The proliferation of jellyfish in oceans around the world, driven by over-fishing and climate change, is a sign of ecosystems out of kilter, experts warn.
“Jellyfish are an excellent bellwether for the environment,” said Jacqueline Goy, of the Oceanographic Institute of Paris.
“The more jellyfish, the stronger the signal that something has changed,” Goy said.
PHOTO: AFP
Brainless creatures composed almost entirely of water, the primitive animals have quietly filled a vacuum created by the voracious human appetite for fish.
HERE TO STAY
And marine biologists say dislodging them will be difficult.
“Jellyfish have come to occupy the place of many other species,” said Ricardo Aguilar, research director for Oceana, an international conservation organization.
Nowhere is the sting of these poorly understood invertebrates felt more sharply than the Mediterranean basin, where their exploding numbers have devastated native marine species and threaten seaside tourism.
And while much about the lampshade-like creatures remains unknown, scientists are in agreement: Pelagia noctiluca — whose tentacles can paralyze prey and cause burning rashes in humans — will once again besiege Mediterranean coastal waters this summer.
That, in itself, is not unusual. It is the frequency and persistence of these appearances that worry scientists.
Two centuries of data show that jellyfish populations naturally swell every 12 years, remain stable four or six years, and then subside.
This year, however, will be the eighth consecutive year that medusae, as they are also known, will be present in massive numbers.
OVER-EXPLOITATION
The over-exploitation of ocean resources by man has helped create a near-perfect environment in which these most primitive of ocean creatures can multiply unchecked, scientists say.
“When vertebrates such as fish disappear, then invertebrates — especially jellyfish — appear,” Aguilar says.
The collapse of fish populations boosts this process in two important ways, he added. When predators such as tuna, sharks and turtles vanish, not only do fewer jellyfish get eaten, they have less competition for food.
Jellyfish feed on small fish and zooplankton that get caught up in their dangling tentacles.
“Jellyfish both compete with fish for plankton food, and predate directly on fish,” said Andrew Brierley from the University of St Andrews in Scotland. “It is hard, therefore, to see a way back for fish once jellyfish have become established, even if commercial fishing is reduced.”
Which is why Brierley and other experts were not surprised to find a huge surge in the number of jellyfish off the coast of Namibia in the Atlantic, one of the most intensely fished oceans in the world.
CLIMATE CHANGE
Climate change has also been a boon to these domed gelatinous creatures in so far as warmer waters prolong their reproductive cycles.
But just how many millions, or billions, of jellyfish roam the seas is nearly impossible to know, said scientists. For one thing, the boneless, translucent animals — even big ones grouped in large swarms — are hard to spot in satellite images or sonar soundings, unlike schools of fish.
They also resist study in captivity, which means few studies.
“There are only 20 percent of species of jellyfish for which we know the life cycle,” Goy said.
The fact that jellyfish are not commercially exploited, with the exception of a few species eaten by gastronomes in East Asia, has also added to this benign neglect.
But the impact of these stinging beasts on beach-based tourism along the Mediterranean has begun to spur interest in these peculiar creatures whose growing presence points to dangerous changes in the world’s oceans.
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