The recruitment of the European eel has been in dramatic decline during the last 30 years, and is at severe low of only 3-5% of earlier magnitude. This change and its influence on the eel fishery have led to an intensified research in the oceanic phase of the European eel. In order to contribute to further understanding of the life cycle of eel the Danish eel expedition – supported by the Carlsberg Foundation – sets out in 2014 for the eel spawning grounds in the Sargasso Sea. A consortium of Danish scientists and international collaborators focused on the linkages between oceanography, biological production, eel spawning, and the growth and drift of eel larvae. The larvae were distributed across a huge area, 500 km high and 2,000 km wide, hence as much as three times the North Sea area. Their densities were generally very low and overall abundances appeared to have declined to about 10% of what was estimated in the period of good eel recruitment 40 years ago. Findings indicate that a number of changes have taken place in the spawning areas that may affect the eel larvae’s chances of survival and their journey to Europe. In the context of increased temperature fronts between the hot and cold water masses – and hence the spawning areas of eel – have spread further north than seen before, a change that might influence both larval feeding and drift pattern.
Danish eel expedition launching the gear for sampling eel larvae in the Sargasso Sea. Photo: Line Reeh.
The SARGASSO-EEL
Project The Carlsberg Foundation supports the project SARGASSO-EEL which investigates the potential role of climate related changes at the spawning grounds of eel in the remote Sargasso Sea. During this project a multidisciplinary research cruise called Danish Eel Expedition was carried out using the only ocean-going research vessel in Denmark, H/S Dana in March-April 2014. The cruise aimed at the spawning grounds in the Sargasso Sea and from here the ship followed the drift route of eel larvae homeward to Europe. The Sargasso Sea is situated between Bermuda and the West Indies, and during the two months of the cruise, no less than 18,000 km were covered.
More Than 20 Research Projects Working for New Understanding
The overall project is led by Senior Scientist at DTU Aqua, Peter Munk, and includes experts from a range of Danish and international universities. The more than 20 subprojects will altogether fill many of the gaps in our knowledge of reproduction and early life of the mysterious eel.
At the cruise, a wide range of organisms was collected: From the smallest plankton of less than a millimeter to very large fish. Eel larvae were distributed across a large area, several hundred kilometers wide. Research groups are now working on samples and data from the expedition and assembling information on key processes in the early life of eels.
One example of research is the intensive work in order to find out what they eat. While there are no identifiable remains in the stomachs of the larvae, researchers therefore use a new advanced technique involving genetic determination of substances from the stomach. Another example is the assessment of larval growth rate variability using microstructures in a little stone in the hearing organ of the larvae for age determination. Preliminary results indicate that these will provide new important knowledge about larval feeding and growth.
Pre-sorting of eel larvae during the expedition. Photo: Line Reeh
Climate Related Changes at Spawning Ground Could Be Behind Eel Decline
Both the adult stock and the amount of new recruits have declined dramatically. The arrival of small eels (glass eels) to coastal areas of Europe has dropped to a level of only 2-10% of the level in the late 1970s. Due to the pronounced changes, there is an urgent need to improve our understanding of potential impact from humans, environment, and climate on the lifecycle of eels. Thus an important goal was to ascertain climate-related changes in environmental conditions at the spawning ground, or in the flow of currents that bring the larvae back to Europe.
Eels – presently at historical low abundances. Photo: Michael Ingemann Pedersen
The European eel has an extraordinary lifecycle where the spawning takes place far from the adult foraging areas in Europe. The route of eel larvae drifting from the spawning areas to the coastal nursery areas is no less than 5,000 kilometres long. When hatched in the saline, open ocean, and drifting for more than a year across the Atlantic Ocean, the eel spends, however, most of its life in brackish and freshwater with only one opportunity to see the ocean again when migrating back to reproduce.
“Our investigations indicate that the larvae are distributed in areas of certain temperatures and salinities in the Sargasso Sea, and apparently great changes have taken place in the environmental conditions during the last decades. The area of larval distribution follows certain patterns in temperature and salinity, and extraordinary high temperatures lead to a northward shift of distribution. This was the case during the expedition in 2014 and we now investigate whether these circumstances have influenced larval growth opportunities and their drift toward Europe,” says project leader, Peter Munk.
