Peek over the parapet of a bridge just as daylight bleeds into dusk, gazing down through the clear layers of a shallow river or chalk stream, and you may glimpse the shadowy, serpentine form of an eel as it stirs for the night.
Darkness is when eels search for food, scavenging or hunting by way of an acute sense of smell. They nudge stones and nose into crevices, seeking invertebrates or small fish.
Theirs is an unfussy, carnivorous diet, with individual preference sometimes dictating the shape of their mouth and head. Each year, however, fewer and fewer eels worm between the watery weed-beds.
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Once common, the European eel (Anguilla anguilla) is in huge decline. Rated as critically endangered by the International Union for Conservation of Nature (IUCN), their estimated population has dropped by as much as 95%, and no one can be sure why.
Despite its appearance, the European eel is a fish, one of around 1,000 eel species worldwide. The dorsal, anal and caudal fins combine to form a singular, ribbon-like fin along much of its back and belly. This can be inconspicuous as the eel slithers through stone and weed, which, alongside a fixed stare and slippery form, adds to the impression of an aquatic snake. Their mysterious existence has long caused confusion, although their edibility has never been in dispute.
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East End delicacy
In his 1653 book The Compleat Angler, Izaak Walton recommended the eel be stuffed with herbs, anchovy and nutmeg before roasting on a spit. He also wrote of the Romans’ fondness for the eel, noting that they “esteemed her the Helena of their feasts”, although he gives heed that physicians deem eel to be “dangerous meat” – reference, no doubt, to the toxicity of their blood.
Such was the eel’s former abundance that numbers could be caught with relatively crude methods, and they became a staple in the diet of Londoners, particularly in the East End where they were jellied or baked in pies.
In the 17th and 18th centuries, eels would be caught from the Thames by eel-bobbers, who would tie worms onto a length of wool which was then ‘bobbed’ from a moored boat. The lack of a hook was not a problem because the wool would tangle in the eels’ small teeth, enabling the bobber to lift them out and deposit them into a tub.
Elsewhere, wicker traps would be weighted, baited with worms or offal and lowered into a watercourse on a stout cord. The design allowed eels to enter via a small orifice in an inverted tunnel which they would struggle to locate in order to escape.
Other fishing methods included the use of an eel-fork, which had narrow gaps between barbed tines and, most commonly, with nets. These could be set at the lowest ebb on tidal water or positioned behind eel-weirs, which were constructed with interwoven willow branches that would channel the eels through specific gaps.
The mystery of how eels breed
Eels are catadromous, meaning that they live in freshwater but migrate to the sea to breed. Although many spend their entire adult lives in freshwater – some remain in estuaries or move in and out of tidal reaches. While living in freshwater, they have dark, olive-brown backs that fade to a pale yellow-brown on the belly.
The male eels lack reproductive organs, something that has long perplexed science as to their manner of reproduction. In the 4th century BCE, the Greek polymath Aristotle believed eels to be born from the “entrails of the earth”, suggestive of spontaneous generation.
In British folklore, it was believed that eels would grow from the hairs of horses, dropped into the water while the animal was taking a drink. In 1876, as part of his medical studies at the University of Vienna, Sigmund Freud dissected around 400 eels in a futile search for male testes, subsequently turning his attention to the human mind. Perhaps aquatic biology’s loss was neurology’s gain.
Unknown to Freud, and to all of those who had previously puzzled, was that he was searching for something that wasn’t there. An eel does not evolve into sexual maturity until it has left freshwater and begun a one-way migration across the Atlantic to breed and then die.
The drivers behind this change are uncertain, with age not seemingly a factor, but eels change in appearance, with their butter-brown bellies becoming silver as they move back towards salt water. The change is prompted by a shift in biological make-up as the eel builds levels of cortisol, the hormone required to survive salinity.
These ‘silver eels’ had long been recognised as fish heading to the sea to breed, while ‘elvers’, the small 7–10cm-long juveniles that would mass into rivermouths on spring nights beneath a new moon, were known to be returning young.
The process between remained a saltwater puzzle until the species Leptocephalus brevirostris, a small semi-transparent oceanic fish, was observed into maturity.
In the late 19th century, studies of specimens taken from the Straits of Messina, by Yves Delage in France, and Giovan Grassi and Salvatore Calandruccio in Italy, revealed that the leaf-shaped leptocephali would metamorphosise into smaller, slimmer, ‘glass’ eels. Grassi concluded that the breeding place of the European eel was within the ‘abyssal region’ of the sea, with the vicious currents of Messina forcing the larvae to the surface where they might be observed.
