THE European eel is in trouble. Its numbers have declined catastrophically since 1980 and the species is listed as ‘critically endangered’ by the International Union for Conservation of Nature (IUCN).
The eel undertakes two enormous journeys, one at the beginning of its life, the other towards the end. How it accomplishes these extraordinary navigational feats is a mystery. If conservationists are to help this iconic creature, they need to better understand its ecology and the difficulties it faces.
A European eel is spawned in the Sargasso Sea between Bermuda and Cuba. Four ocean currents meet there, creating a ‘gyre’ of calm water with extensive growths of brown Sargassum weed. The tiny larva, feeding on plankton, drifts northwards with the Gulf Stream for almost a year. Arriving off our coast, it transforms itself into a ‘glass’ eel and enters an estuary. Then, becoming an ‘elver’, it battles upstream, confronting waterfalls, weirs, and dams. In a brown-coloured phase, it takes up residence in streams ponds and irrigation ditches, where it will live quietly for years or decades.
One fateful day, now silvery and sexually mature, the eel says goodbye to the place it called home for up to half-a-century. Heading back downstream, it swims through the estuary and out into the ocean. Next, it embarks on a 5,000km journey back to the Sargasso Sea, where it spawns. Then, its life’s work done, it dies.
Birds and some insects use the Earth’s magnetic field to navigate. So do sharks and rays. In 1974, German researchers showed that eels taken from the Elbe “changed their preferred [swimming] direction from north or south to east] in response to artificially generated magnetic fields. Now, scientists from Miami University and the Norwegian Marine Institute have discovered that tiny eels have “an internal magnetic compass”.
The team captured 222 glass eels at estuaries in Norway and took them to a magnetic laboratory. The eels were placed in a circular black tank, in which the direction of magnetic north could be rotated artificially using electric coils. The rivers from which the eels were taken flowed in differing directions, some northwards, others southwards, or in between. Some 70% of the little eels ‘oriented to the magnetic direction of the prevailing tidal flow that was occurring at their recruitment estuaries during the tests’, the researchers report in a paper just published.
Evidently, the baby eels had become imprinted on the tidal patterns of the estuaries from which they were taken. They remembered the direction and timing of tidal flows, in relation to the Earth’s magnetic field. “This mechanism could help them to maintain their position in an estuary and migrate upstream.”
This sense of direction, acquired as a youngster in the estuary, may be retained in the eel’s memory throughout its long, sedentary life and deployed again during its last great journey.
“It is possible that adult eels recognise the direction of the alternating tidal flows experienced as glass eels when they descend the estuary, and this constitutes a trigger to undertake the last long marine migration,” the authors speculate.
Alessandro Cresci et al: ‘Glass eels imprint the magnetic direction of tidal currents from their juvenile estuaries’, Communications Biology