RIGHT now, millions of sinuous, silvery fish are swimming determinedly across the Atlantic Ocean. They are snake-like, more than a metre long and have huge, bulbous eyes. They left their homes in Europe in late autumn and have been navigating westwards ever since, often swimming against the currents that once carried them the other way. They travel alone at a languid pace, never stopping to rest. By night they are near the surface; by day in the depths. Their journey will take more than a year. Many won’t make it. But those that do have a reward awaiting them: sex and death in the Sargasso Sea.
This is the ultimate goal, and fate, of the European eel (Anguilla anguilla), a remarkable and enigmatic species that has nourished the human imagination, and belly, for millennia. Their life cycle is fascinating and their final journey, the details of which have only recently been discovered, is jaw-dropping. “This is a species which is notoriously difficult to understand,” says eel expert of the Forth Rivers Trust in Edinburgh, UK. What we do know for certain is that the European eel is critically endangered and needs help to recover, or woe betide it and the ecosystems it nourishes.
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This species starts life far from Europe in the Sargasso Sea, a region in the western Atlantic that is defined by the four ocean currents that form its boundaries. From December to May adult eels spawn there, and their larvae – known as leptocephali – start a long journey to Europe and North Africa. They are largely carried by the prevailing currents, which drag on their leaf-shaped bodies, but they also navigate. “There’s some active swimming,” says Ros Wright of the UK’s Environment Agency in Feering, Essex.
From the Sargasso Sea, the leptocephali fan out and eventually arrive across the western shores of Eurasia and Africa, from Iceland to Morocco, while also penetrating deep into the Mediterranean and Black Sea. By now they have reached the next stage of their life cycle, becoming glass eels, which are less than 10 centimetres long, thin and transparent, but distinctly eely (see diagram below).
For the next few months, glass eels wash in and out of estuaries, feeding and growing and gradually transforming into elvers, which are dark brown and about 12 cm long. At this point they are ready to swap the sea for freshwater and make their way up rivers and streams to find a place to grow up. Once settled in a lake or river, they transform again, into yellow eels. “This life stage can be decades long,” says Wootton. “And this is usually what we see when we see eels within our rivers and lakes and lochs.”
Eventually, though, it is time to return. Following an unknown cue, our yellow eel begins its final metamorphosis into a silver eel. “It doesn’t fully mature yet, but it’s starting the process,” says Wootton. “It has an incredible metallic silver belly, a really dark back, its eyes grow and enlarge, pectoral fins grow, its digestive tract changes and its sexual organs start to develop.” “They’re so silver it’s ridiculous, a really blue silver,” says Andy Don, also of the UK’s Environment Agency. “Incredibly beautiful animals.” The silver eels then head for the Sargasso Sea, where they mate for the first and last time, spawn and die.
Until recently the details of that incredible journey were obscure. Silver eels were known to set out between September and December, but their route to the presumed spawning grounds and how long it took to get there were lost at sea. That wasn’t for lack of trying.
Scientific interest in the eel life cycle goes back to the late 19th century. Elvers and adult eels were a key food in Europe and interest was growing in aquaculture, but nobody had ever seen a larval eel or witnessed sexual activity between adults.
In 1896, Italian biologist Giovanni Battista Grassi, who was working at an oceanographic station in Messina, Sicily, claimed he had solved the enigma. While sampling in the Strait of Messina, he caught a specimen of Leptocephalus brevirostris, a small, transparent, leaf-shaped fish that was believed to be a species in its own right. He dissected it and found it had between 112 and 117 vertebrae, which suggested it wasn’t a separate species at all, but a young European eel. He later observed all the life stages of the eel in the sea around Messina and finally reared specimens of L. brevirostris in an aquarium. When they metamorphosed into elvers he had clinched it. He surmised that European eels spawn in the deep Mediterranean and their larvae are only seen when occasionally brought to the surface by whirlpools, such as are found in the Strait of Messina. The Royal Society and awarded him the prestigious Darwin Medal for his work.
The Sargasso Sea
Grassi was right about his specimen being the larval form of a European eel, but wrong about its origins. Enter Johannes Schmidt, a marine biologist at the Carlsberg Research Laboratory in Copenhagen, Denmark. In 1904, he was aboard Thor, a research vessel, between Iceland and the Faroe Islands. He and his colleagues took water samples near the surface and unexpectedly found an eel larva. Schmidt had found his “white whale” and on subsequent voyages in the Atlantic he pursued it with vigour, always finding smaller and smaller larvae the further west he went. By 1912, Schmidt was convinced that eels spawned not in the Mediterranean, but somewhere in the western Atlantic. He extrapolated his data and marked the spot: the Sargasso Sea.
He in 1912, but it knocked him back on the grounds that it thought Grassi had the eel-origin issue sewn up. However, the journal Nature didn’t have any skin in the game and . In 1930, the Royal Society finally recanted and awarded Schmidt the Darwin Medal.
As a result of Schmidt’s 1912 paper, the Sargasso Sea became accepted as the hitherto-mysterious location of eel reproduction, meaning that European eels must somehow migrate up to 10,000 kilometres on the return journey to complete their life cycle. But there was still a problem: nobody had ever seen an adult eel in the Sargasso Sea or any eel eggs.
In the 1970s, fish scientists to track the first few hours of migration by five eels from the northern Bay of Biscay, off Spain, which confirmed that they at least strike out in the right general direction to reach the Sargasso Sea. But it wasn’t until the advent of pop-up satellite transmitting tags, which can be attached to an eel and collect data on movement, that it became possible to track them for extended periods. After a year, or when the eel dies, the tags float up to the surface and submit their data.