Other scientists, however, believed the solution lay further afield. In 1904, a Danish vessel netted a specimen of Leptocephalus brevirostris to the west of the Faroe Isles. Aboard the research boat was Ernst Johannes Schmidt, who subsequently led a series of expeditions as part of the Dana Investigation.
Schmidt found that the further west they were caught, the smaller the leptocephali were, and he suspected that the larvae originated in the Sargasso Sea, a gyre bounded by four currents in the western Atlantic.
In April 1922, Schmidt travelled from Madeira, via the Canary Islands, to the Sargasso. There he caught thousands of tiny eel larvae, both of the European eel and its close cousin the American eel (Anguilla rostrata). The following year he published his findings, and though he had been unable to record or witness fertilisation, he concluded that the silver eels, having left European waters in the autumn, would travel across the Atlantic to arrive in the Sargasso Sea the following spring.
Almost a century passed before Schmidt’s theory was confirmed, when migrating eels were tagged and satellite tracked. Their journey time varied, but once there the eels would mate and perish, with the fertilised eggs left to the apparent whim of the ocean.
Gulf Stream changes
The sea plays an integral role in the eel’s life cycle, offering the eggs warmth and the developing larvae food in the form of marine snow, a blanket of organic matter that drifts through the water columns.
Meanwhile, the Gulf Stream, the warm current of water that nudges from the Gulf of Mexico to north-west Europe, gently carries the larvae back to the waters where their parents previously swam. After a drift across several thousand miles that may last two years, the larvae develop into glass eels, adept at propelling their own course into estuaries where they then darken into elvers.
Some scientists believe that this extraordinary cycle may have begun shortly after the breakup of the singular supercontinent of Pangaea. With the land masses closer together, the migration would have been shorter and less treacherous.
More recently, as the climate changes, the reliance upon the Gulf Stream will likely impact the number of juvenile eels reaching Europe. Studies suggest the current has weakened, shifting north by approximately five kilometres per decade. This will hinder dispersal of eel larvae and impact the adult eels trying to return to the Sargasso Sea.
Eels have magnetoreception, enabling them to navigate by using the Earth’s magnetic field, in a similar manner to homing pigeons. The forces within this field are not constant, and the flux may alter the eel’s sense of direction, particularly if a long period has passed since the internal maps were formed. And eels have extraordinary longevity.
A Norwegian study found the age of silver eels migrating back to sea can vary between eight and 35 years, while a female eel in a Dutch aquarium was observed to spontaneously mature at the age of 43 years.
So much will change over such a period, including the details held in a magnetic map. Impacts that are decades old may only now be being felt.
Saving the common eel
We know overfishing and bycatch discard are problematic. The formation of bodies such as the Sustainable Eel Group and DUPAN Foundation – collaborations of conservationists, scientists and fish farmers – are hopeful steps.
Many eel netters are turning their skills to conservation, capturing elvers to redistribute above weirs or other barriers. Farming might offer a sustainable source for food production, rearing larvae to maturity or using biotechnology to stimulate sexual maturity in adults. And perhaps we should all consider the ethics of eating eels; although, as they become rarer, so their value grows.
Eels are predated at all stages of their life, by larger fish, aquatic mammals and birds. This is a natural process, and though predation can impact a species in decline, the factors causing that slump must be addressed for recovery in the long term.
More significant was the arrival in the 1980s of Anguillicoloides crassus, a nematode worm native to Asia that parasites the swim bladder of the eel. Its presence may stifle the ability of mature eels to migrate, yet the decline of the American eel, which shares the spawning grounds in the Sargasso, suggests the drop in recruitment is more complex.
Similarly, obstacles to movement, particularly hydroelectric dams, will play a part, but issues in the open ocean are difficult to identify and we have little idea as to the mortality rate of silver eels or larval eels at sea.
The life cycle of the eel is one of nature’s most extraordinary stories. A journey through half a dozen life stages and across thousands of miles of water.
Their plight might be easily overlooked amid so much ecological uncertainty, but it would be a tragedy to lose eels from our seas and rivers. Should you peek over the parapet of a bridge and spot an eel snaking through the water below, wish it well on its odyssey.
