A-Z: We’ve reached E for Eel – and what amazing creatures they are!
This month Dr Ben Aldiss reveals the astonishing secret life of the mysterious eel
Be prepared to be amazed! We’re on letter E this month in my alphabetic look at the nature of Norfolk, and we’re going to look at the incredible life of a mysterious fish.
We have a large and rather beautiful pond in our garden. Amongst the dragonflies and other aquatic creatures it contains, eels can sometimes be seen lurking in the water weeds, despite the closest river being over a mile away. So how did they get there? Unlike other fish, they have an astonishing trick: they can slither, snakelike, over land to reach isolated pools and ditches far away from the nearest stream.
In this way, they have repeatedly recolonised our pond. If that capability surprises you, just wait – the story of eels and their enigmatic habits gets ever more incredible.
Eels spend much of their lives as hermaphrodites – having both male and female sex organs – only developing into males or females as they reach maturity. Whichever they finally become, they tend to stick together in groups of that sex. I’ll come back to a possible explanation later.
The age varies, but usually when they’re between ten and 15 years old, eels suddenly develop the urge to find the sea. One, in the Jardin des Plantes in Paris, was fed on beef for 37 years before it decided to head towards the coast.
Having fattened up on their diet of small birds and fish, insects and crustaceans, they stop eating, change colour from a
golden green to black on top and silver below and their eyes grow larger and protuberant. Then, on a stormy night somewhere between late summer and midwinter, they slide purposefully out of their pond.
We have no idea exactly what stimulates them to do this, but it only happens on dark, wild nights when the moon is in its last quarter and the wind is blowing towards the sea. If all these conditions are met, huge numbers of adult eels migrate simultaneously from their various ponds, ditches, streams and estuaries at the dead of night.
Shutting their gills to avoid drying out, they glide like snakes through the rain-sodden grass until they reach a stream. Making use of the current to save energy, they drift downstream on the start of the second and last major journey of their lives.
It will take them around eight months to swim to their final destination – the Sargasso Sea, south of Bermuda in the Atlantic Ocean. Recent research has shown that they don’t go by the direct route either – they travel first to the Azores off West Africa, then hitch a lift on favourable ocean currents westwards.
Nobody knows how they navigate on this journey of over 4,000 miles, but we have discovered very recently by radiotagging, that the eels swim in the warmer surface regions of the sea by night, then dive to a depth of 1,000 metres by day.
As they eat nothing for the entire duration of their swim, eels have never been caught by rod and line on this epic migration and very few have even been netted as discards by commercial fishing boats.
If eels are a mystery, so too is their breeding ground, the Sargasso Sea. An area 1,000km by 3,000km, it’s renowned for its crystal blue clarity, its huge chains of floating Sargasso Weed and, of course, its eels.
Nobody has seen full-grown eels in this vast area of ocean, despite the millions that come here to spawn every spring.
It’s assumed that they breed en masse at depths of around 200 metres, each female laying between 20 million and 200 million eggs, but none of these eggs have ever been found either.
What we do know is that the eggs hatch into tiny, leaf-shaped, transparent creatures looking nothing like eels. These are called leptocephali – literally meaning ‘thin-heads’ – and it’s in this form that they drift slowly from the Sargasso Sea towards Europe, a journey that takes them around three years.
Meanwhile it’s assumed that after breeding, the adults die in the Sargasso Sea – their birthplace – but no corpses have ever been found as proof of this.
Next, another in the chain of mysteries occurs: each tiny leptocephalus chooses where to leave the sea and which river to swim up. As it approaches the coast of Britain, it changes form again, becoming thin and transparent and only 5cm long.
It’s now known as a glass eel and in this form enters its chosen river estuary – in the case of the eels living in our pond, probably the tiny River Burn. Remarkably, its metabolism then alters to enable it to survive in fresh water, rather than salt, and as it heads upstream it darkens and becomes recognisably a tiny eel.
The elver, as it is now called, has to swim against the current and at times meets obstacles that would seem impossible to surmount. Once again, though, the eel’s remarkable ability to survive out of water, comes into play: if you can’t swim up a waterfall, climb out and thread your way up the surrounding rocks and vegetation.
We’re nearly at the end of this amazing tale. It seems that eels destined to become females are larger and stronger, so tend to swim the furthest and end up in places like our pond, whereas the ones that eventually turn into males congregate in streams or estuaries, but there are lots of exceptions to this rule.
Whatever the case, elvers that have survived the epic journey from Bermuda eventually stop at their favoured place and stay there for many years until they, like their parents, get the urge to swim back to their birthplace one dark, stormy night.
‘It’s assumed that eels breed en masse at depths of around 200 metres, each female laying between 20 million and 200 million eggs, but none of these have ever been found’
Dr Ben Aldiss is a wildlife journalist and broadcaster, deputy managing editor of World Agriculture and an adviser in biodiversity and education on farms. waspsandwildlife.co.uk
ABOVE:An eel caught near Surlingham