Significant Contributions to the Understanding of Eel Spawning and Early Life
In total, the scientists on Dana launched equipment more than 400 times in order to collect water samples and samples of plankton organisms of all sizes. Eel larvae were sampled using a large, specially designed net.
“Even when the densities of larvae were generally low, we were able to collect hundreds of larvae, sufficient for the many subsequent analyses. We have had good progress in our analyses of e.g. their genetics, morphology, growth, and food preferences. The early life stages of eels are special in many aspects and the findings we have now assembled provide interesting new information on important life cycle aspects of this remarkable fish,” comments Peter Munk.
Historical Traditions for Sargasso Sea Research
The Danish Eel Expedition 2014 was carried out about 100 years after the first cruise targeting the spawning areas of the eel. This first cruise was organised by a Danish scientist working at the Carlsberg Laboratory at the time, Dr. Johannes Schmidt, who as early as 1912 suggested that the spawning areas of eels were to be found in the Mid-Atlantic. In 1923, he published a famous paper based on expeditions in 1921-22, which provided evidence that the spawning areas were to be found in the Sargasso Sea area. Later surveys to the spawning areas have confirmed much of Schmidt’s early discoveries, but there are still many unanswered questions regarding the eel reproduction and larval growth and survival.
Considering the poor status of European eel population, and the consequential severe restrictions in the fishery, new initiatives for improving our understanding of the eel life cycle are urgently needed. The eel has been a traditional species in the European fishery during centuries, and the declining fishery is of much concern to the society.
An understanding of the life cycle of eels and the variability in stock sizes and recruitment inevitably will have to include investigations on the spawning grounds of the eel. Due to the scale of expeditions to the Sargasso Sea, such initiatives strongly depend on external funding. The support from the Carlsberg Foundation was essential for the establishment of the present project and expedition. Further support was achieved from the Danish Centre for Marine Research and national universities.
Facts:
Dana towards harbour in Bermuda during the expedition. Photo: Line Reeh
The research vessel Dana is 78 metres long and is Denmark’s largest research vessel. It is equipped for ocean going research and has five dry- and wet-laboratories with a wide range of scientific instruments. Moreover, it has gear for trawling, water sampling and bottom coring, and its capacity of people is set to be 38 including a crew of 12-15 people.
Eel of an age of about a month in the so-called leptocephali stage, caught during the expedition. Photo: Sune R. Sørensen
Despite decades of intensive research, it has not yet been possible to organise large-scale reproduction and rearing of eels in captivity. DTU Aqua has had success in obtaining eggs and larvae from artificial maturing of adults, and the larvae survived for up to 26 days - a world-record for European eels and a major step towards sustainable aquaculture of eels. The next step will be to find appropriate feed for the larvae, and rear them beyond the first larval stages. The studies of eels’ feeding preferences and opportunities in the Sargasso Sea will provide new information of great value for the preparation of appropriate feed for reared larvae and hence for a production of glass eels in captivity.
Further Reading
Munk P, Hansen MM, Maes GE, Nielsen TG, Castonguay M, Riemann L, Sparholt H, Als TD, Aarestrup K, Andersen NG, Bachler M. (2010). Oceanic fronts in the Sargasso Sea control the early life and drift of Atlantic eels. Proc Royal Soc London Biol Sci 277:3593–3599 doi:10.1098/rspb.2010.0900
Miller, M J. ; Bonhommeau, S ; Munk, P ; Castonguay, M ; Hanel, R ; McCleave, J D. (2015) A century of research on the larval distributions of the Atlantic eels: a re-examination of the data. Biological Reviews 90(4): 1035-1064 doi: http://dx.doi.org/10.1111/brv.12144).
Ayala, D J ; Munk, P ; Riemann, L (2016) Species composition and diversity of fish larvae in the Subtropical Convergence Zone of the Sargasso Sea from morphology and DNA barcoding. Fisheries Oceanography 25(1): 85-104, doi: http://dx.doi.org/10.1111/fog.12136
Other (selection):
Article in ICES Insight Issue No. 51 September 2014: Searching for eel larvae in the Sargasso Sea. ISBN 978-87-7482-148-9
Interview in Natursyn: Den gådefulde ål, http://www.dr.dk/p1/natursyn/natursyn-2015-12-21
List of press releases: http://www.aqua.dtu.dk/Forskning/Marine-oekosystemer/Aaleekspedition/Presseomtale