The group says reduced rainfall and other climate changes in the region have raised the temperature and salinity of coastal waters.
These warmer, saltier waters allow open-sea jellyfish species to reach the shore, where cooler waters had once acted as a barrier to the marine creatures, researchers explain.
Scientists in Italy also attribute the jellyfish menace to higher sea temperatures, which they say have allowed the animals to migrate north from Africa.
Several other recent studies conducted throughout Europe have similarly concluded that increases in jellyfish populations are being fueled by global warming.
For instance, research by the Pelagic Ecology Research Group at the University of St. Andrews in Scotland has linked rising sea temperatures in the North Sea to a northward shift in plankton and a growth in jellyfish abundance.
Peter Richardson of the Marine Conservation Society, based in Ross-on-Wye, England, says warmer seas are thought to boost the production of the plankton on which jellyfish feed.
"In cooler waters like the northeast Atlantic and the Mediterranean, these warmer water temperatures are thought to lead to greater numbers of jellyfish," he added.
"With climate change it's likely that we will see larger numbers in the future, as sea temperatures increase."
Beware Lion's Mane Jellyfish
Richardson runs a national survey, started in 2003, to monitor jellyfish populations off U.K. shores.
While based on public sightings rather than scientific studies, the survey has identified large blooms of several jellyfish species.
Of these, Richardson says, by far the most potent stinger is the lion's mane jellyfish.
"It can be 2 meters [6.6 feet] in diameter and have 15 meters [49 feet] of tentacles hanging below it," he said. "It will ruin your day if you touch one."
In the Merseyside area of northwest England this summer there have been reports of up to 80 people a day being stung by lion's mane jellyfish on a single beach.
Richardson adds that one of his colleagues has firsthand experience of this year's Mediterranean jellyfish invasion.
The researcher was badly stung by a purple jellyfish while visiting the Spanish island of Minorca (Spain map).
"The whole side of her face and her arm were in agony and then went numb for several hours," he said.
Other recent evidence supports the idea that overfishing is also contributing to the jellyfish's rise.
Oceana's scientists attribute the Mediterranean invasion partly to the overfishing of jellyfish predators there, including moonfish, triggerfish, and leatherback turtles, which are often caught accidentally as bycatch.
Likewise, a study led by University of St. Andrews marine biologist Andrew Brierley found that heavy fishing in the Atlantic Ocean has caused a huge increase in jellyfish numbers off southern Africa.
The study, published last month in the journal Current Biology, estimates that the combined weight of jellyfish in seas around Namibia (Namibia map) is 13.4 million tons (12.2 million metric tons), far more than the total weight of commercial fish stocks in the area.
Brierley says that fish, including sardines and anchovies, have been heavily exploited since the 1960s, and stocks have become strongly depleted.
"Because fish and jellyfish essentially compete for similar food resources, a dramatic decline in fish populations could theoretically contribute to a substantial increase in the abundance of jellyfish," he said.
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