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Mystery has been solved: the Moon prevents glass eels from dying in the Arctic


During their journey from the Sargasso Sea, the larvae develop into post-larvae. They now resemble a thin, almost transparent ribbon, which is why they are known as “glass eels” during this life stage. 

Photo: Alessandro Cresci/HI

A new study shows how young eels find their way from their birthplace in the Sargasso Sea to Norwegian fjords and river mouths. 

The life of the European eel starts in the Sargasso Sea, a large area of water south of Bermuda.

Their parents spend at least six months travelling from there. They cross the Atlantic Ocean to spawn for the first, and only time, in this precise location. 

But how do the baby eels find their way back to Europe from the Sargasso Sea? 

New research shows that the Moon directs the young eels towards the coast. 

A “Lunar compass” guides eels 

From the Sargasso Sea, European eel larvae are carried across the ocean by the Gulf Stream, and then onwards by the North Atlantic Current, which goes up the Norwegian coast all the way to the Arctic. 

During their journey from the Sargasso Sea, the larvae develop into post-larvae. They now resemble a thin, almost transparent ribbon, which is why they are known as “glass eels” during this life stage.

The question the researchers asked themselves was how glass eels find their way from the North Atlantic Current and into the river mouths, fjords and coastal areas around the North Sea, where they will live and grow until they are ready to swim off to spawn. 

“Glass eels have an internal ‘compass’ that directs them towards the Moon. So when the Moon is above the horizon, the eels swim towards it. That is generally south. This allows them to ‘get off’ the North Atlantic Current”, explains Cresci. 

“And that’s a good move, because if they continued going with the flow, they would end up in the Arctic and die.”

Instead, of course, glass eels reach the North Sea.

Combined experiments with modelling 

In order to understand this, the researchers performed experiments where they looked at the behaviour of glass eels under controlled conditions at the IMR’s research station at Austevoll.

Afterwards, they incorporated their observations into a computer simulation of glass eels and ocean currents. 

“In the model, we ‘released’ glass eels north of Scotland. In one simulation we looked at where the eels would end up if they just drifted passively with the currents. In the other ones, we simulated the Moon coming up over the horizon, and the eels starting to swim towards it. We ran lots of these simulations, giving the eels various different swimming speeds.” 

What did you find out? 

“We found that whatever speed we gave the eels, the big factor was whether or not the Moon was over the horizon. When glass eels swam following their ‘lunar compass’, the proportion of glass eels that reached coastal areas around the North Sea greatly increased. The higher their swimming speed, the greater the number that arrived, and the fewer that were carried onwards to certain death in the Arctic.” 

The North Sea: a key area for the European eel 

The North Sea is an important area for the European eel. From the North Sea, the glass eels spread out across large areas. 

“There are lots of countries around the North Sea basin, and eels are found in many places, particularly in the south, but also in Norwegian fjords and in the Baltic Sea. Understanding how glass eels get to the North Sea is an important step towards understanding eel recruitment”, says Cresci, who is a postdoc. 

“And if we understand eel recruitment, we can become better at protecting and managing them.”  


Alessandro Cresci  Anne D. Sandvik  Pål N. Sævik  Bjørn Ådlandsvik  Maria Josefina Olascoaga  Philippe Miron  Caroline M. F. Durif  Anne Berit Skiftesvik  Howard I. Browman  Frode Vikebø. «The lunar compass of European glass eels (Anguilla anguilla) increases the probability that they recruit to North Sea coasts». Fisheries Oceanography (2020). 

LINK: https://doi.org/10.1111/fog.12521