British Wildlife of the Week

European Eel

The European eel is a surprisingly enigmatic creature. Its breeding behaviour in particular has confounded people for over 2,000 years. To some, its proliferation seemed inexplicable. The great Greek thinker Aristotle, for example, believed that eels spontaneously emerged from mud. A few hundred years later, Pliny the Elder had his own imaginative ideas about eel reproduction – he thought they bred by rubbing their bellies against rocks. Later, others believed that these animals simply spawned from dew. In fact, the origin and life cycle of the European eel remained largely a mystery for a very long time – a mystery that wasn’t solved until well into the 21st century.

An Eel Enigma

So what caused all the confusion? For a start, most of the eels being studied seemed to be sexless; repeated dissections failed to reveal any gametes (sex cells). Secondly, although these eels spent a lot of time in the rivers of Western Europe, they eventually headed downstream and vanished before they could breed, seemingly without a trace. Several months later, young eels reappeared in these same rivers, swimming upstream from the sea, and there they remained until it was time for them to reproduce for themselves.

Some fish, such as salmon, migrate from the sea into freshwater in order to spawn. This had been known for a long time. Some people, therefore, proposed that the European eel, unconventionally, did the reverse: it left freshwater and headed out into the open ocean to breed. It seemed like a sound suggestion – but there was no evidence to support it. No baby eel had ever been found in the sea. And no one knew where their breeding ground might be located.

In 1904, Danish explorer and oceanographer Johannes Schmidt was commissioned to find the obscure origins of the eel. He was given a small ship and set off to trawl the Atlantic Ocean. His quest was to locate the smallest possible eel larva, which would, he logically thought, lead him to their place of birth. It took him almost 20 years to find out where they bred.

The eels are born in the Sargasso, a vast expanse of warm, seaweed-strewn sea in the North Atlantic, bordered on four sides by oceanic currents. The tiny, transparent, leaf-shaped larvae are called leptocephali (‘slim heads’) and are physically so different from adult eels that until 1896 were believed to be an entirely different type of marine life. Driven by an ancestral urge, these minute, fragile creatures embark on an epic odyssey to Europe, 6,000 km away across the Atlantic. The vast majority will not make it. But those that do survive transform, thanks to chemical and temperature cues as they near land. From tiny, laterally-compressed larvae, they become ‘glass eels’ – still small and largely see-through, but now recognisable to most people as eels.

Finally, as they leave the sea and start swimming upriver, changes in the gut and kidneys equip them for freshwater. They transform again, this time into ‘elvers’ – miniature versions of the adult eels – and forge inland. Those rivers with wide, west-facing estuaries like the Severn in England usually get large quantities of young eels swimming up them. These fish will go to any length to reach their goal; sometimes they pile their bodies up in their hundreds to climb over obstacles, and their thick coating of mucus means they can leave water entirely and slither over wet grass at night. They have remarkable navigational skills. Eels in Scandinavia have been caught, tagged, and then released into an estuary hundreds of miles away, yet they reappeared within their original river within weeks.

The eels spend many years within these rivers, feeding and growing more opaque as they mature. Whereas elvers only eat microplankton, adult eels are voracious carnivores, feeding on invertebrates and small fish. They’re also very cannibalistic, frequently devouring their own kind.

Only after living in freshwater for between 5 and 20 years do European eels become sexually mature. This is partly what had stymied Aristotle and the other early naturalists. The eels they had been examining, though fully-grown, hadn’t yet reached this stage in their life, so their reproductive organs hadn’t developed.

In order to breed, the eels must make the incredible journey back to the Sargasso Sea, all the way across the Atlantic. An unknown stimulus triggers a relentless downstream migration to the estuaries. Many changes are made to their physiology to prepare them for oceanic life: their backs turn darker and their bellies silver to provide camouflage from seafaring predators; they toughen up and store fat, for they will not eat once they start their migration; their eyes enlarge so they can cope with the ocean darkness; the salt content in their blood decreases; and their gas bladders grow, allowing them to withstand the pressures of a ton per square inch in the ocean depths.