In 2008, a project called used pop-up tags to track 80 silver eels from all over western Europe over six months – that being the limit of tag battery life at the time. All the eels headed towards the Azores archipelago, around 1400 km off Portugal; one actually made it there before its tag failed. “That’s the furthest that eels had been tracked from Europe going towards the putative spawning grounds,” says Don. “It really was quite ground-breaking.”
That led to a hypothesis that the Azores served as a way station on the migration, and hence would be a good place to try to finally track eels all the way to the Sargasso Sea. Wright wanted to test that idea, but there was no funding available, so, in 2017, she went on holiday there and asked around. On a birdwatching trip, a local guide told her that there were freshwater eels in the Azores, but that not much was known about them, not even what species they were.
Later that year, Wright, Don and others went on an expedition, setting off with just a few essentials in their hand luggage. “Where some people might pack their Speedos and some beach clobber, because we were doing it on a shoestring, our suitcases were just full of fisheries management paraphernalia,” says Don. During a long and arduous trip, they found silver eels in Azorean waterways and confirmed that they were European not American. The project became official and got a name: . In 2020, they caught 23 silver eels in the Azores, tagged them, released them into the Atlantic and waited on tenterhooks for data to come in.
Over the following months, many of the tags failed or became detached from their owners, leaving just a handful in action. The day of reckoning – when the remaining tags would give their data – was 27 November 2020. It was a breakthrough moment, says Wright. Five of the eels had reached the edge of the Sargasso Sea and . “This is the first time we’ve been able to track eels to the Sargasso Sea and we are delighted,” says Wright.
One big surprise was that the leg of the journey from the Azores to the Sargasso Sea takes the best part of a year. The presumption was that silver eels go hell for leather to reach the Sargasso Sea for the first spawning season after their departure. But the truth is that they dawdle and meander for months, travelling an average of just 6.5 kilometres a day, skipping the first season and joining the next one.
Eel Trek isn’t just about closing the loop on a long-standing mystery, but also about the urgent task of saving the European eel from extinction. This species has been in decline for centuries as wetlands were drained. “Italy has just 5 per cent of what it had in Roman times,” says Andrew Kerr of the , a pan-European conservation organisation based in Brussels, Belgium. But since 1980, the population has slithered spectacularly downwards and the European eel is now . The number of glass eels arriving in Europe – termed recruitment – has fallen by 90 per cent or more, and has almost dried up completely in some places.
Eel conservation
“This is an incredible, monumental crash in the species,” says Wootton. “And we’re not 100 per cent sure why this happened. What we do know is that river barriers, that pollution, that illegal fishing, that 101 other causes are still impacting them today.”
Wright suspects climate change is also a culprit, perhaps by weakening the ocean currents the eel larvae use to surf to Europe, or by changing the nature of the Sargasso Sea. This is why her team’s results may help with conservation. The worry, she says, is that so few silver eels will make it back to breed that the population is no longer viable.
There are actions that we can take. One is to remove or modify river barriers such as weirs and dams. These are a major problem for eels, preventing them from swimming up rivers as elvers and escaping again once they are ready to breed. There are some 1.2 million barriers in Europe, many of them no longer serving a useful purpose. Wootton leads a project removing these or installing structures such as eel passes – a set of water-filled steps that eels can leapfrog up or down.
Illegal fishing is another major issue. The black market for European eels sold in Asia is vast, worth an estimated $3 billion a year, according to Europol. In 2018, an estimated to Asia. That translates into 350 million glass eels, says Kerr, or about a quarter of the 1.3 billion that the International Council for the Exploration of the Sea estimates make it to Europe after each breeding season. “This is the biggest wildlife crime on the planet for a living creature,” says Kerr.
The eels are usually packed in suitcases and smuggled from European airports to Japan, South Korea, China and Malaysia. The profits are vast – an eel that sells for 10 cents in Europe can eventually fetch 100 times that value in Japan. Authorities are increasingly taking the problem seriously. A decade ago, smugglers were routinely given a slap on the wrist and a small-fry fine. Earlier this year, a smuggler in Spain was hit with . Trafficking is now a fifth of what it was, but that may be more down to covid-19 travel restrictions than policing. “It’s been an incredible battle to expose this criminality and it’s not over, we’ve got many more fights,” says Kerr.
The navigation enigma
There are tentative signs that eel populations have already stabilised. From 1980 to 2010, glass eel recruitment declined 15 per cent a year. In 2011 the decline stopped, and recruitment has been bumping along the bottom ever since, but it is still in a terrible state, says Kerr. “This is a critically endangered fish”.
The consequences of this decline ripple through the whole ecosystem. Eels once represented a huge influx of nutrients into wetland environments, providing food for species such as otters and bitterns. “All this work that we do on eels has got wider biodiversity benefits and that’s just so important now,” says Wright.
In the meantime, the remaining silver eels continue to make their epic return journey. In 2021, Eel Trek tagged another 16 from the Azores and they are currently en route to their spawning grounds. Wright says they have extended the life of the pop-up tags to 18 months, so they can discover what the eels actually do once they arrive in the Sargasso Sea.
Even then, the eel will remain an enigma. Nobody has ever found eel eggs in the Sargasso Sea, though they must be there. That would truly close the loop. Their amazing, metamorphic life cycle is also not well understood. “We still don’t know what controls a lot of their life cycle systems,” says Wootton.
Then there is the small matter of how they navigate back to their birthplace, across up to 10,000 km of open ocean. “The navigation, I think, is still something we really don’t know,” says Wright. It could be guided by magnetic fields, or maybe visual cues from the sky – hence the huge eyes and shallow swimming by night. It could be genetic: American eels also spawn in the Sargasso Sea, but head east as their European counterparts head west; hybrids occasionally arise and head north. Those route-finding skills could be down to something else entirely. But that is a mystery that the European eel may well take to its watery grave.
