The highly venomous Portuguese Man o" War has an air-filled top membrane that acts as a sail.© NOAA
Large-scale jellyfish population growth followed by rapid decline used to occur periodically in cycles of several years. It is now much more frequent and large blooms of jellyfish have almost become a permanent fixture in the Mediterranean. Recently, a swarm of more than 500 of the dreaded Portuguese Man o" War was even reported off the coast of Andalucia. Thanks to their air-filled top membrane that acts as a sail, they can also use winds to drift across the oceans. These highly venomous creatures are not usual residents of the Mediterranean, and were blown in from the Atlantic through the strait of Gibraltar.
The jellyfish"s enterprise of colonization has been going almost unchecked, as their natural predators such as swordfish or tunas are being dangerously overfished, partly because of the high rates of illegal or underreported catches. Since stocks of smaller fish are also dwindling, the stinging invertebrates have free access to an ever-greater share of the plankton and small marine organisms that they otherwise had to compete for with other inhabitants of the sea. They also feed on fish eggs and larvae, further undermining their rivals. Similar phenomena have been reported in other parts of the world.Perhaps this same mechanism that causes the exponential expansion of a species thanks to the lack of predators and an abundance of food could work in reverse and help regenerate big fish populations? But for that, a moratorium, or drastic reductions on fishing would need to be implemented. Even so, scientists point out that once one species has achieved predominance in an ecosystem, it is quite difficult to restore the lost equilibrium.
Sadly, a halt to fishing activities seems unlikely at present. Last year in November 2008 an international conference organized by ICCAT (International Commission for the Conservation of Atlantic Tunas) took place in Morocco to agree on a reduction of Mediterranean bluefin tuna catch quotas. It was another disappointment to environmentalists. A moratorium, demanded by many environmental groups, was rejected. The new quotas, which will reduce permitted volume of catches by a total of 30% by 2010, still exceed even ICCAT"s own scientists" recommendations. A small extension of two weeks on the fishing ban during the spawning period was also granted.
Other factors that play in favor of the jellyfish and contribute to the explosion of their population are the run-off of residues from agricultural fertilizers into the sea. These effluents provide nutrients for the organisms on which jellyfish feed and can cause a drop in oxygen levels in marine environments that are harmful to fish, but in which jellyfish can still thrive.
The natural habitat of jellyfish is in the high seas, 20 to 30 nautical miles off the coast. So why are they now making a habit of visiting the shore? According to scientists like Josep-Maria Gili, research professor at the Institute of Marine Sciences in Barcelona, this is due to climatic and meteorological conditions. As overall levels of precipitations in southern Europe have fallen in recent years, the reduced influx of fresh water into the sea through rivers has caused the salinity of coastal waters to rise.This less-saline coastal fringe acted as a barrier, but now oceanic waters can periodically reach continental areas, bringing the swarms with them. A rise in sea water temperatures due to global warming is also said to add to the problem by changing current patterns, as well as favoring jellyfish population growth.
After several years of observing the phenomenon, the Spanish authorities have started to take concrete measures to try and defend the beaches against the gelatinous invaders, fearing their impact on a tourism industry already weakened by the global crisis and the low value of the British pound against the euro. Lifeguards have been issued with jellyfish alert flags to warn bathers. The Plan Medusas ("Plan Jellyfish"), launched by the Ministry for the Environment in 2007 has been extended to the entire coast for this summer. Marine biologists and volunteers are monitoring swarm movements and trying to remove them from the water with specially equipped boats if they get to within 200 meters of a busy beach. But the phenomenon of recent jellyfish proliferation has so far not been studied in sufficient depth to allow researchers to predict future trends.
For Josep-Maria Gili, these events are a clear indicator of the poor state of health of the oceans, and the only solution lies in a change in the way in which marine resources are managed.