Changing Planet

Why Are Jellyfish Swarming This Summer?

Moon jellyfish swarm in Ketchikan, Alaska, in 2008. Photograph by Chip Porter, AlaskaStock/Corbis

Swarms of jellyfish that have appeared recently in the Pacific Northwest and the United Kingdom are not unusual, but may signal an ocean out of balance, experts say.

This summer, huge numbers of blue Velella velella have been washing onto beaches of the Pacific Northwest, where they die and decompose into cellophane-like corpses. The East Coast isn’t immune, either, as moon jellyfish (Aurelia aurita) swarmed off the coast of Maine. The southwestern coast of England has also been hit by explosions of huge barrel jellyfish (Rhizostoma pulmo). (Related: ” ‘Immortal’ Jellyfish Swarm World’s Oceans.”)

Both areas have seen blooms—the term for an explosion of the gelatinous creatures—before. But in recent years, some scientists have begun to suspect that they’re becoming bigger and more common.

For one, agricultural runoff carrying fertilizers and other chemicals is fueling growth of algae and plankton, some of jellyfish’s favorite prey, says invertebrate biologist Jim Watanabe of Stanford University. Overfishing has also wiped out many jellyfish predators.

“Year-to-year differences in jellyfish blooms are normal—we have ‘good’ years and ‘bad’ years,” Cathy Lucas, a marine biologist from the U.K.’s University of Southampton who recently received a National Geographic grant to study blooms, said by email. (See National Geographic’s pictures of jellyfish.)

“People tend to remember the years when there are lots of jellyfish, possibly because their beach holidays have been affected or they have seen more from boats, but we must not forget that there are years when jellyfish can be less common,” Lucas said.

Watch a video of stunning jellyfish.

Explaining Blooms

As far as predicting blooms goes, “we can’t yet look at the ocean and say, ‘Gee, this is going to be a jellyfish year,’ ” said Watanabe.

But there are factors that scientists watch to keep tabs on jellyfish populations.

Ocean currents and winds, for example, can affect jellyfish numbers. Although nearly all jellyfish can move independently, using a bell-shaped top that propels the animal through the water and tentacles that capture prey, they are also transported throughout the ocean.

When winds blow just right off of the California coast, for instance, they can blow the jellyfish-like Velella velella onto shore.

A photo of sand sail jellyfish washed ashore.
Velella Velella, a jellyfish relative, can be seen on Mediterranean beaches. Photograph by mauritius images GmbH, Alamy

This is considered an apparent bloom, when jellyfish are pushed together into groups by tides, winds, or currents. The other type, a true bloom, occurs when jellyfish simply mate and make more jellyfish.

Plankton, on which Velella likes to feed, also go through boom-and-bust cycles, and this could partly explain why these creatures appear onshore some years more than others. (See “Huge Swarm of Gelatinous Sea Creatures Imaged in 3-D.”)

Another factor is temperature. In the U.K., the unusually warm winter of 2013-14 may have led to a population boom in barrel jellyfish. Die-offs had occurred during the extremely cold winter the previous year.

Measuring nearly six feet (two meters) across, the animals’ sting isn’t considered severe, though experts still advise not to handle them. (See pictures of colossal sea creatures.)

A photo of a Rhizostoma Jellyfish
A barrel jellyfish floats in Slovenia. Photograph by WaterFrame, Alamy

Jellyfish in Our Future

Whatever the reason for jellyfish blooms, marine biologists say we should get used to it.

“The human population is growing exponentially, and more than 40 percent of people live within 93 miles [150 kilometers] of the ocean,” said Kelly Sutherland, a jellyfish scientist at the University of Oregon.

“As humans have more and more contact with our oceans, we are more likely to notice or perceive problems with jellyfish.”

Follow Carrie Arnold on Twitter and Google+.

Carrie is a freelance science writer living in Virginia. When she's not writing about cool critters, she's spending time outside, drinking coffee, or knitting. You can visit her website at
  • tejas dolariya


  • Emmy Green

    The intro picture are, in contrast to the caption, not moon jellies. They are a strong stinging hydromedusa called Aequorea. Please get your fact straight, especially writing on this level! ( I am 14 and I saw it straight away -.-)

  • Christian Jacobsen

    I found millions of the Velella Velella on the beaches of Washington last week while camping. My understandingwas that they were set adrift by the 2011 Tsunami affecting Japanese waters

  • Stephen Coulter

    Would all these current changes in behavior of marine life be caused by all the recent underwater volcanic activity we have been having over the past few years…we are hearing more and more stories of marine life trying to get to shore…example, dolphins?…just a thought

  • Jadene Lum

    We witness a swarm of Velella Velella beached on East Beach of Graham Island, Haida Gwaii, B,C, It was quite a devastating site, albeit they were beautiful. One guide said she had only seen this jellyfish twice in 18 years. Thanks for the post. It explains a lot.

  • vicci

    I have been seeing these Velella on the northern California coast for years. They aren’t new.

  • Paul

    Same all along the west and South coast of Ireland. Never seen so many jelly fish washed up on our shores.

  • Tom

    On-shore winds, King tides and intense low pressure storm systems always bring the jellies to the beach. Here in Eastern Australia the most common is: Physalia utriculus (Blue Bottle or Portuguese Man of War) cover the beaches after all of these naturally occurring events.

  • Cal Crowley

    per [ ] Introduction to the Hydrozoa, some … hydrozoans have developed pelagic (floating) colonies that are often confused with jellyfish, but unlike jellyfish they are composed of many individuals, all specialized for various functions. The “Portuguese man-o’war” and “by-the-wind-sailors” [Velella Velella] that often wash up on beaches are examples of these unusual colonial hydrozoans. So each “purple sailor” [Velella] you see on the beach is actually a colony, either all males or all females, rather than just one individual creature.

  • molly cruz

    The turtles are all gone; they used to eat the jelly fish. Simple.

  • Omar Hassanein

    The southern medeterenian coast line has been also the affected by this wave of jellyfish. Millions of barrel type jf have been washed out on the shores of the North coast of Egypt. This barrel type jellyfish has a very aggressive effect on the skin , once it touches you , your skin swells as if you have been burnt. It is keeping all the holiday makers away from the sea . Locals say that it is the overfishing of the turtles that is bringing these vast amounts out to the shore , but I believe it has something to do with the water temperature and current. We have seen this incident 10 years ago , but the numbers this time are may be doubled or even tripped.

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