The production of eels is based on wild catches of glass eels (elvers) that are used for further on-growing. Glass eels and elvers are best cultured in indoor tanks before being moved into grow-out facilities. Gro-wout of elvers to market size can be achieved in either tank systems or earthen ponds.
Glass eels are captured around the shores of China, Taiwan Province of China, the Republic of Korea and Malaysia, and are either used nationally or exported to eel farmers in other countries.
At first the glass eels (~0.2 g each) are kept in smaller tanks of 80-100 m3
for quarantine purposes. The density at this stage is 0.3-0.5 kg/m3
. Water temperature is kept stable at 25-29 ºC. The eels are examined for diseases and, following diagnosis, treated. They are also weaned onto artificial diets with bloodworm and, later on, dry starter feed. When the eels reach approximately 5 g they are transferred to a juvenile production unit with larger tanks (300-600 m³) at stocking densities of 0.8-1.0 kg/m2
. At this point the eels can digest dry feed pellets (1 mm).
Extensive pond systems
The traditional form of eel culture in the People's Republic of China is in earthen ponds, which should be constructed on non-porous soils. It is possible to line ponds so that they do not leak; however, the costs of construction are higher. Ponds range in size from 3 000-5 000 m2
, and are 1 to 1.5-2.0 m deep. When eels reach marketable size, they are transferred to larger ponds (5 000-10 000 m2
). The ponds may be static or flow-through. The best temperature range in ponds is 20-30 ºC. Individual growth rates vary substantially; grading every five weeks is necessary in order to reach a high overall growth performance.
Cement tanks systems
These systems consist of square or circular tanks from 100-300 m2
, usually built of cement. The eels are stocked at a size of 50 g. Densities reach up to 1-5 kg/m2
. Extruded dry feed (1.5-3 mm) is fed several times a day. Grading every 6-7 weeks is necessary.
Eel feeds are high carbohydrate (about 22 percent) and have a high fish meal content (65-70 percent) and crude protein level of 50 percent. Besides fish meal, other common ingredients are yeast, wheat, soybean meal, starch, corn, dicalcium phosphate, vegetable or animal fat, and trace mineral and vitamin premixes. Most eels (approximately 95 percent or more) are fed powdered feeds, and are fed 2-3 times daily. As the powered feeds are dry, water and fish oil are added to the mixture and the mixture is passed through a milling machine on-farm. In the People's Republic of China there are more than 100 feeds mills that produce a total of 350 000 tonst of eel feed.
Eels are harvested at a weight varying from 150 g to several kilograms, depending on the target market. Feeding is stopped a minimum of 1-2 days before harvesting. Harvesting can be carried out by draining the pond, using a seine net or (at feeding time) using a scoop net. The eels are then sorted into different sizes using a grading system. Eels that have not reached market size are returned to the rearing tanks for further ongrowing.
Handling and processing
After harvesting, eels are rapidly sorted into different sizes, using grading systems. The eels are then placed in holding tanks for several days without feed to purge their stomachs. They are chilled and then packed into strong plastic bags with just enough water to ensure that their skin remains moist. The bags are then filled with oxygen and transported to the market. If they are destined for processing they are transported live to the processing plant.
The cost of glass eels (elvers) varies significantly depending on annual catches and the interest from other eel producers. The price of Japanese glass eels has risen enormously; sometimes they cost well over USD 10 000/kg. The eel farmers of the People's Republic of China produce eels considerably more cheaply than in other producing countries. Whereas production costs in there are USD 3.6-4.2/kg, they are USD 4.2-5.6/kg in Taiwan Province of China and USD 8.4/kg in Japan.
Japanese eels are susceptible to some bacteria, numerous parasites, viruses and fungi. However, in aquaculture only a few disease agents result in disease outbreaks that decrease growth or increase mortality. Commonly seen disease agents are listed in the following table.
In some cases antibiotics and other pharmaceuticals have been used in treatment but their inclusion in this table does not imply an FAO recommendation.
Suppliers of pathology expertise
||Skin lesions; necrosis of liver and kidney; friable liver; fin haemorrhage
|Red fin disease
||Red spots on ventral & lateral areas; haemorrhagic septicaemia
||Improved water quality; antibiotics
|Red eel pest
||External symptoms are in the form of haemorrhagic dermatitis or enteritis, with lesions and development of deep ulcerative myositis
|Viral erythrocytic necrosis
||External signs may be subtle or nonexistent; sick fish anaemic, which may result in pale gills and internal organs
||White to brown cottony patches on the skin, fins and gills; usually secondary infection
||NaCl bath (0.1 %)
||Pseudodactylogyrus anguillae; P. bini
||Invade the gills; gill tissue is distinctly dropsical; gill filament stretches out with much mucus & becomes anaemic & dull in colour; respiratory distress
||30 ppm formaldehyde (40 %) bath; mebendazole
The following can provide expertise in this topic:
- Institute of Hydrobiology, CAS, 7# Donghu South Road, Wuhan City, Hubei Province, P.R. China 430072.
- Shanghai Fisheries University, 334# Jungong Road, Shanghai, P.R. China 200090.
- Pearl River Fisheries Research Institute, CAFS, Xilang, Fangcun District, Guangzhou City, China 510380.
- Freshwater Fisheries Research Centre, CAFS, 9# Shanshui East Road, Wuxi, Jiangsu Province, China 214081.