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

EDP Norfolk - - INSIDE - Dr Ben Ald­iss wildlife jour­nal­ist and broad­caster, deputy man­ag­ing edi­tor of World Agri­cul­ture

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

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