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Europe's largest wildlife crime is the illegal trade of the critically endangered European eel. It's an industry that many people are completely unaware of and may be unwittingly contributing to.
Europe is at the centre of an illegal wildlife trade operation worth up to €4 billion (£3.5 billion).
But rather than illicitly moving ivory or rhino horn, international criminal organisations are dealing in European eels. Gangs are thought to be smuggling up to 350 million live eels from Europe and shipping them to Asia every single year.
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This multibillion-euro enterprise is having a devastating impact on the fish. Since as recently as the 1980s, it is thought that the European eel population has crashed by as much as a staggering 96%. The combination of blocked migratory routes, pollution and insatiable demand for young eels is pushing them ever closer to extinction.
The species is now listed by the International Union for Conservation of Nature (IUCN) as critically endangered.
'When talking about the trade, with the focus is very much on juvenile eels known as glass eels,' explains Matthew. 'Within the EU it is legal for states to trade the eels with each other, but if they leave the EU it becomes illegal.
'The eels are usually sent to east Asia for farming where they are grown. From there, both live and processed eels are sent all over the world for consumption. Historically there was a big demand in Japan, but that seems to have declined slightly in recent years.'
As the global popularity of sushi has increased over the last few decades, this has opened up a whole new market for farmed eel fillets.
Florian Stein, director of scientific operations for the Sustainable Eel Group, explains that traditionally the Chinese eel farmers raised more local species such as the Japanese eel.
'When that species declined they started to import other species, particularly the European eel,' he adds.
There used to be legal trade of eels between the Europe and China, but in 2010 the EU shut this market down.
'Before 2010 there were huge numbers of eels being shipped to China,' says Florian. 'Combined with the fact that the eels were declining and that the trade was so big, the EU decided to halt the trade to Asia.'
But it has now become clear that the trade didn't stop, as despite the downturn in the Japanese markets the demand for eels remains high.
With the global spread of Japanese culture and restaurants, the markets have once again expanded and criminal gangs have taken to smuggling the eels from Europe to China, making billions in the process.
Japanese restaurants typically sell two main eel dishes: unagi kabayaki and sushi. In many Western countries, particularly the USA, customers could be funding criminal gangs by eating the illegally trafficked and critically endangered European eel. Every country that serves eel from China has the same chance that it is buying these fish.
European eels have one of the most extraordinary life cycles in the animal kingdom.
Unlike many migratory fish, the young are born at sea and travel to coastal and freshwater where they feed and grow. When ready to breed the adults, known as silver eels, make their way down the rivers of Europe and out into the ocean.
They then swim roughly 5,000 kilometres across the Atlantic Ocean to get to a region of seaweed off the eastern coast of the USA called the Sargasso Sea. It is here that the adults mate and lay their eggs, before all of them die.
The eggs then hatch into near-transparent, leaf-shaped larvae known as leptocephalus, and drift on ocean currents back towards Europe. This can take up to two years, with less than one in 500 thought to survive the journey.
By the time they reach the coastal waters of Europe and the Mediterranean - ranging from North Africa, to Norway and Turkey - they will have undergone their first major change, turning into glass eels about six centimetres long.
The glass eels then start searching for a suitable river to swim up, darkening in colour to become elvers as they head toward freshwater. Swimming upstream, the elvers will cross over obstacles and even go over land to find a suitable patch of river to mature in.
Here, they become yellow eels. They will then spend up to an astonishing 20 years living in the rivers and lakes, until some still unknown stimulus marks the final change in the eels life, as the yellow eels turn silver.
While scientists have never actually witnessed this, it is then assumed that the silver eels make their long and arduous journey back to the Sargasso Sea, where they spawn and then die - and the cycle begins anew.
It is when the eel larvae reaches Europe and turns into a glass eel that they are caught.
An estimated 60 tonnes of glass eels are caught legally, largely by France in the Bay of Biscay, but also to a lesser extent in Spain and the UK. Each kilogramme of glass eels contains thousands of fish.
But it is thought that only half of these eels are then traded within the EU. The other half of the catch simply disappears.
'There are obviously a number of actors in many different countries,' says Matthew, 'and there has definitely been an increase in illegal activity in the past ten years.'
But getting a real picture of the scale is very hard. According to Florian, this year authorities seized around eight tonnes of glass eels as various agents tried to smuggle them out of Europe.
When it comes to these sorts of seizures, it is often thought that only around 10% of product is detected, meaning that the real scale of the illegal trade could be up to 100 tonnes a year, according to Europol. This means that the illegal market could be pushing on three times the size of the legal one.
'There are two different methods to get the eels to Asia,' explains Florian. 'You have mainly Asian gangs smuggling eels in suitcases. But the other one is to send them in airfreight cargo. They are either mislabelled or hidden under other seafood.
'We have to assume that is the bigger part of the trade.'
The criminal syndicates responsible for this are clearly very organised, as they have been changing their routes and their methods as seizures are being made. This indicates that there are a lot of resources behind the groups, which is perhaps unsurprising for a business worth billions of euros every year.
'The unsustainable and illegal exploitation and trade is one of the things that harm these fish, but you have to put it in the context of all the other threats,' says Matthew. 'Because of their lifecycle, they're being exposed to other pressures in the oceans and freshwater.'
This means that unlike many species of commercially important fish, the fisheries are not the only threat they face.
'The barriers to migration and pollution in rivers are big problems, while changes in oceanic currents are another,' explains Matthew.
As ocean currents shift - something which climate change is expected to exacerbate - then it's possible the eel larvae won't be taken to the parts of the coast where they need to be in order to thrive.
Another big issue is barriers across rivers, and in Europe there are thousands of such obstacles and it's not always easy to manage the impact of these, particularly if large hydroelectric projects are involved.
'Arguably, the legal fisheries and associated trade are the aspects that tends to get focused on when it comes to management, as in relative terms they are the threat that is most straightforward to regulate,' says Matthew.
What needs to now happen to help tackle the illegal trade is tighter law enforcement and management of stocks, including the improved traceability of the eel trade. But this will only happen if European governments acknowledge this problem and put the right policies in place.