The oceanic life of eel is still a mystery. New findings from the Danish Galathea 3 Expedition to the Sargasso Sea now shed light on the conditions for larval growth and feeding and point to an alternative route for larval drift towards Europe
As both European and American eel populations are in a drastic decline, there is an urgent need for a better understanding of the early oceanic phase of their life cycle.
Two new findings are now being published from a research project which collected data on the eel’s spawning sites in the Sargasso Sea on the Danish Galathea 3 Expedition in 2007.
European eel larva (Anguilla anguilla) (10 mm). Photo Peter Munk.
Climate change may influence life cycle completion of eel
Scientific results published in Proceedings of the Royal Society show the importance of including the climatic influence on oceanic processes when assessing the conditions for the early life of eel and the background of a declining recruitment to the eel populations.
"Our studies in the Sargasso Sea demonstrate a significant relationship between the physical and biological conditions in the area. A front established where warm tropical waters meet colder North Atlantic water has a key role in the larval life of eel" says Senior Scientist Peter Munk from DTU Aqua.
The front retains eel larvae within a zone of enhanced feeding conditions and influences their drift towards the continent.
European eel larvae are generally believed to initially follow a westerly drift route from the spawning site and subsequently drift with the Gulf Stream. But the study’s demonstration of a strong linkage between larval distributions and the fronts in the area suggests another possibility.
The front between the warm and cold waters leads to a current which, contrary to the predominant currents in the area, is directed eastward, and the distribution of larvae indicates that they could use this "subtropical counter current" as a shorter and faster route towards Europe.
These oceanic processes are affected by climate change, and the study shows the importance of including them in the understanding of the life cycle completion of eel and the fluctuations in stock sizes.
Map of the surface temperature in the Atlantic Ocean. Study stations (white dots) along the eel’s spawning area in the Sargasso Sea (ellipsis). The arrow shows the potential drift of the eel larvae in the ‘subtropical counter current’ following the fronts (black contours of lines placed closely together). Click to enlarge the picture.
Insight into the diet of eel larvae
Other research finding from the Galathea 3 Expedition has just been published in Biology Letters and describes the diet of eel larvae. The scientists brought home very small eel larvae (5-25 mm) from the Sargasso Sea to find out what they actually eat.
It has been assumed that the eel’s long journey to the Sargasso Sea was linked to the feeding opportunities in the area for the newly hatched eel larvae. So far, knowledge about what and if eel larvae eat has been sparse, as only very few larvae have been found with identifiable prey items in the gut.
The eel larvae from the Galathea 3 Expedition have now been analysed using DNA barcoding, which makes it possible to identify the various plankton organisms found in the otherwise unidentifiable gut contents.
The findings show that even the smallest eel larvae eat remarkably diverse marine organisms. Gelatinous zooplankton, in particular, e.g. small jellyfish, play an important role in their diet. The study suggests that the frontal zone in the Sargasso Sea provides ample feeding opportunities for the eel larvae.
The findings relating to larval diet may be very useful in connection with artificial reproduction and rearing of the European eel, where one of the major questions is what to feed to the eel larvae.
The studies of larval diet were conducted by Associate Professor Lasse Riemann from the University of Copenhagen in collaboration with, amongst others, scientists from DTU Aqua.
About eel larvae growth areas and drift
Senior Scientist Peter Munk, DTU Aqua, tel.: +45 33 85 34 09,
The scientific article in Proceedings of the Royal Society: Peter Munk et al.: Oceanic fronts in the Sargasso Sea control the early life and drift of Atlantic eels: http://rspb.royalsocietypublishing.org/content/early/2010/06/22/rspb.2010.0900.full.pdf+html
About eel larvae diet
Associate Professor Lasse Riemann, University of Copenhagen, tel.: +45 24 78 29 05,
The scientific article in Biology Letters: Lasse Riemann et al.: Qualitative assessment of the diet of European eel larvae in the Sargasso Sea resolved by DNA barcoding: http://rsbl.royalsocietypublishing.org/content/early/2010/06/15/rsbl.2010.0411.full.pdf+html