Magnetic orientation & navigation system of European eels

Translated from : Bulletin Electronique

A dark and rainy, eels leave fresh water to begin a long and last trip of 6000 km to its breeding ground, the Sargasso Sea. How are the eels they can find their way into the Atlantic Ocean? Researchers may have found the answer: the eels would use the Earth’s magnetic field.

Place of reproduction

European and American eels breed both in the deep water and salt. The spawning area and the zone of growth have not always been so remote, but the drift of continents has forced the eels to dramatically increase their travel time (5-6 months) to reach the Sargasso Sea. To date, no adult eel has been caught or even seen in the Sargasso Sea.

This area is extremely broad and deep, making it expensive and difficult expeditions. Several attempts at tagging were held but no individuals have been followed long enough to determine the exact location of reproduction. However, these markings have shown impressive swimming depths of up to 600-700 meters, “says Caroline Durif, a researcher at the Institute of Marine Research in Norway Austevoll (Havforskningsinstituttet).

Mythical

Today, the eel has kept all its mysteries. For 2,000 years, scientists are asking questions about its reproduction. Heeding neither larvae nor eggs, Aristotle thought that the eel had no gender (neither male nor female), it was an example of spontaneous generation, caused by the putrefaction of algae. In the late 1800s, the first larva of eel has been identified in the Mediterranean. From other eel larvae (leptocephali) were harvested in Atlantic. It was not until the early 20th century a Danish oceanographer (Johannes Schmidt) found, after years of sampling, that smaller (younger) larvae were located in the Sargasso Sea and that s’ therefore acting in the place of reproduction. But how eels are they then able to find these clear waters? Starting from the Norwegian coast, this is a journey of about 6000 km.

Using Earth’s magnetic field

Animals have many ways of moving in space. Some sail through the stars, others through their nose, or by following the ocean currents. But for a fish swimming at great depths, the earth’s magnetic field provides a reliable and this day and night.

The eels, which come from localities far apart from each other (northern Norway, Morocco, not from Mediterranean countries) require a navigation system for effective and accurate return to the same place and same time for reproduction . The perceived intensity of Earth’s magnetic field is theoretically possible to sail to the Sargasso Sea. It is likely that eel integrates a sort of map for the journey, she will use to return, “said Caroline Durif, is the assumption that it has tested with his colleagues at the Institute of Marine Research .

From North to South

This lab is almost unique in the world (this study and the experimental setup have been funded by the Council for Scientific Research Norwegian: Norges forskningsråd), magnetic north is changed each night to test the effect of orientation of the eel. Eels are isolated in a basin in the dark during their migration period in autumn. The coil giant to change the direction and intensity of the magnetic field, then by an infrared camera to observe the behavior of the eel. Caroline Durif was initially surprised when eels oriented themselves preferentially towards the east and north, which was simply not the direction of the Sargasso Sea (southwest). However, these directions correspond perfectly to those that have been taken out of the fjord or the eels were caught. It seems that the young eels learn the way to go back to the opposite when they leave the rivers and inland waters, “she says.

It is a discovery that has important implications for the management of this endangered species. The current management measures advocated moving the young eels which are located in areas at risk to less polluted areas, where fishing is less important and dams absent. But if these individuals are suddenly moved to unknown areas they would not find no way back and are therefore unable to reproduce.

A GPS

The study showed that the eel was sailing through the magnetic field. This intensity varies with latitude and allows the eel and other animals to determine their position on the earth. It’s a bit like a GPS, “says Caroline Durif. For now, researchers do not know the precise physiological mechanisms behind this system or any component of the magnetic field is perceived by the eels.

Eels are not sexually mature when they leave the inland waters. The factors that trigger this maturation are not known and is one reason why the European eel farming is far from being controlled. Experiments have shown that the magnetic field can affect the physiology of migratory species. Durif intends to examine the effect of the magnetic field present in the Sargasso Sea on the onset of sexual maturation.

European eel

• catadromous (breeds in salt water, grows in fresh or salt water)
• the European eel is highly endangered: recruitment is currently between 1 and 5% of pre-1980 levels
• on the red list of threatened fish species
• the sharp decline is due to overfishing, pollution, disease, construction of dams in the context of hydropower development and climate

Links :

• Caroline Durif, PhD Thesis (in French) : the downstream migration of the European eel
  
• Magnetoreception and magnetic compass orientation in European eel
• BBC News : Eel reveals its migration secrets
• ICES : Eel stocks dangerously close to collapse
• Eels french cooking video with Maïte