From the Sargasso Sea to Lough Furnace, our eels face an increasingly perilous pilgrimage

A narrow neck of land separates Lough Feeagh from Lough Furnace in Burrishoole, just north of Newport, Co Mayo. But two rivers along this isthmus, known as the Mill Race and the Salmon Leap, ensure that the water flows freely between them.

This watery route links the salty vastness of the Atlantic to the cool freshwater of Feeagh via the brackish Furnace, and it’s used by some of our most critically endangered fish, such as the sea trout, the Atlantic salmon and the European eel, who migrate to take advantage of different habitats.

Luckily for scientists, whichever direction they are travelling in – from freshwater to the sea or vice versa – they must travel through one of the two rivers. And so, 68 years ago, when a research station was established in Burrishoole, a decision was taken to build traps at the Mill Race and the Salmon Leap to monitor the fish moving in and out of the system. In these traps, fish are temporarily held, checked and released.

One cold morning in 2011, while visiting marine biologist Dr Russell Poole, who headed up the research centre, I went into the Salmon Leap to see what might be there. Swimming alone in the water was an eel; it looked like a piece of thick, dark rope, with mucous-covered brown skin, tinged with green, and a creamy white ridge along its back. It glided around the water in a figure-eight movement, a powerful display of strength and agility.

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This was a female eel, about 20 years old, and she’d spent most of her life feeding and growing in Lough Feeagh. That morning, spurred by an overwhelming genetic instinct to reproduce, she was leaving Mayo to swim thousands of miles away to the Sargasso Sea, where she’d face her inevitable fate: to create new life, then die.

If a scientist catches an eel at sea, is it possible to know whether it has already been in freshwater or not? There are hints

It wasn’t my first encounter with an eel. Another was an unexpectedly intimate affair late one autumnal evening as I stood at the side of a muddy ditch along a rural road in the west. Wearing flip-flops, I stood in on my tippy-toes as I reached across a hedgerow to pluck a few flowers when I felt something slimy on my left foot. I looked down to see a young eel, in no hurry whatever, slithering around my toes.

[ Seeing fish trying to jump a weir isn’t impressive – it’s tragic ]

I’ve always understood that the basic life cycle of the European eel goes something like this: They are born in the Sargasso (an egg-shaped sea off the eastern coast of the US, flanked not by land but by four ocean currents that create a whirling gyre). The larvae move with the currents across the Atlantic, where they transform into rounded juvenile “glass eels” off the Irish coast in autumn.

They enter estuaries in November, and once the temperature reaches 10 degrees they swim into freshwater rivers and lakes as “elvers”. They then grow into adult “yellow” eels where they can spend decades feeding and growing, before leaving as “silver” eels to reproduce and die in the Sargasso.

Research suggests that their lives may be more complex than this, however. Some eels don’t bother entering freshwater systems, preferring to be “marine residents” instead. They might spend a decade in the ocean before entering freshwater, while others, known as “habitat shifters”, will move several times between marine and freshwater habitats.

Why wouldn’t they enter freshwater? It takes a lot of energy for an eel to move from salt into freshwater, and if they’re not in great condition or haven’t had enough to eat, it makes more sense to conserve energy by remaining in the sea.

[ Michael Viney: Final proof of Irish eels’ long Sargasso swim ]

Understandably, scientists still don’t fully understand what eels are up to in the Atlantic. Sampling them at sea is challenging; it was only two years ago that a team of British and Danish scientists successfully tracked them to the Sargasso for the first time. (Until then, they had only been able to track them as far as the Azores.)

If a scientist catches an eel at sea, is it possible to know whether it has already been in freshwater or not? There are hints. Some parasites carried by eels are only found in freshwater, so their presence indicates the eel has spent time in lakes and rivers. Scientists can also take non-lethal samples of their muscles and look at their fatty acid profiles, which differ depending on whether they’ve been feeding in freshwater or marine habitats.

Why does it all matter? The number of European eels that return from the Sargasso has declined by 95 per cent in the past four decades. This is an astonishing collapse. But the number of young eels returning to northern countries, such as Norway, has declined by a factor of 10 compared with southern parts of its range, and much of this data comes from freshwater studies.

As temperatures increase, eels grow faster and mature earlier

Could a proportion of these juveniles prefer marine life? If so, how many? And how might this shape future plans to restore their numbers?

None of this takes away from the dramatic decline, and we must continue do everything possible to boost their chances in our estuaries, rivers and lakes. Barriers that impede their migration must be swiftly dealt with – most pressingly, the eel death trap that is Arnacrusha power station on the Shannon, where eels are crushed to death in the turbines. We also need urgent, effective action to reverse increasing levels of water pollution.

[ ‘More than a quarter’ of species in Ireland at risk of extinction ]

Added to these pressures is the ecological turmoil from climate change, which may already be changing eel migration patterns in Burrishoole. As temperatures increase, eels grow faster and mature earlier. A recent analysis by Russell Poole and his colleagues reveals that the start of the seaward migration of eels has advanced by one month since 1970. Scientists have yet to understand the consequences of this for their survival rates at sea.

On that 2011 morning in the Salmon Leap, it was a privilege to be present as the female eel embarked on the marathon journey to reproduce in the Sargasso. Since Aristotle, the lives of eels at sea have perplexed generations. Closing that knowledge gap may help us understand how we can better restore their numbers in the future.

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