Article from the Guardian(UK).
Just when you thought it was safe to go back in the water, scientists in Spain are warning that the plagues of jellyfish that have been the scourge of Mediterranean swimmers in recent years will return this summer.
In November, scientists at the Barcelona-based Institute of Marine Sciences (ICM) began studying the life cycles of jellyfish off the Costa Brava, and were alarmed to detect large numbers of the Pelagia noctiluca, commonly known as the "mauve stinger", growing in the winter, ready for an assault on Spain's beaches.
The study revealed that jellyfish proliferate throughout the year, not just in the summer. Between November and January, scientists discovered 30 colonies, or blooms, ranging in size from four to 10 jellyfish per cubic metre of water, all along the Catalan coast.
According to Josep-María Gili, research professor at the ICM, these groups were born last autumn, and the summer tides will carry them inland from deeper waters, causing the plagues that have seen millions of jellyfish wash up on Spain's beaches in recent years. "The problem seen on the beaches is not the main concern for scientists," said Professor Gili, "For us the major worry is the global disequilibrium in the sea caused by over-fishing."
As a result of over-fishing, the jellyfish do not have to face their usual predators and competitors, which usually regulate population growth. Numbers of large fish such as swordfish and red tuna, which eat jellyfish, have been drastically reduced by bad fishing practices, as have the smaller fish, such as sardines and whitebait, which compete for food with the stingers.
Global warming has also brought about the ideal conditions for jellyfish to breed: mild temperatures, little rain and a lack of the usual winter rainstorms. Plagues of jellyfish are nothing new - they often recur in cycles of up to 10 years, but recently, these cycles have become ever shorter, and the blooms more widespread and populous.
According to Gili, the recent growth in jellyfish numbers "is a message from the sea that something is wrong. People need to realise that fish, especially adult fish, play an important role in the sea - they are the principal carnivores. We must change the laws about over-fishing and the type of fishing."
The problem is not restricted to the Mediterranean. "Spectacular growth has been found in jellyfish populations in Japan, Namibia, Alaska, Venezuela, Peru, Australia ... this is an international ecological problem," Gili said...
Dr Reyes Tirado, at the Greenpeace research laboratories in Exeter, said the plagues were not just caused by over-fishing: "Our activities on land also play a big part ... overloading of coastal waters with nutrients both from sewage and from agricultural fertiliser runoff are also important," she said. "Excess nutrients can have disastrous effects on estuaries and coasts, causing blooms of harmful algae and helping jellyfish populations to increase.
"Add to these factors the warmer waters and changing marine currents caused by climate change and the problem of jellyfish invasions seems set to get much worse in the future."
More info @ the Independent (UK)
And from allafrica.com - Jellyfish Population Worries Fish Industry in Namibia:
As the effects of climate change continue to impact on major industries in the world, the Namibian fishing industry could become one such casualty.
The ever-presence of jellyfish in Namibian waters, which feed on fish eggs of commercial species, has had the line ministry worried. .
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