European Eels – slipping towards extinction?

Until as late as the 1950s the European Eel (Anguilla anguilla) was a cheap, nutritious and readily available food source. Britain’s largest rivers used to glitter silver with the spring tides as young ‘glass eels’ pushed upstream in their hundreds of millions. The slippery little fish were once so common in the Thames estuary that nets were set as far upriver as London itself, and eels were a staple for London’s poor. The first “Eel Pie & Mash Houses” opened in London in the 18th century, and ‘jellied eels’ were as closely identified with cockneys as Bow Bells, rhyming slang, and Pearly Kings and Queens (in fact, though, eating eels peaked a time when the Thames was so polluted that few actually came from the river: they were brought in by eel barges from Holland instead).

Vast numbers were trapped as they moved through Europe’s freshwater systems, and eel fisheries were well-established from Ireland and the UK to France, Germany, the Netherlands and up into Scandinavia. Eels were thought to be the most widespread, single fish ‘stock’ in Europe, occurring all over the European continent and the Mediterranean. Trade between the Netherlands and London dates back to the 1400s, and by the 1920s several thousands of tons of eels were being traded internationally.

Overfishing was recognised as problematical as early as 1865, when an anonymous report from France suggested that “the eels, that feed us, have almost disappeared from our small waters”. In 1910 discussions around options to develop an eel fishery, described “an increasing depletion of the inland waters of eels” in Germany. But this was long before the absolute collapse in numbers that has occurred over the last fifty years. European eels have declined to such an extent that they are now considered to be in danger of extinction.

How did a fish that was once so abundant and so familiar become listed in 2008 by the International Union for Conservation of Nature (IUCN) as Critically Endangered (the highest threat level), leading to headlines in the media this week which included “Man found guilty of smuggling £50m worth of live eels out of UK“?

Unfortunately it’s down to a combination of human-related causes that have impacted so much of the world’s wildlife: widespread habitat change, agricultural and industrial pollution, and exploitation and wildlife crime.


A curious life history

While many people might think of an eel as just ‘that slippery thing’, the European Eel is a fascinating animal. Its life history was poorly understood until the 1920s, but incredibly it all begins near the island of Bermuda in the Sargasso Sea. Even today the exact spawning grounds are unknown, but eel eggs laid here hatch into transparent larvae that then drift for two or three years along the Gulf Stream as it slowly sweeps 4000 miles across the Atlantic Ocean towards Europe and North Africa. On arrival the larvae undergo metamorphosis, changing into see-through miniature eels called ‘glass eels’ (image above copyright BBC). These glass eels continue their long journey inland up river estuaries, migrating upstream to traditional freshwater habitats where they slowly mature over many years transforming from ‘elvers’ into ‘yellow’ then ‘silver’ adult eels. After 5–20 years in fresh or brackish water, they begin their migration back to the Sargasso Sea to spawn – and die.

And it’s at either end of that long journey that the demise of the European Eel seems to be found. Researchers discovered that young eels were not being ‘recruited’ to the adult population, leading to long-term declines in the overall numbers of eels as adults died and were not replaced by enough maturing eels to maintain population numbers.It was this imbalance between young and old that led to the IUCN decision  in 2008, citing “a steady and almost continent-wide decline of ~90%…observed in the recruitment of glass eels [the youing sexually immature stage of the eel” since the 1980s.


Disappearing eels

Isolated island species can be lost by to a single introduced predator or disease (the story of Lyall’s/Stephen’s Island Wren makes fascinating if sombre reading), but population declines in such widespread populations are typically driven by a combination of factors. This is true for European Eels.

A major factor impacting eels is habitat alteration: wetland reclamation and conversion (which has impacted wetlands around the world) means habitats have been lost, and infrastructure developments along river systems – weirs and dams, hydropower and water-pumping stations driven by turbines slicing through the water, one-way flood defences which use flaps to shut pipes closed – are blocking the young eels’ migration pathways from the sea into the freshwater catchments where they mature. Many of Europe’s major rivers are now ‘contained’ in some way, and while salmon, for example, can leap obstacles, glass eels can only wriggle: a metre high barrier may be unpassable unless tidal rises allow them to make their way over it.

Our oceans and rivers are heavily polluted too. Fertilisers and farm waste (animal sewage) still leach into the UK’s rivers, and pesticides – which are poured onto crops in almost unbelievable quantities (listen to a podcast on this here) – and industrial chemicals are also believed to be part of the eels’ problem. While loads varied from country to country, in 2008 researchers reported that  “eels from rural areas presented higher loads of crop protectors (pesticides/herbicides), while individuals from industrialized areas were characterized by higher levels of [now banned Polychlorinated biphenyls] PCBs and heavy metal”. In the 2010 review ‘Contaminants in the European Eel’, the authors reported that “that spawner quality might be an essential element in the decline of the species since pollution by bioaccumulating chemical substances may have a large impact on the reproduction success of the eel” and that “contaminants may cause disturbance of the immune system, the reproduction system, the nervous system and the endocrine system and effects were reported on several levels of biological organization, from subcellular, organ, individual up to even population level”.

Oceans and estuaries are also increasingly noisy places. It’s hard at first to imagine how the sound of a ship’s engine might have an impact on an eel, but a 2014 study by researchers from the University of Exeter and the University of Bristol looking at how noise from shipping might stress eels or slow their reactions to predators, found that “when surrounded by ship noise, eels were half as likely to respond to a predator’s ambush than they were under normal circumstances. Those that did react to impending death did so at rates about 25 per cent slower than usual“.

