Eels, already renowned as among the planet’s most extraordinary creatures, have once more confounded experts by proving they can escape the Mediterranean Sea for the Atlantic Ocean.
It had been thought they were navigationally ill-equipped to be able to find the narrow Straits of Gibraltar, and even if they did the currents would be too extreme for them to pass through.
However, experiments in which silver eel, the mature form of the fish, were tagged and released showed that they are clearly well up to the task of swimming from shallow lagoon areas of the Mediterranean and out through the Straits.
“One of the really fascinating things about eels is the mystery that surrounds them,” said Dr David Righton, Principal Cefas Scientist and one of the research team. “They are able to do things that are quite incredible. An amazing species.”
European eels have fascinated mankind for centuries, with the first study of them published, by Aristotle, as long ago as the fourth century BC.
Despite attracting such longlasting attention, their secrets are having to be dragged from them one by one. It was only a century ago that their spawning grounds were discovered by Danish scientist Johannes Schmidt and still no one has seen them breed.
Until now, researchers were unable to say if the fish which grew to maturity in the Mediterranean were able to get out again to form part of the spawning stock.
Of eight tagged female European eels, two managed to swim more than 2,000km from the Salses-Leucate and Gruissan regions of southern France and up to 600km into the Atlantic before the tags detached themselves from the fish, as programed, and floated to the surface.
The two eels known to have reached the Atlantic were still several thousand kilometres short of reaching the Sargasso Sea, where eel are believed to breed, when they lost the tags but having reached the Ocean they demonstrated that the Mediterranean stocks are likely to be among those that spawn.
Five of the other eels appear to have been eaten by predators while the sixth was still in the Mediterranean when the tag detached, but all of them had swum westwards from the release point towards the Atlantic.
“Many scientists have questioned whether eels could escape the Mediterranean because finding the Straits of Gibraltar is like finding a needle in a haystack,” said Dr Righton. “All of the eels migrated west after release, and two of them made it more than 2000km from release and into the Atlantic.
“The fact we have tracked them out of the Mediterranean is significant. It proves that they aren’t trapped.”
European eel are classified by the International Union for the Conservation of Nature as critically endangered, following a slump of at least 95 per cent since 1979 of glass eel recruitment, the juvenile stage in which they reach Europe’s shores after being washed by currents eastwards across the Atlantic – though no one knows how long the journey takes.
The team that carried out the tagging experiment, published in the journal Nature, said that the better eel biology is understood, the better scientists will be able to help the fish recover across Europe as part of the European Union’s Eel Recovery Plan.
Despite being a well-known fish and a traditional dish in many parts of Europe – in the UK eel pie and mash shops proliferated, the Belgians love it in green sauce, and in Spain glass eels are flash fried and brought sizzling to the table – it remains a mysterious creature.
Data collected by the tags showed that the Mediterranean eel behaved in similar ways to those tagged further north. In particular, they swam at huge depths – reaching more than 1,000m down – during the day and came up to about 300m during the day.
What drives them to switch depths is another mystery about the eel, and one still to be solved. Temperature regulation, and predator avoidance are among the possibilities.
Similarly, the route silver eels take to reach their spawning grounds in the Sargasso Sea is, as yet, unknown.
“There’s a lot of speculation about what eels can and can’t do, and how long and how far they have to travel to get to their spawning grounds,” said Dr Righton. “No one has ever caught an eel in the ocean. The adult eels that we recognise are in effect invisible.”
The silver eels that the tags showed had escaped the Mediterranean provided data that further complicates theories about how long and what route they take to reach the Sargasso. According to the time they left and the speed they were travelling at, they would have missed the spawning season. This suggests they might take longer, and perhaps a more circuitous route than previously supposed.
“There’s more to this story to come,” added Dr Righton. “They are international creatures of mystery!”