European eel larvae are generally believed to initially follow a westerly drift route into the Gulf Stream, but new research results on bio-physical linkages in the Sargasso Sea point to a shorter route 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.
New findings from the Danish Galathea 3 Expedition to the eel's spawning sites in the Sargasso Sea now point to an alternative route for larval drift towards Europe and shed light on the conditions for larval growth and feeding
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 -- National Institute of Aquatic Resources, Technical University of Denmark.
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.
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.
P. Munk, M. M. Hansen, G. E. Maes, T. G. Nielsen, M. Castonguay, L. Riemann, H. Sparholt, T. D. Als, K. Aarestrup, N. G. Andersen, M. Bachler. Oceanic fronts in the Sargasso Sea control the early life and drift of Atlantic eels. Proceedings of the Royal Society B: Biological Sciences, 2010; DOI: 10.1098/rspb.2010.0900
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