish around aquaculture, where extra fish feed and fish waste produce eutrophication, and pens and structures provide polyp habitats. In addition, each year the Chinese and Malaysians, who regard jellyfish as an epicurean delicacy, release millions of tiny baby jellyfish into the waters to later harvest.

The introduction of alien species via ballast water in ships, the aquarium trade, or through changing ocean currents caused by climate change, have often resulted in large jellyfish blooms because non-native species can thrive in areas where they have no competitors or predators.

In fact, many regions that are experiencing jellyfish blooms face a multitude of manmade problems. For example, China contends with warming temperatures, eutrophication, overfishing, construction and invasive non-native species. Eutrophication, construction, as well as habitat modification is occurring in Mar Menor, Spain. And overfishing and climate variation contribute to the Black Sea’s jellyfish blooms.

The U.S. has not faced jellyfish problems as serious as those of other countries, but with climate change and warming temperatures, the rising demand for energy that could result in the construction of more power plants, and increasing coastal development, we may yet. Purcell warned that, “ As we continue to degrade our coasts, we will have more jellyfish.” Thus far only a few coastal species of jellyfish have been causing problems, but hundreds of other jellyfish species exist, so “the potential for further problems with jellyfish may be very great.”

Despite all the circumstantial evidence, scientists acknowledge the need for more research in order to determine conclusively if jellyfish blooms are a function of natural cycles or the result of human impacts on the environment. The National Center for Ecological Analysis and Synthesis is working on a global synthesis of reports of jellyfish blooms to examine their causes, effects on ecosystems, consequences for various industries, and to notify the public. Citizens can help by reporting any sightings of jellyfish to Jelly Watch, which is creating a long-term database.

There is little we can do now to curb jellyfish blooms, except to take better care of our oceans. In a striking video about jellyfish swarms, Monty Graham, senior marine scientist at the Dauphin Island Sea Lab in Alabama, said, “If ecosystems are healthy, then jellies will do their normal thing…But if you nudge the system towards the more perturbed side, the jellyfish will take off. And so we do sort of think of jellies as that kind of canary, that bellwether of change.”

Jellyfish are ancient creatures, and were here long before the dinosaurs. Over millions and millions of years, they and other plants and animals evolved into precisely calibrated and balanced parts of an ecological tapestry. As humans continue to disrupt these interdependent relationships, we are seeing profound effects on our environment and on human life. Jellyfish blooms are likely the result of just one of countless eco-niches we have unraveled. Who knows what other havoc human actions will set in motion?

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8 years ago

Jellyfish loves warmer oceans and we are making it easy for them. We are also killing smaller fishes and lobsters who feeds from the same food sources. As a result, they are getting more foods. More food means more growth, it is true for every living form in earth, including us humans.

Reply to  Nomura
3 years ago

Have any of you heard of the irukandji? If not, its worth looking up. One of the most deadly animals. Also, a good book is The Thing About Jellyfish. Anyone (like me) who is obsessed with jellies, will love this book!

Reply to  Madison
3 years ago

omg that book is amazing! I’ve read it twice you should also look up Turritopsis dohrnii the immortal jellyfish they mentioned in the book its really interesting

Reply to  Nomura
3 months ago

sooooooo sad

1 year ago

Surprisingly I have been to many spots where Jellyfish could be seen but never actually seen one, bummer, as many bad things they can do I’m obsessed with them.
I enjoyed reading this on them though. It helps me understand these things.

Last edited 1 year ago by Ayla
5 months ago

Interesting. I believe humans may be the cause seeing as we are just making the oceans warmer and more suitable for jellyfish.