But this slimy creature, usually found only off the East Coast of the United States, has devastated fish stocks in the Caspian Sea since it was accidentally brought to the region in a ship’s ballast water.

Fishermen and government experts now fear that it could destroy the lucrative caviar industry, which has already been crippled by overfishing and pollution. It is not just that mnemiopsis feeds on the same plankton that nourish the kilka sprats on which larger fish, such as the caviar-producing beluga sturgeon, live. The voracious stowaway, Mnemiopsis leidyi, can also produce 8,000 eggs a day and eat so much plankton that its body weight can double in 24 hours. And, crucially, not a single local predator feeds on it.

“It is a monster,” said Rufat, who has fished for Caspian sturgeon for more than 20 years. “If it carries on feeding like this, there will be no fish left.” Since mnemiopsis was first discovered in the Caspian in 1999, its population has risen by an estimated 5,000 per cent.

In the same period, kilka stocks have dropped by 50-80 per cent. Starved of their main food source, beluga sturgeon are becoming smaller and producing fewer eggs by the year, said Mehman Akhundov, of the Azerbaijan Fishery Research Institute. “You can imagine how hard it is for the beluga to feed now that the kilka have gone,” Dr Akhundov told The Times. “When we catch them, we see that their stomachs are empty.”

A recent study found an average of 37 mnemiopses in every square metre of water in the southern Caspian, he said. The plague has affected millions of people in fishing communities in the five countries that surround the sea — Azerbaijan, Iran, Turkmenistan, Kazakhstan and Russia.

But now Iran has come up with a secret weapon to wage biological war on the invader. Iranian scientists have proposed introducing another kind of American jellyfish, Beroe ovata, which feeds on only one thing — the mnemiopsis. They have been breeding the gelatinous assassins in special tanks to adapt them to Caspian waters, which are less salty than their normal habitat. Since the beroe feeds only on the mnemiopsis, they say, it will simply die out once it has consumed them.

They cite the example of the Black Sea, where mnemiopsis devastated anchovy stocks after arriving from America during the 1980s. After beroe arrived in 1997, the mnemiopsis population began to decline rapidly and plankton and fish stocks stabilised. The Azeri Government has called for further research to be done before undertaking such a bold experiment, which risks upsetting the Caspian ecosystem.

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