Eel Encyclopedia
The Biological Characteristics & Breeding Captivity Of Eel

Most eels having long, cylindrical flat tail body & some species are with colour spots on the body. The upper & lower jaws are full with fine teeth. Scales are very small, hidden and buried subcutaneously with low dorsal & anal fin; the back ended with caudal fin. The pectoral fins are small, round and without pelvic fins. Eel is river & sea migratory fish. The mature eel will swim back to the deep sea to spawn. Eel larvae are transparent and are known as Glass Eels. During spring, eel larvae will follow the sea current and reached to the seashore of certain selected region and when they growth to the size of elvers then they will swim back to the rivers and settle down at the tributaries or lakes to feed on small fish, shrimp and aquatic insects. Eels are nocturnal based, fast growing animals with tender & greasy meat, rich in nutrition.

There are 18 types of eels around the World, 4 types are found in Taiwan inclusive of Japanese eel, Perch eel, Celebes eel & Short-finned eel. They have been survived in Earth for thousands of years but we only get to know them in recent decades. In 1991, we have discovered the spawning grounds and noticed that the sex of the eel is much depending on the population density and the environment factors. When the population density is high and lead to insufficient of food, then they will become male fish and contrary to become female mother eel. In Taiwan, the eel population density in the rivers is low, and therefore most of the eels are female.

The Morphology Of Eel

The elvers, each with the body length of about 6cm, weight 0.1gm, small head, thin & transparent body is also called Capelin. Its body fluids density is same with the sea water; this makes it easily to drift with currents to travel around. Some elvers travel with the Kuroshio Current for about six months from the spawning ground to reach to the beach of Taiwan. One month before arriving to the shore, it began to metamorphosis for a slender and transparent body which is known as the glass fish. In the month of December to January, the Taiwan fishermen were in busy hand to start fork net fishing at the coasts between the river mouths hunting for elvers and sold them to the eel farms. The elvers under human breeding captivity will start changes slowly to yellow colour appearances and then turned into silver adult eels.

The Natural Living Habitat Of Eel

Eel is a migratory fish, originating from the sea; grew up in the fresh water and return to the sea for spawning. During spring time, large group of eel larvae (also known as “white glass fish”) flows with the Sea-Current reach to the coast and enters the estuary. Male eels mostly stay and grow within the estuary, and female eels swim against the freshwater current of the river to travel up to the upper stream of the river, the tributaries and lakes that connected to. Some may even travel thousands of kilometers to reach the upper stream of the river. They will be settled down, grow & develop at the place where they have finally chosen. Eel is often nocturnal and hunts for small fish, crab, shrimp, crustaceans and aquatic insects during the night with low moon light. It has a strong absconded ability and able to swim against strong flow current, and likes burrowing and slow water flowing environment.

During the autumn, the female eels that have reached the age of sexual maturity will swim down in large groups to the down river stream or river mouth to meet up with the male eels and then they will travel all the way to the deep ocean sea for mating & spawning. At approximately 30 degrees North Latitude of the South East of South China Sea, with temperature between 16 - 17° C, a 30% salt content of sea water with the depth of 400-500M, female eels most likely chosen this type of environment as their spawning ground. Each time a female eel may lay small eggs between 700 million to 1 trillion of each at about 1 mm in diameter. The eggs are floating and incubate within 10 days. After hatching, the larvae gradually rise to the water surface, and later to be drifted by the currents to the coast of China, Japan, Korea, Taiwan & South East Asia.

Now, the larvae are about a year old, and will develop into white transparent elvers one month before they reach to the coast. They start migrating to the river mouths when they are still in white & transparent colour during the beginning stage of the migration, and turns into black eels mixed with small amount of white eels at the last stage of the migration. The gonads of the eel cannot be developed well and breed in the fresh water. The gonads development of the female eel will be completed only during the time they returned to the river mouth and heads towards the deep ocean sea. Eel will loss of appetite when temperature drops to 15°C or higher than 30°C and stop feeding totally when the water temperature drops to 10°C and to start sneaked into the mud for hibernation. Eel can use the skin to breathe; it will not be dead as long as the skin is keeping wet & moist.

The Breeding & Reproduction Of Eel

Eel larvae cannot be nurtured apart of getting from the natural wild. This is because eel has a very unique life cycle and it is very difficult to analog a man-made environment like that within today’s technology.

Eels grow in the fresh water river up to mature size and start migration to swim down the river mouth and heads to the deep sea for spawning. Unlike the trout and salmon; adult trout & salmon will swim back from the sea to the upper stream of the freshwater river for spawning.

Eels will travel thousands of kilometers to the deep ocean to seek for a suitable environment for spawning. The sea between Philippines, Indonesia & Malaysia also found their spawning ground. The scientist found these spawning grounds because of the large amount of hatched eel larvae are found there. Eel breeds in the deep sea and growth in the freshwater with ferocious temperament, bulimia, hyperactive, nocturnal, loves fresh, warm, slow flowing current and burrowing characters makes scientist difficult to imitate its breeding environment.

Adult eels are round mediocre in appearance; like a conical shape, black colour and some species with colour spots and commonly found throughout the year, but most plump and delicious during summer or winter season. It is nutrient-rich, less bone, fleshy and mostly farmed by eel farms.

About Intensive Aquaculture In Eel Bleeding Program

Intensive Aquaculture Farming
Depending on the sizes and conditions of the proposed farm, basically intensive aquaculture farms are using fiberglass made breeding tanks to breed glass eels (approximate 0.03g each) with area of 3 – 4 m² and density of 10 – 15kg per m². When glow to 5gm, elvers will be transferred to larger tanks with area of 6 – 8 m² and density of 50 – 75 kg per m². In Malaysia, most fiberglass tanks are manufactured with basic measurement of 1.219m ( 4ft ) wide X 2.438m ( 8ft ) long X 1.219m ( 4ft ) height.

Carbonate Hardness, PH & Living Environment For Eel
The most suitable living environment for eels is hard water with Carbonate Hardness of not less than 8.5, PH ranging from 6 to 7.8 and the water temperature of 26°C to 28°C. Eels will stop feeding if the water temperature is exceeding 30°C and if the temperature between day and night is difference by more than 3°C; diseases such like white spots, gill diseases and tail rotting diseases may be infected throughout the entire breeding tank and system.


Water Quality Control
Water quality control is the key towards the success of high productivity and efficiency in eel farming. First, we have to change the water at least 2 to 3 times monthly, each time at approximately 15CM. This practice must be increased to more frequently change of water during the later breeding stage. Secondly, the paddle wheel oxygen aerator must be switch on for at least 8 hours daily during the hot and warm days and to be regulated accordingly to the changes of temperature and weather conditions.

Thirdly, regular use of biological agents; at the later part of the breeding stage, because high quantity of baits, foods & excreta remains in the pond, the water quality will easily turns bad. Biological agent such like EM micro plasma will be used at a ratio of 500gm /acre, twice a month to be added to the ponds. Others such like Photosynthetic bacteria and Bacillus will be used for disinfection purposes. Intensive farming system will be equipped with precision instruments such like water filters, water circulation systems, oxygen & ozone generator, protein, colloids and toxic agent removers to meet the international intensive aquaculture requirements.