In France – which catches more of the fish than any other EU state – the issue has taken on political dimensions.

“There’s around 10 per cent of stocks left compared to 30 years ago due to habitat loss and what we’ve done to the migration pathways in Europe,” Andrew Kerr, chairman of the Sustainable Eel Group (SEG) that works to conserve the species, said.

The eel’s vertiginous decline has provoked some action from governments and law enforcement agencies. It is now listed in the CITES international convention on trade in endangered species, resulting in strict national catch quotas.

The problem, according to Michel Vignaud, head of fishing regulation at France’s National Biodiversity Agency, is exploding Asian demand for a product viewed as both a delicacy and an aphrodisiac.

That eel you’re eating in Japan: it may be endangered species smuggled from Spain and farmed in China

“We cannot legally export eels outside the EU, but the prices are different in Asia. There is a real Asian demand for eel,” he said.

The UN’s Food and Agriculture Organisation said that in 2016 China produced close to a quarter of a million tonnes of eel for consumption, far ahead of Japan – where eating eel is seen as bringing good luck and fertility – and the EU.

The bloc’s law enforcement agency Europol estimates as many as 100 tonnes of baby eels – known as glass eels for their translucent skin – are trafficked abroad each year: equivalent to around 350 million fish.

“Glass eels trafficking involves environmental crime, smuggling, document fraud, tax evasion and money laundering,” a spokesman said.

The live eels are largely caught – legally or otherwise – in western Europe before being smuggled eastward in vans or lorries, often falsely labelled as non-endangered fish, police and conservationists say.

Criminal gangs then divide the eels into suitcases, up to 50,000 of the tiny fish per bag, which are then flown by commercial airliner to Asia.

The fish are grown in special farms to their full size -up to a metre and a half – and then sold to market for the equivalent of 10 euros each.

“Prices vary so you can only come up with bracket figures, but we’re talking billions (of euros). It’s the biggest wildlife crime by value on Earth,” said Kerr.

Four Chinese arrested for smuggling glass eels in Spain-Portugal bust on ‘the biggest wildlife crime in Europe’

“It’s the most trafficked and travelled animal on the planet.”

The European eel’s extraordinary life-cycle begins in the topaz waters of the Sargasso Sea. The eggs drift on the current across the Atlantic, often taking up to two years to reach the feeding grounds of Europe.

The baby eels swim up rivers where they live for up to 25 years, feeding on larvae and worms until fully grown before embarking on the 6,500km (4,000 miles) journey back home to the Caribbean, where they breed and finally die.

But they are under threat from a host of man-made dangers, including illegal fishing, pollution, and the estimated 1.3 million river barriers blocking their path across Europe.

“Our hope is that any eels still alive are left alone by humans,” said Charlotte Nithart, head of the Robin des Bois conservation group.

She said that France’s current legal eel quota – 60 tonnes per year, of which 60 per cent must go to restocking efforts – was contributing to the species’ decline.

“We have never said that trafficking alone is responsible for the disappearance of eels,” she said.

“We want to cancel or at least dramatically reduce fishing quotas and reinforce the means to fight trafficking.”

For Guillaume Le Priellec, who represents the local fishing committee in Brittany, quotas are “justified” for those whose livelihoods depend on the eel fishing season, which began across France last week.

“If you are in commercial fishing people tell you that when you fish more you earn more. So the fishermen always want more,” he said.

As the numbers of fish arriving on Europe’s shores goes down, smugglers are taking greater risks to sate voracious Asian demand.

“Trying to control the traffickers is getting more dangerous. These are people who operate on the sidelines of mass organised crime,” said Vignaud.

Europol has scored a number of major hauls in recent years, including Spanish police busting an eel-smuggling gang in possession of 350kg of glass eels in April.

But despite such seizures and a handful of ongoing trials, campaigners say the penalties remain flimsy compared with those for other trafficking crimes.

“Europol can act on the level of customs but there’s not a European intervention force to fight trafficking and traffickers,” said Nithart.

Eels are mentioned in England’s 11th-century Domesday Book and used to be an accepted tax payment in medieval times.

But as mass trafficking continues to undermine EU-wide

efforts to save the threatened species, there are fears for the future of one of the world’s most storied fish.

“One of the sad things about today in general is how human beings are losing touch with nature, and the eel really symbolises this,” Kerr said.