Which fish species migrates from European rivers to the Sargasso Sea to spawn?

Where eels came from was an open question for well over two thousand years. Aristotle concluded they formed spontaneously out of mud, which was a reasonable inference given that nobody could find an eel with eggs or with testes. A young medical student named Sigmund Freud spent months in Trieste in 1876 dissecting some four hundred of them looking for the gonads, and found nothing; he left the field shortly afterwards.

The answer was assembled by the Danish biologist Johannes Schmidt, who spent nearly twenty years trawling the Atlantic for eel larvae and following the ever-smaller ones like a trail of breadcrumbs. They led to the Sargasso Sea, a becalmed patch of the western Atlantic thousands of kilometres from any European river. The larvae are transparent leaf-shaped things that drift on the Gulf Stream for a year or more, arrive as glass eels, and turn yellow as they move into fresh water, where they may stay for twenty years.

When the moment comes they change again - they stop eating, their eyes enlarge for deep water, their bodies silver - and swim five or six thousand kilometres back. No one has ever witnessed European eels spawning. Satellite tags only tracked adults as far as the Sargasso for the first time in 2022. The species is now critically endangered: the number of glass eels arriving in Europe has fallen by well over ninety per cent since the 1980s.