Blue patrol boats crisscross the swimming areas of beaches here with their huge nets skimming the water's surface. The yellow flags that urge caution and the red flags that prohibit swimming because of risky currents are sometimes topped now with blue ones warning of a new danger: swarms of jellyfish.
In a period of hours during a day a couple of weeks ago, 300 people on Barcelona's bustling beaches were treated for stings, and 11 were taken to hospitals.
From Spain to New York, to Australia, Japan and Hawaii, jellyfish are becoming more numerous and more widespread, and they are showing up in places where they have rarely been seen before, scientists say. The faceless marauders are stinging children blithely bathing on summer vacations, forcing beaches to close and clogging fishing nets.
But while jellyfish invasions are a nuisance to swimmers and a hardship to fishermen, for scientists they are a source of more profound alarm, a signal of the declining health of the world's oceans.
Never miss a local story.
“These jellyfish near shore are a message the sea is sending us saying, ‘Look how badly you are treating me,'” said Josep-Maria Gili, one of the world's leading jellyfish experts, who has studied them at the Institute of Marine Sciences of the Spanish National Research Council in Barcelona for more than 20 years.
The explosion of jellyfish populations, scientists say, reflects a combination of severe overfishing of natural predators, like tuna, sharks and swordfish; rising sea temperatures caused in part by global warming; and pollution that has depleted oxygen levels in coastal shallows.
These problems are pronounced in the Mediterranean, a sea bounded by more than a dozen countries that rely on it for business and pleasure. Left unchecked in the Mediterranean and elsewhere, these problems could make the swarms of jellyfish menacing coastlines a grim vision of seas to come.
“The problem on the beach is a social problem,” said Gili, who talks with admiration of the beauty of the globular jellyfish. “We need to take care of it for our tourism industry. But the big problem is not on the beach. It's what's happening in the seas.”
Jellyfish, relatives of the sea anemone and coral, are the cockroaches of the open waters, the ultimate maritime survivors who thrive in damaged environments, and that is what they are doing.
Within the past year, there have been beach closings because of jellyfish swarms on the Cote d'Azur in France, the Great Barrier Reef of Australia, and at Waikiki and Virginia Beach in the United States.
In Australia, more than 30,000 people were treated for stings last year, double the number in 2005. The rare but deadly Irukandji jellyfish is expanding its range in Australia's warming waters, marine scientists say.
While no good global database exists on jellyfish populations, the increasing number of reports from around the world have convinced scientists that the trend is real, serious and climate-related, although they caution that jellyfish populations in any one place undergo year-to-year variation.
“Human-caused stresses, including global warming and overfishing, are encouraging jellyfish surpluses in many tourist destinations and productive fisheries,” according to the National Science Foundation.
The foundation is issuing a report on the phenomenon this fall and lists problem areas as Australia, the Gulf of Mexico, Hawaii, the Black Sea, Namibia, Britain, the Mediterranean, the Sea of Japan and the Yangtze estuary.