DTU's Dana -- Denmark's largest marine research vessel -- has spent three weeks exploring and gathering samples in the spawning grounds of the European eel in the Sargasso Sea, between Bermuda and the West Indies. The first phase of the Danish Eel Expedition 2014 has been successfully completed.
The expedition -- led by DTU Aqua and comprising experts from a number of Danish and international universities -- is in the Sargasso Sea to investigate whether climate-related changes to the eel spawning grounds or the ocean currents that carry the eel larvae to Europe have caused the dramatic decline in eel numbers. The expedition is also investigating the feeding conditions for the eel larvae, as no one knows what they actually feed on. This ignorance thwarts efforts to grow larvae in aquaculture, which would otherwise be an option to ensure the supply of eels, which is considered a delicacy in Denmark.
Unique study of prey
A distinctive feature of the European eel is that it spawns far from its nursery grounds in Europe, requiring the eel larvae to ride the ocean currents for their 6,000-kilometre trip back across the Atlantic. Today, the number of young eel returning to the coasts of Europe is just 2-10 per cent of the quantities seen in the 1970s. In 2008, the International Union for Conservation of Nature (IUCN) added the eel to its list of critically endangered species.
The first leg of the expedition investigated conditions in the eels' central spawning areas in the Sargasso Sea at 41 carefully chosen locations where cold and warm waters meet, as this is believed to create good conditions for eel larvae.
As something unique, the expedition was able to perform a comprehensive '24-hour vertical study' of a hotspot with a particularly high concentration of eel larvae. The study showed that by far the most eel larvae were collected over a 24-hour period in a small band at a depth of 150 metres, just below where the water has the highest salt level. For 24 hours, the expedition performed measurements on organisms and the environment around the larvae to work out what makes that specific location so attractive to the larvae.
The expedition mapped the distribution of marine snow (organic detritus) and jelly plankton (primarily medusa jellyfish) at various depths in the nursery grounds of the eel larvae. Marine snow and jelly plankton such as appendicularians are likely candidates as an eel larvae food source.
A total of 21 researchers and 17 crew members participated in the first leg of the exhibition.
Fishing for adult eels No one has yet succeeded in catching a mature European eel in the Sargasso Sea. The expedition therefore trawled for three nights around the new moon -- when it is assumed eels spawn, so the eggs have the best chance of being concealed in the dark from hungry predatory fish. However, no adult eels were caught this time either, although many smaller fish, particularly deep sea fish, were caught.
Extensive new knowledge During the first phase of the expedition, Dana deployed instruments in the water 463 times. These included 61 CTD water measurements, 95 deployments of the large larvae net, 3.5 metres in diameter, and 49 deployments of the fine-meshed multi-net, which collects plankton at five different depths. Each instrument was in the water for about an hour at a time, and the crew worked around the clock in 12-hour shifts in order to fully utilize the time and collect as much information as possible.
DNA analyses During the first leg of the expedition, over 400 eel larvae were retained for further on-shore DNA analysis. This might reveal whether there are genetic differences in the populations, and to what extent the two species which spawn in the Sargasso Sea -- the American and European eel -- interbreed.
DNA analysis of the contents of the eel larvae's intestines will also be matched against the 250 samples of plankton DNA and 80 samples of marine snow collected so far. Many of the organisms living in the Sargasso Sea are not yet covered in DNA databases.
Continuing east After successfully completing the first leg, Dana will continue on leg two of the expedition. The key objective is to map the eastern frontier of the eel spawning area, as this has not yet been determined. On the third and final leg, Dana will map the route of the eel larvae's return to Europe, taking samples along the way. On 5 May Dana returns to Hirtshals, Denmark, marking the end of the expedition.
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