Another Life: Final proof of Irish eels’ long Sargasso swim

Autumn gales spark the nightly mass movement of migrating Irish eels, their skins already silvered for the ocean

Just a century ago a research paper by Danish biologist Johannes Schmidt was read at a meeting of the Royal Society in London. He reported netting leaf-shaped larvae from the Sargasso Sea that seemed to be the young of the European eel, Anguilla anguilla. This distant, seaweed-strewn gyre of the western Atlantic, he theorised, must be the eels’ breeding ground.

Schmidt’s key to an ancient mystery of ocean migration earned him the Darwin Medal. But scientists still had to track the eels’ amazing journey across thousands of kilometres of ocean. It took until this October to announce the final evidence.

Ireland was one of four countries in an EU-funded project that, beginning in 2006, released more than 700 silver eels, fitted with pop-up satellite tags, to estuaries from Sweden to Spain. Among them were 44 Irish eels from the Shannon and other western estuaries.

Theories of European eel migration had pictured the fish swimming at high speed for six months, on a fairly straight line across the Atlantic, then meeting up in the Sargasso for a single, springtime orgy. Instead, as the project’s scientists reported in 2016, most headed half way, to the Azores, then paused.


They spent their days in deeper, safer water, surfacing at night, and migrated on a much slower schedule. This was despite findings that eels swim with remarkably high efficiency, four to six times that of other fish. One Irish eel, indeed, took 10 months to swim more than 6,900km, having doubled back to the Azores after swimming south.

The study concluded that the Sargasso spawning season must begin in December and that a lot of eels don’t make the earliest round of mating.

With another 1,000km to go, a set of adult eels were tagged at the Azores and the final tracking made to the Sargasso. It was confirmed on BBC News on October 16th by Ros Wright of the UK’s Environment Agency, who led the study.

Autumn gales, just begun, spark the nightly mass movement of migrating Irish eels, their skins already silvered for the ocean. An approaching storm sends vibrations – “microseisms” – into the land that may act as a trigger. But movement can begin far inland, in lakes and streams, and even send silver eels wriggling through rain-soaked grass to reach the next stream towards the sea.

As thousands are swept on in swollen rivers, their exit to the sea is blocked by dams. Almost half the inland “wetted habitat” available to Ireland’s eels is above major barriers – six big hydropower installations on the Shannon, Erne, Liffey, Lee, Clady/Crolly and Ballysadare rivers.

The mincing of migrating eels in power station turbines is a problem across Europe, prompting a variety of mitigating measures. At Ardnacrusha on the lower Shannon, the ESB relies on a costly “trap and transport” scheme to move up to 30 per cent of the eels around the dam. Most are large females taking millions of ova to their mating rendezvous in the Sargasso.

The toll of eels at Ardnacrusha has brought the ESB into years of dispute with Dr Will O’Connor, an eel expert whose European Eel Foundation is part of Ecofact, an Irish ecological consultancy.

Their latest confrontation arose in the wake of Storm Barra in December 2021. As it approached Ireland, a red weather warning was in place for the lower Shannon. The high river flows on the night of December 7th, plus the imminence of the storm, created ideal conditions for the exodus of eels.

Next morning, Dr O’Connor relates on his website, “large numbers of dead and dying eels were found all along the river downstream of the hydroelectric station”. The eels were killed, he charged, because ESB was abstracting 95 per cent of the flow with no fish screens in place to stop eels being swept into the turbines. He posted videos and photos of the kill on social media.

His subsequent, fraught dealings with ESB and Inland Fisheries Ireland (IFI), and the conflicts of their relative accounts, may be inspected on the parties’ websites. Among Dr O’Connor’s urgings is that turbines should be stilled on nights of such evident mass migration .

Research by the IFI estimates that Ireland’s eels are now down to a critically endangered 8 per cent of those of the 1970s. The Marine Institute in 2016 estimated some 8 million yellow eels and more than 200,000 silver eels in the country.

Among my early ventures in self-sufficient living was the use of a fyke net to catch eels for home-smoking. A fyke has two tapering funnels of net joined by a net in between, and it was staked overnight in lakes behind the shore.

How we get on and coped with our slippery catch (smoked eel is delicious) is related in the handsome new edition of my book A Year’s Turning, published for Christmas by New Island Books.