Jellyfish thrive in the artificial disruption of the oceans

Thousands of them invade our beaches to the horror of vacationers who dread their bite, but thanks to the disruption of the oceans by humans, jellyfish thrive.

Jellyfish have been on Earth for longer than us, they are thought to have roamed the oceans for nearly 600 million years.

But human activities, from overfishing to plastic waste to plastic waste, have created an environment in which they are even more at home.

The proliferation of jellyfish could lead to what some observers call the "jelly" oceans, which are facing profound changes according to a draft UN report to be released on Wednesday.

Fabien Lombard, a French marine biologist from the Sorbonne specializing in the ecology of plankton and jellyfish, would not go that far.

"There are more jellyfish in some areas of the world," he told AFP: in the Black Sea, off the Namibian coast and in the Sea of ​​Japan.

It is not clear if their presence has increased in other parts of the world because it is difficult to actually count them, although a global database was created in 2014 to track it. .

Jellyfish, which were among the first inhabitants of the planet, live today in all the seas and oceans of the world and at all depths.

Invertebrates do not have brains, are composed of 95-98% water and float and swim with ocean currents.

These are incredibly diverse creatures, appearing in a wide range of colors and sizes.

"Absolutely unbelievable quantities"

Jellyfish breed even in different ways during their life cycle.

Adults spawn and release a huge amount of eggs and sperm in the water. The fertilized eggs flow into the oceanic depths before hatching a small polyp, which can then clone.

When many jellyfish are massaged, we speak of "flowering". In the past, this was happening at regular intervals, said Anais Courtet, a biologist at the Paris Aquarium, every 12 years in the Mediterranean, for example.

"Today, this cycle is no longer respected and we see it every year," she added.

For Philippe Cury, marine ecosystems specialist at the National Research Institute for Development (France), this is explained by anthropogenic factors such as overfishing, trawling on the high seas and the warming of the oceans.

"These three factors cause eruptions of the jellyfish population," he said. "It always happens," he added, "but they are much more frequent and we sometimes see absolutely incredible quantities."

Overfishing has eliminated some of their natural predators, such as tuna and sea turtles that are often accidentally caught in nets, but also fish that feed on plankton.

With their predators reduced in number, jellyfish have more plankton to feed on and thrived without control.

The human factor

Trawling on the high seas also helped them. The trawlers drag giant nets on the ocean floor, pulling everything without distinction: sponges, worms and corals.

This leaves an environment in which jellyfish polyps can reproduce without control, Cury explained.

They have also integrated human objects into their own habitat, from buoys to oil rigs.

"They love plastic," Lombard said. Plastic waste of just a few centimeters can be used as breeding colony.

And while global warming and ocean acidification may have affected some species, jellyfish have not caused any damage, Courtet said.

Their proliferation is at the point where they interfere even now with human activity – and it goes far beyond reckless vacationers.

This is a problem for fishing, fish farming and desalination plants. They can even obstruct the cooling systems of nuclear facilities.

For example, in 2007, jellyfish decimated salmon on a fish farm off Northern Ireland, pricking the fish, which could not escape.

In Japan, fishermen sometimes cancel fishing trips if there are too many jellyfish in the water, lest the weight of the creatures make them lose their nets or even capsize them.

"We need ecosystems that function normally, with extensive biodiversity," said Cury.

Any idea of ​​catching the jellyfish themselves – whether to eat them or just to get rid of them – is not practical, he added. "They breed very quickly."

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© 2019 AFP