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Chapter: Aquaculture Principles and Practices: Eels

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Collection and rearing of seed eels

As indicated earlier, eel culture is based on seed eel collected from the wild. Several attempts have been made to propagate A. anguilla and A. japonica artificially, starting as early asthe 1930s.

Collection and rearing of seed eels

 

As indicated earlier, eel culture is based on seed eel collected from the wild. Several attempts have been made to propagate A. anguilla and A. japonica artificially, starting as early asthe 1930s (Boucher et al., 1934). Spontaneous release of eggs was obtained by Fontaine et al. (1964)         in the European eel. 

Boëtius and Boëtius (1967) were able to mature males of the species by weekly injections with carp pituitary and maintaining them in sea water at a temperature of about 14°C. Several other workers have subsequently succeeded in

maturing the males, as well as obtaining the release of mature eggs from females, but artificial fertilization of the European eel has not been a success. The Japanese eels have been stimulated to spawn by hormone injections and by keeping the brood fish in sea water at a temperature of about 23°C (Yamamoto et al., 1975), but the larvae could be reared only until the sixth day.

 

In Europe, the collection of elvers is done either during winter and spring or in the beginning of summer in June and July, when they ascend the rivers. They seem to be able to migrate up the rivers at lower temperatures of 2–10°C, unlike the Japanese eels. The early migrants are smaller in size (about 7 cm in length), but the later ones are larger (15–20 cm in length). These are probably the elvers that hatched out the previous year and they ascend further up the river than the smaller ones. The elvers for restocking or rearing are collected during the earlier migration which starts around December. The best catches are generally obtained from February to May. They are captured with large wire-meshed sieves or large nets similar to a plankton net dragged from a powered boat.The catches are stored in aerated tanks for transport ashore.

 

In Japan, the elvers enter the rivers from October through to late May, when the water temperature reaches 8–10°C.They are caught in scoop nets at night, using bright lights as attractants, or by using fine-meshed bag nets set across the river. Special elver traps may also be set near obstructions across the rivers, where the elvers are likely to congregate. In Taiwan, the elvercatching season is from October to March. They are caught with scoop nets, drag nets or eel traps.

 

Elvers need careful handing after capture and during rearing. It is a common practice to condition them for a day after capture, in special bamboo baskets or tanks. They can be transported to distant farms, packed in wooden boxes. Intercontinental shipments of elvers have also been made in polythene bags, after conditioning at low temperatures of 4–7°C.

 

Before release of elvers into nursery ponds, many farmers give them a bath of malachite green to prevent infection. To protect the elvers from cold winds, the ponds may be covered with vinyl sheets. Some farmers use electric

heaters to maintain the water temperature at about 10°C. A common system of rearing elvers in Japan is to stock them initially at higher densities in a series of ponds, and when they grow larger to transfer them to a series of progressively larger ponds at lower densities. The first series is about 165 m2 in size, with a depth of about 40 cm, and the stocking rate is about 500–600 g elvers per m2. The larger ponds are about 200 m2 in area. Most eel ponds have vertical concrete or brick walls and sandy soil bottoms, although there are some with steep mud walls and mud bottoms. The seed eels can climb considerable distances up the wall, especially during heavy rains, and so it is desirable to have some protective devices at the top of the walls to prevent their escape.

 

Before the elvers are released, the nursery ponds are disinfected with lime. As mentioned earlier, the stocking density in Japan is 500– 600 g per m2, but in Taiwan it may be almost 10 times that. European elvers are stocked at the rate of 3500–10 000 g per ha. Feeding is started when the water temperature is about 15°C. As they are nocturnal in feeding habits, the feeding spot is covered with boards or other suitable material, to make it as dark as possible. Small worms are considered a suitable first feed for elvers, and after two or three days fish flesh is added in progressively increasing quantities until about the tenth day, when a paste of minced fish is given. Formulated eel diets are now in use in many farms. The recommended protein level in practical diets for elvers is 50–60 per cent and for subadults, 40–45 per cent. Even though the amount of feed consumed by elvers depends on the condition of the water and the temperature, the recommended normal daily ration is about 30 per cent of the total weight of the released elvers, fed in several lots. Adequate feeding is important to reduce cannibalism. When the elvers overgrow the capacity of the pond (indicated when they congregate near the surface and breathe atmospheric air), the stock is thinned out. Eels can be scooped out easily from near the feeding place and transferred to larger ponds. The stocking rate then is about 150–200 g per m2. In about four months the elvers grow to around 7 g, and in another four months they reach about 100 times their size at stocking. It is essential to ensure that no filamentous algae develop in the ponds, as elvers are likely to hide under them and refrain from feeding. Usually elver farmers start catching and selling some of the stock as they grow in size, according to market demand.


 



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