At long last, the eels travel out into the vast sea – and then they disappear. No adult eel has ever been caught in the mid-Atlantic. Presumably, this is because they make the journey at great depths, far below the reach of any net or trawl, and since they no longer feed there is little chance of catching them on baited lines. But six months after they vanish, their gruelling odyssey is complete and the eels arrive back at the Sargasso Sea. Perhaps they follow the imprinted memory of the smell of their larval hatching grounds, carried on the currents. Or maybe they use the Earth’s magnetic field. In any case, the surviving females spawn and the males fertilise their eggs. Afterwards, the eels, which haven’t eaten since the rivers of Europe, die of starvation and exhaustion.

At least, that was the commonly-held theory. For a very long time, although the circumstantial evidence certainly pointed to the Sargasso Sea being the eels’ spawning grounds, there was no definitive proof. It wasn’t until 2015 that adult eels were actually observed in the area, finally proving, after a century of speculation, that they do indeed reproduce there.

So why should the eels travel all the way to the Sargasso Sea just to lay their eggs? Why partake in such a long and hazardous voyage? Why force their young into making the reverse journey when they hatch? The answer lies in the distant past. It is highly likely that this epic migration is a relict behaviour originating in a distant geological period when the continents of North America and Europe were much closer together and the Atlantic Ocean was nothing more than a narrow strip of water between the two. Since then, however, the forces of continental drift have pulled the two landmasses apart, widening the Atlantic considerably. Yet the habit of returning to the Sargasso Sea each year to spawn has never been broken, even though now it involves such an immense journey.

The Endangered Eel

The European eel, globally, is classified as Critically Endangered. The number of glass eels arriving at our coasts in spring has declined by more than 90% since 1980. Overall, in the UK, we may have just 2% of the population that lived here just 50 years ago. Overfishing and pesticides are thought to be contributing factors to this precipitous decline, while climate change may be shifting the track of the Gulf Stream so that fewer glass eels are hitching a trans-Atlantic ride to Europe. And the ones that do make it back to our shores are finding it increasingly difficult to make their way upstream, thanks to the huge amount of dams, weirs, sluices and lock gates that we have installed on our rivers to control and channel their flow.

Meanwhile, vast quantities of elvers – around 300-350 million fish, equivalent to 100 tonnes – are illegally shipped to China and Korea to be eaten as a delicacy every year. In terms of numbers taken, the European eel has become the most trafficked species on the planet. Sadly, it’s much harder to drum up sympathy with the general public for a slimy, snake-like fish that lurks at the bottom of rivers than it is for, say, rhinos, tigers or even pangolins.

To make the issue even worse, the European eel currently cannot be bred and raised in captivity. This is because the development of ovaries and testes is controlled by a complex hormonal mechanism. At the start of their migration to the Sargasso Sea, an inhibitory mechanism kicks in to prevent the development of these organs. But as the fish approach their (final) destination, this mechanism is deactivated, allowing the development of reproductive organs to continue and the eggs and sperm to form.

Only by using hormone treatment have scientists been able to successfully get eels to mate and spawn in captivity, and for the eggs to hatch. However, because the diet of the leptocephali is still poorly understood (they are believed to eat floating particulate matter rather than plankton) the larvae do not survive for very long – rarely more than 20 days. This means that, as of this writing, the full life cycle of this species has never been completed in captivity.

The genesis and life history of the European eel has remained shrouded in mystery for millennia. Today, we know much more about their epic migration and their breeding habits than ever before – but even so, many unanswered questions remain. No one has ever seen European eels mating in the wild, for example, nor have eel eggs been found in the ocean. Finding such things remains a zoological Holy Grail.

In the next British Wildlife of the Week, we’ll be looking at a bird that, for me at least, signifies the true arrival of summer – the swift.