During the summer months the European media is full of reports of how holidaymakers are stung by jellyfish in ever increasing numbers. The NGO coalition Ocean2012 sets out to explain why these jellyfish blooms are reaching plague proportions in some areas.

In a briefing, The Battle for Ocean Supremacy, the Jellyfish Conquests, the coalition argues that halting overfishing – one of the key causes of rising jellyfish numbers – would reduce the ecological space that jellyfish need to survive in marine ecosystems.

For many years researchers have been studying the causes and consequences of the rise in jellyfish numbers. Three key human- induced factors are said to have caused the rise in numbers – pollution, climate change and overfishing.

Pollution from the land runs off into the sea, causing increased nutrients in the water, increasing plankton, a food source for jellyfish and fish. Many scientists claim that climate change creates favourable conditions for jellyfish in European waters. Overfishing takes out of the ocean jellyfish predators and competitors, leaving them room to thrive. The briefing gives the example of Namibia, where sardine overfishing is thought to have caused the fish rich ecosystem to become dominated by jellyfish. The shift in the ecosystem is amplified as the large numbers of jellyfish outcompete the fish by preying on the same zooplankton species.

The briefing warns that in a worse case scenario the oceans could reach a point where highly evolved ecosystems are destroyed and revert to a less evolved gelatinous state, known as a “phase shift”.

The Ocean2012 briefing is part of a series putting forward arguments for an effective reform of the Common Fisheries Policy (CFP).

Julie Cator

Links:
Ocean2012 briefing: The battle for ocean supremacy, the jellyfish conquests (pdf)

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