The incredible migration of the European eel is a feat that has long fascinated scientists. European eels (Anguilla anguilla) are catadromous, which means that they spend their adult lives in freshwater or estuaries before migrating to the ocean to spawn. And not just any sea will do for laying their eggs – the eels will travel for thousands of kilometres from Europe to the Sargasso Sea in the North Atlantic Ocean.
While the role of the Sargasso Sea as an important spawning site has been known for some time, there has been much debate as to whether eels from the Mediterranean were capable of joining eels from the rest of Europe to journey across the Atlantic. To escape the Mediterranean Sea these eels must pass through the Strait of Gibraltar, the name given to the strait separating the Atlantic Ocean from the Mediterranean. The strait is just 14.3 kilometres wide at its narrowest point, marginally dividing the continents of Europe and Africa. Many doubted whether eels would even be able to find the strait, let alone navigate through its strong currents to reach the other side.
But now, new research published in Scientific Reports claims to have recorded evidence of Mediterranean eels achieving what was thought impossible. The study involved satellite tagging eight female silver eels as they escaped through lagoons into coastal waters off the south of France. Five of the eels were ultimately eaten by predators in the Mediterranean, but, after six months, two had survived their epic journey and arrived in 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 David Righton, who contributed to the study. “This is a great achievement for the team, and a significant scientific standing.”
The study also went some way to helping us better understand the swimming patterns of the eels. Those within the study were recorded following a movement pattern that alternates between swimming in deeper waters (up to 800m) during the day and ascending to shallower waters (around 350m) at night, a behaviour otherwise known as ‘diel vertical migration’ (DVM).
DVM is believed to have been adopted by eels for a number of different reasons, including predator avoidance, temperature regulation, navigation, or to reduce pressure on the gonads to allow them to develop. Scientists are still unsure as to exactly what drives this behaviour, though the study suggests that the role that thermoregulation plays in DVM may be more minimal than previously thought. Once they reached the Straits of Gibraltar, the Mediterranean eels briefly abandoned DVM, and instead descended to the seabed in a likely attempt to avoid strong currents. With the strait safely navigated, they returned to their previous swimming behaviour.
Eels are difficult to study, particularly in deep ocean waters, which explains our current lack of knowledge on some aspects of their biology. This new research is important for understanding the relevance of eels in the Mediterranean for contributing to the future of European eel stocks. The species is categorised as Critically Endangered by the International Union for the Conservation of Nature, and consequently appears under Appendix II of the Convention on International Trade in Endangered Species (CITES).
Elsa Amilhat, Kim Aarestrup, Elisabeth Faliex, Gaël Simon, Håkan Westerberg, David Righton. First evidence of European eels exiting the Mediterranean Sea during their spawning migration. Scientific Reports, 2016; 6: 21817 DOI: 10.1038/srep21817
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