On top of everything else, it appears that these benighted animals are increasingly being stricken by a nematode worm parasite as well. The invasive eel parasite Anguillicoloides crassus, according to another 2014 paper,is considered one of the major causes for the decline of the European eel”. A. crassus seems to be using what researchers have termed a ‘Trojan Horse’ strategy, invading the cysts of a parasite of the Round Goby which eels prey on. Once inside the eel, crassus makes its way to the eel’s swimbladder, reducing the swimming performance of its host. With a 4000 mile trans-oceanic crossing to complete, any impairment on swimming ability could have a serious and deleterious impact.

So, unable to reach the freshwater habitats they need to grow and mature, bioaccumulating contaminants which may impact reproduction, unable to respond properly to predators, and attacked by parasites – it’s little wonder that eels are in trouble. That’s not all, though. Like so many other species across the planet, it appears that the European Eel is also being affected by climate change. Water temperatures in the Sargasso Sea, where larvae develop and adults return to spawn, are changing. As long ago as 2008, ‘Fluctuation in European eel (Anguilla anguilla) recruitment resulting from environmental changes in the Sargasso Sea‘ warned that “sea warming in the eel spawning area since the 1980s has modified marine production and eventually affected the survival rate of European eels at early life stages“. Which would tie in with the declines noted in European populations.

It’s entirely possible, too, that climate change may be shifting the track of the Gulf Stream so that fewer glass eels are able to successfully hitch that trans-Atlantic ride described above.


Wildlife trade and wildlife crime

How does this all bring us back to headlines of smuggling eels worth such enormous amounts? Rarity value, market forces, and wildlife crime.

As with any ‘stock’ or ‘animal product’ the rarer something is, the more you can charge for it if you have it (or, more accurately, if you steal it or poach it). And of course the more the consumer wants something, the more they will pay for it anyway. Especially, as in the case with the European Eel, if that ‘stock’ species is Critically Endangered and at risk of extinction.

Despite an EU-wide export ban in place since 2010, in a highly disturbing example seafood salesman, Gilbert Khoo, was caught trying to smuggle live eels out of the UK to Hong Kong. Border Force officers found 200kg of “glass eels” at Heathrow Airport hidden underneath other fish in the first seizure of its kind in the UK. European Eels, the same eels eaten for hundreds of years by the urban poor, now fetch sky-high prices on the east Asian black market, where they are considered a delicacy and – as they get rarer – are in increasingly high demand. According to Marion Longford, the unit head of the CPS specialist fraud division, “Khoo was trading these animals purely for a financial gain. He had no regard for the controls in place for trading endangered European eels, which are vital to safeguard animals increasingly at risk of being wiped out completely.”

Reports suggest that every year 350million live eels are being smuggled out of Europe to the Far East, an illegal trade supposedly more lucrative than cocaine. And these are young eels which, of course, have never had the opportunity to reproduce, lost to a breeding population already in enormous decline.


Conservation efforts

Conservation of the European Eel is tremendously difficult. The migratory nature of the eel’s life cycle means they are exposed to natural and human pressures both at the local and global scales (from individual wetland changes to rises in ocean temperatures). Large-scale habitat changes largely can’t be reversed, infrastructure developments can’t simply be removed (smaller weirs can be altered perhaps, but not massive dams) and the value of eels means illegal trade pressure is increasing rather than decreasing.

Management of existing fisheries is touted as part of the answer – closing them down entirely would be a better answer, but few (if any) governments would put a fish above economic concerns and lobbying from fishermen. Once the fish are gone of course there will be no choice, but until that happens countries all across Europe insist on allowing elvers to be taken from the wild and ‘ranched’ for export. As the Marine Conservation Society puts it, “Eel ranching contributes to the depletion of endangered wild stocks and does not provide a farmed alternative to reduce pressure on wild stocks.”

The Zoological Society of London (ZSL) has been at the forefront of eel conservation in the UK. In 2005 ZSL established a monitoring project to determine the recruitment of elvers into the River Thames catchment area and found that up to 99% fewer eels were arriving into the River Roding catchment than in the 1980s. ZSL is now a key partner in delivering the Environment Agency’s Thames Eel Management Plan, which outlines key targets and measures to improve the population and conservation of the European eel. They are also part of the IUCN’s Anguillid Eel Specialist Group, which “brings together conservationists, academics, business interests and governments and advocates science-led conservation of these species“. From March to June each year ZSL and project partners now recruit citizen scientists at sites across the Thames region to help check eel traps and measure and release any eels present. Data on the number of young joining the adult population can highlight the impact of barriers on the Thames, which ZSL considers to be one of the principal threats to eels in the freshwater part of the river.

In 2018 the Wildfowl and Wetlands Trust (WWT) began to microchip eels to track their movements around Slimbridge on the River Severn, once a vital waterway for the species. The study, according to WWT, “is part of a broader eel conservation project being undertaken by WWT in partnership with Bristol Water. The project aims to improve eel access around the site, and will eventually extend to the wider Severn Vale“.


Every effort helps, but as memories of those ‘silver tides’ fade, there is a real possibility that shifting baselines (the phenomenon in which over time knowledge is lost about the state of the natural world, and people don’t recognise the changes that are actually taking place) mean that eels will simply fade away unmissed. Perhaps understanding that the decline of a once-abundant species over such a huge area is a warning to us humans that we are increasingly making the environment ‘unfit’ for ourselves as well might make a difference – but right now, it really does look as if the European Eel is in danger of slipping away altogether.


  • Header image copyright Derek Middleton/Sussex Wildlife Trust