Boom in jellyfish: overfishing called into question

300px-Phyllorhiza_punctata_(White-spotted_jellyfish)
White-spotted jellyfish off the North coast of Haiti. Français : Phyllorhiza punctata, un méduses photographiée au large de la côte nord d’Haïti. (Photo credit: Wikipedia)

Will we soon be forced to eat jellyfish?

Since the beginning of the 2000s, these gelatinous creatures have invaded many of the world’s seas, like the Japan Sea, the Black Sea, the Mediterranean Sea, etc. Is it a cyclic phenomenon, caused by changes in marine currents or even global warming? Until now, the causes remained unknown. A new study conducted by IRD researchers and its partners, published in Bulletin of marine science, exposes overfishing as the main factor.

Jellyfish have free rein

Jellyfish predators, such as tuna and sea turtles, are disappearing due to overfishing. However, jellyfish are primarily taking advantage of the overfishing of small pelagic fish . Just like these cnidarians , sardines, herring, anchovies and more feed off zooplankton. Thus, they represent their main competition for food. In areas where too many of these fish are caught, they free up an ecological niche . Jellyfish now have free rein and can thrive. Furthermore, small fish eat the eggs and larvae of jellyfish. Therefore, under normal conditions, they regulate the population . In their absence, there is nothing to stop the proliferation of these gelatinous creatures.

Comparison as proof

In order to demonstrate the major role played by overfishing, researchers compared two ecosystems belonging to the same ocean current, the Benguela, which flows along the south of Africa. The first ecosystem is located off the coast of Namibia. Here, fish stock management measures are not very restrictive. The stocks are barely restored before fishing activities start up again. Jellyfish are currently colonising these coastal waters. The second ecosystem is located 1,000 km further south, off the coast of South Africa. Here, the opposite is true: fishing has been tightly controlled for 60 years. The jellyfish population has not increased.

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