EXPERT TOPIC EELS
US $1 billion dollars a year market
by Matt Holmes, Features editor, International Aquafeed
Eel farming is worth over US $1 billion dollars a year, with eel farms producing around 60 percent of all eel meat consumed. European and Scandinavian countries along with China, Taiwan, Australia and Morocco all produce eels with the largest single producer being Japan.
Eels are fish known for their round, slender and elongated bodies. They are a carnivorous, long-living species that spend most of their life in fresh water but return to the sea to breed.
Eels live in the fresh waters of rivers and streams and only breeds once during their lives. On average, young eels live for up to 12 years for males and up to 18 years for the females with some species reaching greater ages.
Species of eels include:
The Longfin eel (Anguilla reinhardtii) and the Shortfin eel (Anguilla australis) which are found in brackish and fresh waters of Australia and New Zealand. These are the most commonly farmed species in the countries
European Eel (Anguilla Anguilla) is found in fresh and brackish water in the UK, Ireland, Mediterranean, Northern Africa the Baltic Sea, and Iceland up to mid-Norway
American eel (Anguilla rostrata) is found in brackish and fresh water on the eastern side of the United States, the south- east of Canada and the Gulf of Mexico
Japanese eel (Anguilla japonica) is most common in the fresh and brackish waters of Japan, Korea, China and Taiwan.
As they reach sexual maturity the pigment in the skin of the eel becomes silvery, they put on weight and migrate miles out into the seas to find the spawning grounds to breed.
The fertilised eggs are carried by the ocean current as they change into larvae, after around 18 months they have developed into 'glass eels'. These are juvenile eels that have an under-developed, transparent appearance. When the glass eels reach two-to-three years old, the ocean currents carry them towards the shores.
Their pigmentation becomes darker and they resemble adult eels, only much smaller at around 8-20 cm in length. These young eels are called elvers. They must migrate further up into the fresh water rivers to feed and grow for some years, beginning the cycle once more.
The farms begin by sourcing stock, usually obtained by purchasing the wild, glass eels which are sold on and used to replenish the stock on the farms. Once the juvenile eels reach the glass eel stage of development, they are much closer to the shores and can be captured in nets.
The young eels, sometimes called fingerlings, are sold and brought to the farms to restock the supply. It is important for them to be quarantined for several weeks and carefully inspected for any signs of pest or disease.
After the fingerlings have been in quarantine, they can then be grown-on in ponds or in specialised tanks that recirculate the water. The temperature of the water must be kept between 23°C and 28°C to ensure optimal growth and health of the fish. This means that in hotter climates, growing eels in pond set-ups may be more suitable as the water will naturally be at a warmer temperature.
For the most intensively farmed eels, tanks that incorporate recirculation systems are mostly used. The tank system requires access to a large amount of clean water, needed to recirculate around the tanks to keep them aerated and clean. An electricity supply is also required to heat the water to the optimal temperature and to pump the water around the system.
Another way of cultivating eels is by a method called valliculture. Rather than using ponds or tanks, farmers use the natural areas of the coast to grow the eels. This might be from making use of natural lagoons and by setting up a weir to keep the eels from escaping into the open waters, keeping them contained so that the can be harvested once they reach the desired size.
Harvesting the eels is done by grading the sizes to make sure each eel is large enough to reach market value. It is important to keep stress to a minimum while grading, so as not to affect the health of the eel. The large eels that are ready for harvesting are put into tanks with clean water to flush out any possible impurities in their systems that could affect the taste.
Eels that are to be freshly consumed are chilled and put into bags that are filled with oxygen. There should also be enough water in the bag to keep their skin moist. They are then ready for transportation to market.
Common Ailments and Treatment
The young glass eels most commonly come from the wild, so they are more prone to carry parasites or diseases. It is important to quarantine the glass eels before introducing them to tanks with other fish. The change from salt to fresh water also helps to eliminate many parasites naturally. However, here are some parasites and diseases that need further attention.
• Fungal Infections – Symptoms include swellings on the body, gills or fins. There can also be white or brown fibrous patches on the skin, which can cause the eel to die when on the gill area. Treatments include salt water solutions or the removal of the infected eel, so the fungus does not spread to other eels
• Parasites – Symptoms of a parasitic infection include an increase in mucus, frayed fins, lethargy, respiratory distress and white patches on skin. The eel may display changes in behaviour such as attempting to rub off parasites on the surface of the enclosure. Treatments for parasites include formaldehyde or salt solutions
• Red Fin – This is a bacterial disease that affects the fins. The symptoms are a rotting of the tail and fin areas. Treatment is with a salt solution
• Red Eel Pest – This bacterial disease causes symptoms of swelling, red spots and ulcerated lesions on skin. Treatment is with antibiotics
During times of stress, such as when being handled, graded or if the water becomes poor quality, eels can also become more susceptible to infections. Care must be taken to keep the stress levels to a minimum at all times in order to produce healthy fish. Prevention is always better than cure, so regular maintenance and cleaning of equipment are recommended, and also routine inspections of eel stock.