Researchers at The Danish Institute for Fisheries Research, Technical University of Denmark have succeeded in producing viable eggs and cultured larvae of captive European eel for up to 12 days. During this period, the larvae completed the critical period, where they completely rely on the nutrition from the egg and reached the stage where they are ready to start feeding. The result is a breakthrough, because reproduction of eels in
captivity has proven difficult. The results were presented in October at a conference held by The European Aquaculture Society. The long term goal is to produce glass eels for self-sustaining aquaculture and thereby help the endangered European eel.
The research project: “Artificial reproduction of Eels - ROE III” is financed through the Marine Research Programme 2006 of the Danish Ministry for Food, Agriculture and Fisheries. The project is led by the Danish Institute for Fisheries Research at the Technical University of Denmark and is conducted in close collaboration with the Faculty of Life Sciences, Copenhagen University (KU-Life) and two SMEs, i.e. Billund Aquaculture Service and the Danish Eel Producers Association (part of the Danish Aquaculture Organisation).
The reproduction of European eels only takes place in the Sargasso Sea and eels do not mature naturally in captivity. In order to reproduce captive eel, female and male maturation is therefore induced by hormonal control and fertilisation of eggs is made in vitro.
Larvae survived up to 12 days
French researchers were the first to successfully induce maturation in males (Fontaine 1936) and females (Fontaine 1964) in captivity however it was not until the 1980’s that larvae were produced for the first time. These larvae obtained by a Russian team (Prockhorchik 1986) survived for 3.5 days. This record remained unsurpassed until a Danish research team lead by the Technical University of Denmark last year succeeded in producing 5 days old larvae and extending it to 12 days this year. The larvae thereby completed the first larval stage i.e. the yolk sac stage.
When the larvae hatch around 2 days after fertilisation, they are poorly developed. During their first life stage they live from the nutrition they carry from the egg, i.e. yolk and an oil droplet, while they develop eyes, jaws, digestion system, etc. After 5 days the jaws and mouth start to form and after 8 to 9 days the larvae reached the youngest stages known from the Sargasso Sea. At day 12 the larvae were fully developed with large pigmented eyes, the characteristic protruding teeth and a functional digestive tract. All yolk and lipid was used and the larvae are ready to start feeding on their own.
Nutrition is important
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The nutrition of the female broodstock is important, because the yolk sac larvae feed exclusively on the nutrition from the egg. In the successful experiments, the females were fed a diet optimised in lipid composition to meet the nutritional requirements observed for other marine fish larvae in culture. This is of particular important because the broodstock originated from farmed eels that do not receive the varied food of wild specimens.
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Endangered species
The new results on artificial reproduction of eels are of importance because eel has been classified as an endangered species. Recruitment of glass eels from the Atlantic in some areas of Europe has fallen over recent decades by over 90% and stocks, especially at the extremes of the eel’s range, have declined. Recruitment and stocks have been low in the past, but in June this year, the European eel was included in the CITES Appendix II list of endangered species. CITES is an international convention established in 1973 to protect wild animals and plants against overexploitation. Of particular concern have been recent major increases in the export of glass eels as seed stock for eel farms in the Far East.
Dependence on glass eels
At present, eel farming completely relies on glass eels that are caught when Atlantic currents bring them to the European coasts. The long term goal is therefore to produce glass eels for self-sustaining aquaculture. The next challenge will be to find suitable feed for the larvae, because even though European eel larvae have been found in nature, it remains unknown what they feed on. Experiments with Japanese eels have shown that is difficult to develop feed that fulfils the requirements of the larvae.
Further information
Project leader: Senior scientist Dr. Jonna Tomkiewicz
Institute for Fisheries Research at the Technical University of Denmark
jt@difres.dk
Eel life cycle
The earliest larval stages caught in the Sargasso Sea (a large area south east of Bermuda) have remains of the oil droplet from the egg. When the larvae start feeding they transform into glassy leaf-shaped larvae, called leptocephali. In nature, the Gulf Stream and North Atlantic Drift current bring the leptocephali larvae from the Sargasso Sea to Europe over a 1 to 2 year period. In the coastal areas the larvae transform into glass eels, some may remain in coastal and estuarine waters but many migrate into freshwaters.
The immature growth phase is called the yellow eel due to the yellow-green colour of the back and sides. When the eels are between 6 and 16 years (or much older in more northerly cold areas) they again change their appearance. The head becomes narrower, the eyes enlarge, the colour changes from yellowish to silver and females begin to develop millions of eggs. In this life stage eels are called silver eels. The silver eels migrate towards the sea to return Sargasso Sea to spawn. The eggs mature during or after the migration. Maturing or spawning eels have never been caught. It is assumed that the European eel spawns only in the Sargasso Sea.