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Food, Health, and Eco-news
Jellyfish Boom Signals Ocean Warming and Overfishing

Massive swarms appear for eighth year in a row, defying historical boom-bust population trends

by Craig Weatherby

Last December, a dense, 10-square-mile pack containing billions of stinging medusa jellyfish rode warm currents to Ireland.

These Mediterranean jellyfish had rarely been sighted in large numbers so far north, until shortly before the turn of the 21st century.

After arriving in bays north of Belfast, Northern Ireland, they stung some 120,000 farmed Salmon to death (See “Jellyfish Swamp Irish Salmon Farm”).

While the slaughter was unintentional on the part of the jellyfish, it seemed a sobering harbinger for the fate of our fragile seas.

Sadly, that odd occurrence may represent only the tip of an ecological iceberg, built out of overfishing and global warming.

Research on this topic is sparse, but one of the few studiesa literature reviewwas authored by Claudia E. Mills, Ph.D.

Dr. Mills works at the University of Washington's zoology lab in Friday Harbor, San Juan Island… a two-hour ferry ride from our backyard in Bellingham.

As she wrote in a landmark 2001 paper, “Massive removals of fishes from ecosystems might be expected to open up food resources for gelatinous predators, which seems in some cases to be what has happened.”

“Over recent decades, man's expanding influence on the oceans has begun to cause real change and there is reason to think that in some regions, new blooms of jellyfish are occurring in response to some of the cumulative effects of these impacts” (Mills CE 2001).

This concern is one reason we've begun to offset the global warming effects of our shipmentssee our VitalGreen™ pageand why we only sell fish and shellfish that are harvested sustainably.

Now, recurring blooms in the Mediterranean are raising concern that they represent another “canary in the coalmine” sign of mankind's negative impactsan early warning we'd be wise to take seriously.

Spanish finding raises red flag

In recent years, jellyfish blooms have driven vacationing bathers out of warm Mediterranean waters and back onto white-hot summer beaches.

Last November, Spanish scientists expressed alarm over the large numbers of medusa jellyfishthe so-called “mauve stinger”growing off Spain's Costa Brava, primed to bring misery to its beaches this summer.

Significantly, their study revealed the unprecedented finding that medusa jellyfish were growing throughout the year… not just in the summer.

The jellyfish that have swarm Spanish beaches in recent summers can reach the size of a dinner plate, and their purple tentacles contain chemicals that deliver a nasty sting.

If someone get enough stings, or suffers from sensitivity to the stings, they can be seriously injured, scarred, or even killed.

In summer 2006, the Red Cross reported treating more than 10,000 jellyfish stings in Spain's eastern Catalonia region.

And last year in Australia, more than 30,000 people were treated for stings… twice the number treated in 2005 (Rosenthal E 2008).

This summer the jellyfish were kept further out to sea for a time, thanks to runoff from heavy spring rains in Europe.

But as of late July, the stinging jellyfish had begun to swarm Mediterranean shores, including the Côte d'Azur, the west coast of Italy, Sardinia, parts of Italy's east coast, and much of the southern and northern coastlines of Spain.

As a consequence, beaches have been filled with hot, frustrated, fearful vacationers.

Recently, some 300 people on Barcelona beaches were treated for stings, and several were taken to hospitals.

The Portuguese Man o' War (Physalia physalis), whose sting can be fatal, is crowding Spain's northern coast at Cantabria and Asturias, and winds have blown some ashore (Rosenthal E 2008).

Officials in Cannes and Monaco have installed booms and nets on several beaches, and authorities in Antibes employ a “jellyfish Hoover” boat to patrol the coastline and suck up any invaders.

But the risk of injury by jellyfish is not the biggest concern.

The steadily increasing numbers of jellyfish in seas worldwide signal undesirable changes in sea life driven by human activity.

Human impacts may boost jellyfish blooms

Marine biologists attribute the swarm of icky invertebrates to two main factors:

  • Warming oceans allow jellyfish to grow year-round and promote a glut of plankton – the favored food for jellyfish.
  • A decline in natural predators like tuna, dolphins, and turtles.

The primitive but persistent creatures consist almost entirely of water, and are filling a vacuum created by the voracious human fish consumption.

The impact of increasing numbers of these ancient animals is hitting the already warm, overfished Mediterranean basin especially hard.

Summertime swarms of the offending medusa jellyfish (Pelagia noctiluca) are unusual, but the frequency and persistence of these invasions set off alarms among scientists.

Records dating back more than 200 years show that jellyfish populations rise naturally every 12 years, remain stable four or six years, and then subside.

But 2008 is the eighth consecutive year that medusa jellyfish populations have exploded.

The collapse of fish populations means that fewer jellyfish get eaten, and the larger numbers of survivors have less competition for food.

Like salmon, tuna, and many other predator species, jellyfish feed on small fish and zooplankton.

And jellyfish also feed on fish larvae before they can hatch.

As professor Andrew Brierley of the University of St Andrews in Scotland told Reuters, “Jellyfish both compete with fish for plankton food, and predate directly on fish. It is hard, therefore, to see a way back for fish once jellyfish have become established, even if commercial fishing is reduced.”

This hypothesis is backed by the enormous surge in jellyfish swarms off the Atlantic coast of Namibia, which is one of the most heavily overfished areas in the world.

European fishermen have been scouring these African waters to escape legal restrictions and scarce pickings in their own waters.

And as Spanish researcher Josep-María Gili said back in February, “Spectacular growth has been found in jellyfish populations in Japan, Namibia, Alaska, Venezuela, Peru, Australia... this is an international ecological problem” (Hamilos P 2008).

While historically there has been relatively little demand for jellyfish as food, enormous numbers are now being harvested in Southeast Asia, and this may help control jellyfish populations in that region.

Research and sound regulationnot panicis the prescription

No one knows how many jellyfish float the seas, as they are very hard to spot in satellite images or sonar soundings.

As Dr. Miller noted in her review paper, the scientific literature contains very little information on jellyfish populations or behavior.

And these ancient, odd animals are hard to study in captivity, so we have much to learn about their behavior and needs and know very little about how to combat jellyfish overgrowth.

We report this story not to add to anyone's eco-anxiety, but to underscore the unforeseeable effects of man's impacts on the environment… especially an impact as comprehensive as global warming.

In fact, the picture is mixed, with human impacts cutting jellyfish numbers in some areas, including Puget Sound (Mills CE 2001).

No matter what we humans do, we will exert some eco effects. The challenge is to detect and correct any seriously damaging ones.

Let's hope society will fund the necessary research… and summon the political will to act on any significant findings.


  • AFP. Jellyfish outbreaks a sign of nature out of sync. June 17, 2008. Accessed online August 7, 2008 at
  • Hamilos P. Scientists warn of new plague of jellyfish. February 29, 2008. Accessed online August 7, 2008 at
  • Mills CE. Jellyfish blooms: are populations increasing globally in response to changing ocean conditions? Hydrobiologia 451: 55–68, 2001. Accessed online August 7, 2008 at
  • Rosenthal E. Stinging Tentacles Offer Hint of Oceans' Decline. The New York Times, August 3, 2008. Accessed online August 7, 2008 at
  • Spain: Heatwave brings jellyfish plague to beaches. July, 28 2008. Accessed online August 7, 2008 at


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