Anja Murray: Epic eel journey from Sargasso Sea to Ireland — and back again

Last summer, I got to assist a team of researchers from the Marine Institute carrying out a fish count in a mountain river in Mayo, while filming with Eco Eye. I was intrigued to see young eels up close, wriggling about in trays of river water while being measured and then returned to the water. 

The young eels we assessed were 15 centimetres long or less, slender and delicate looking. It’s hard to imagine how these little creatures had already completed epic journeys of between 5,000 and 10,000 kilometres to reach the gurgling waters of an Irish river.

People have been catching and eating eels in abundance for millennia. Rivers such as the Bann, the Shannon, the Blackwater, the Lee, the Nore, the Suir, the Barrow, and the Boyne have all been of considerable importance for eels and eel fisheries for centuries. 

1.Friday’s 🧵is all about Eels. The species we have in 🇮🇪 is Anguilla anguilla, the #EuropeanEel, not to be confused with other Anguilla species (such as the American eel), or indeed the mighty Congor eel (Congor congor). The photo is a bit blurred because....😱 pic.twitter.com/eG8ANL0LjL

— Ireland's Environmentalists (@IrelandsEnviro) October 28, 2022

Lough Neagh has a particularly illustrious history of eel fishing, with centuries-long battles for eel fishing rights being fought. Eels were a staple, accessible, nutritious food enjoyed everywhere from Sweden to Italy for thousands of years. 

Up until the 19th century, ‘excess’ catches of small eels were spread on fields as fertiliser, such was their great quantity. But up until recently, nobody had ever seen anything recognisable as a young eel, nor found any indication of reproductive organs or eggs in adult eels. 

In the dearth of any credible evidence, people came up with their own stories. Some claimed that these animals emerged spontaneously from mud, others that they originated from sea foam, while others insisted that eels started out life as beetles.

In the early 20th century, an intrepid Danish scientist, Johannes Schmidt, was determined to find out where elvers (young eels) came from. Through decades of determined research, gathering, measuring, and logging eels from boats all over the Atlantic Ocean, he concluded that eels originate in a part of the western Atlantic called the Sargasso Sea, near the Bahamas. 

In making this discovery, he resolved one of the most intriguing and enduring mysteries of the time — that of the eel life cycle. But over the hundred years since then, details of how they achieve such an epic migration have remained elusive. 

Only with very recent developments in tracking technology have scientists been able to map the eels’ migration routes and unravel outstanding unknowns about their life cycle. Right now, during February, eel spawning in the Sargasso Sea is at its peak.

Eel eggs hatch in the depths of the Sargasso Sea, where each eel begins life as a minute transparent leaf-like larva, drifting the currents of the open sea. When still out at sea they metamorphose into tiny glass eels, still transparent but now with the ability to navigate to European coastlines. 

Like salmon and sea turtles, eels use magnetic fields to orient during migration. When they reach the coast of European landmasses, they have an incredible ability to synchronise their movements with the tides so that they can navigate through the strong currents of tidal estuaries and find their way into the fresh water of their next habitat — rivers.

As all this is happening, their transparent little bodies slowly adjust to the changing chemistry of the water, before they begin the freshwater leg of their migration. As they enter freshwater rivers, where waters tend to be browner, glass eels change from having silvery–transparent bodies to becoming pigmented brown elvers, blending better into their environment, lessening the chances of being caught and eaten by bigger fish, otters, herons, and other predators. 

These are the kind of eels that can shimmy over grass and mud on a dark, damp night, even nibbling on, earthworms, slugs, and snails if they come across any. This is also how they find their way from streams into ponds. When a brown eel finds a place that suits it well, whether river, lake or stream, they can live there quietly for decades.

Once mature, after 6–20 years, eels are ready to return to the Sargasso Sea to spawn. Their bodies become silver once again for the journey, and they will typically wait for dark, moonless nights when water levels are high to begin swimming downstream to the sea.

Just last year, tracking the return migration revealed that the routes adult eels take across the Atlantic to return to the Sargasso sea are not ‘as the crow flies’ but rather are indirect, suggesting that migrating eels are using olfactory cues carried on ocean currents that were imprinted or learned during their larval phase, when much of the journey followed the curve of the currents around the north Atlantic. 

Eels appear to ‘remember’ the route that they took as minute larvae crossing the Atlantic Ocean decades previously. 

After millions of years of evolution, developing such a wondrous adaptations and complex life cycles, our European eels are now critically endangered. Populations began to plummet in the 1970s and the rate of decline has continued to be dramatic. Barriers to migration, overfishing, and changing ocean currents due to climate change have all been contributing to their demise.

There is some chance for recovery. Huge swathes of eel habitat in Irish rivers are upstream of impassable barriers such as damns, weirs and bridge ‘aprons’, which can be removed so that eels and other migratory fish will once again be able to access these crucial habitats. 

Eels can be very curious and I’ve found many over the years will actually approach me to have a better look at the monkey swimming with them. #eels #underwater #wildlifephotography pic.twitter.com/prtvYLz1xe

— Jack Perks (@JackPerksPhoto) February 6, 2023

Limiting climate change to less than 2°C may limit the extent to which ocean currents change. Ending overfishing will also give these incredible creatures a chance of survival.

Eels are strange and wonderful animals. Their final migration and spawning still holds mysteries we don’t understand. We do, however know enough to urgently implement every possible conservation measure, to give these amazing animals every possible chance of swimming through ocean, river and lake in decades to come.