6. Oct, 2016

The long and dangerous road to the Sargasso Sea

The European  eel (Anguilla Anguilla) has always fascinated and puzzled  biologists.  Unlike many other migrating fish, eels begin their life cycle in the ocean and spend most of their lives in fresh water, returning to the ocean to spawn and then die. The eel  life cycle begins as larvae (leptocephali)  born on the continental shelf of  the Sargasso sea not so far from the Bermudas.  The Sargasso sea is known for its still waters and dense seaweed accumulations. They then are carried with the gulfstream towards Europe in a 300-day migration. When approaching the European coast, the larvae metamorphose into a transparent larval stage called glass eel, enter estuaries, and start migrating upstream (see upper picture). Glass eels have also been seen gliding through wet meadows to reach water.

Growing up on land  After entering fresh water, the glass eels change  into elvers, miniature versions of the adult eels. As the eel grows, it becomes known as a yellow eel due to the brownish-yellow color of their sides and belly. After a period of around 1o-20  years spent in fresh water, the eels become sexually mature, their eyes grow larger, their flanks become silver, and their bellies white in color. In this stage, the adult eels are known as silver eels and they begin their migration back to the Sargasso Sea to spawn.

 Back to sea: mapping   the spawning migration route The return of the eels after  a lifetime of  around 20 years spent in European lakes and rivers  is the most dramatic and mysterious part of  their migrations,  due to the length and duration of their trip in the immense Ocean.Their natural departure dates  from Europe  are  between the months  October to December.  An international  team of investigators  was recently  able to describe in detail the first stage  in the migratory route using modern techniques.*  In their 5 years study they  used tags that were attached to 700  female eels, that popped of the eels after some time.   The tags then  drifted with the currents along the surface where they were collected  by  attentive beach visitors,  who  posted them to the investigators.  Some interesting  results:

-Eels  migrated  at  a depth of 200 meters crossing  a  distance of between 5000 and 10,000 km (depending on their departure locations) to reach their final destination.

- Overall, the migratory trajectories of eels released from different   European countries  converged on the Azores region (see lower picture), at approximately half the distance to the spawning area in the Sargasso Sea.  Thereby following the reversed currents of the  northern part of the subtropical Gyre  in the North Atlantic.  

- Eels exhibited  regular vertical migrations, moving from deeper water during the day into shallower water at night. The range of migration speeds was 3 to 47 km per day

- the period  estimated  to achieve their spawning migration varied between 80 to 170 days .

- The eels adopt a mixed migratory strategy, with some individuals able to achieve a rapid migration, whereas others arrive only in time for the following spawning season.

 Implications  For quite some time, the population number of European eels has been falling.  Contributing factors include overfishing, pollution,  parasites such as Anguillicola crassus, barriers to migration such as hydroelectric dams (in Spain), construction of dikes and ‘polders’ (in Holland)  and natural oscillations in the North Atlantic Gulf Stream, and North Atlantic drift.  The  superfical layers of the Sargossa sea  used to suffer from lead contamination from American  industries, which decreased in the last 20 years. The investigators feel that their study could have practical consequences for eel stock management in the future, now environmental conditions  have listed  the European eel  as a critically endangered species

Sources and links

Jacoby, D. & Gollock, M. (2014). "Anguilla anguilla". IUCN Red List of Threatened Species. Version 2014.1. International Union for Conservation of Nature. Retrieved 30 June 2014.

*Righton et al. (2016). Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea. Science Advances, October, 2.  



Isotopic evidence of pollutant lead transport from North America to the subtropical North Atlantic gyre". Geochimica et Cosmochimica Acta. 1997. Retrieved 7 December2013.