Eels are generally classified as a warmwater fish, and 19 species (including sub-species) in the Anguilla genus are distributed throughout the world. One of the species, the European eel Anguilla anguilla, is found in Greece. In Japan, farmers had tried to culture European eel with the traditional Japanese eel culture methods, because of the shortage of Japanese eel elver and the low price of European elver. However, most of the farmers failed to culture European eel. It was found that in Japan, the European eel was more sensitive to high water temperatures than the Japanese eel and death often occurred in the summer in stillwater ponds. The eel also appeared to be highly susceptible to parasitic disease. Some farmers have succeeded in culturing European eel in flow-through water system ponds. Success has been attributable mostly to the lower water temperature than that used for Japanese eel and constant water conditions in the flow-through water system.
1. Culture methods for European eel
Suitable water temperatures for Japanese eel culture are thought to be 23–30°C, and the lower limit of water temperature for feeding is about 12–13°C. However, A. anguilla seems to have more lower water temperature adaptation, as it is distributed from 30–70°N latitude, while Japanese eels range from 20–40°N. Trials with cultured European eel in Japan suggested that their ideal water temperature is 20–23°C and that they may feed at temperatures as low as 7–8°C. However, culturists in Europe frequently use temperatures of 22–28°C as optimum rearing temperatures for European eel. Water temperatures in ponds are a very important factor in practical eel culture; these should be kept within a suitable range for at least six months for commercial success. In this respect, there are many suitable water resources in Greece.
Eel culture methods are classified broadly into two types of water system, namely still water and flow-through water. Flow-through water systems have the advantages of maintaining the water temperature and quality in the pond1, however, a plentiful water supply is needed. On the other hand, a still-water system may not need such a plentiful water supply, but conditioning of water in the pond is difficult. As for water resources, ground water is particularly suitable for intensive eel culture in Greece, because of the ease of controlling water quality. River water can also be used for culturing eels; however, protection from water pollution is needed. Generally, the oxygen content of underground water is lower than required, and so before use the water should be exposed to the air for a while, and should be aerated with aeration devices.
The training of eels to feed at fixed feeding areas is an important technique in eel culture, and this feeding would not be possible under low-stocking densities. As eel culture is basically an intensive fish culture, aeration and water supply facilities are indispensable for controlling the water quality in both the still-water system and the flow-through water system, and for increasing the productivity of the pond. Below 1 mg/l oxygen in water, eels cannot breathe and come up to the surface. This phenomenon is a sign of a dangerously low oxygen content in the water. Oxygen supply devices such as paddle-aerators, vertical pumps and bubbling aeration systems are usually installed in each pond. Paddle-aerators are frequently recommended, as these can supply oxygen and can create a current in the pond.
1 “Pond” is taken to mean pond or tank.
According to the growth stage of eels, the use of several sizes of pond are recommended, for example glass eel or elver in nursery ponds (first pond, about 30–300 m2), elver or medium-sized eel in rearing ponds (second pond, about 100–1 000 m2), and commercial-sized eel in rearing ponds (third pond, about 300–3 000 m2). After sorting of the eels over set time intervals, bigger-sized eels are put into their respective ponds. The water depth is usually 40–50 cm in the first pond and 80–100 cm in the second or third pond.
In order to prevent small-sized eel from escaping during rains, an over-hanging lip is necessary on the top of the pond wall. Ponds are usually constructed using earth and/or concrete. Net-pen culture in reservoirs or lakes is also possible for commercial-sized eel production.
The stocking rate of seedfish in ponds should be decided by the supply of water and oxygen content. About 0.2–0.4 kg/m2 of glass eels can be stocked in the first pond, 0.5–1.0 kg/m2 in the second pond and 1–3 kg/m2 in the third pond. In flow-through water systems, about ten times this level can be stocked. The oxygen consumption rate of eel (for an eel weighing about 5 g, at 15°C) is 150 ml O2/kg/h, and this consumption rate increases as the water temperature increases.
In practical eel culture in Japan, farmers specialize in two types of production: (1) short-period culture from glass eel and elver to medium-sized eel (harvesting size 2–50 g), and (2) long-period culture from glass eel and elver or medium-sized eel, to commercial-sized eel (150–250 g). In Europe, farmers frequently grade out intermediate-sized eels for restocking in lakes, lagoons, or other farms.
2. Feed preparation
The composition of the typical eel diet in Japan is shown below in Table 1:
Composition of typical eel grower diet
(46% protein diet)*
|Ingredients (air dried)||Percentage|
|White fish meal (65% protein)||62–69|
|Yeast and liver meal||4–6|
* Nitrogen × 6.25 × 100
From National Research Council, 1977
Five to ten parts oil are added to 100 parts of the mash-type diet feed and this is mixed well. Then 80 to 100 parts water are added and blended vigourously for 30 to 60 seconds in a mixer just before feeding to make a good consistency. This consistency of the diet feed, due to the alfa-potato starch, will be weakened gradually. Thus the diet feed should be used as soon as possible after preparation. The breakdown of diet feed due to weakened binding ingredients may cause pollution of the pond water.
The composition of Japanese eel feed can be modified by using local ingredients such as animal by-products from slaughter-houses or trash fish. Alfa-potato starch as a binder in the feed can be substituted by other binders such as alginic acid or carboxymethylcellulose (CMC). To make a good consistency of the feed, it is said that about 23–25 percent alfa-potato starch is necessary; however, this may be more for the benefit of the eel farmer, not for the fish. The most important factor is that of avoiding the breakdown of the diet feed. Five to eight percent of CMC, 2–3 percent of alginic acid, and smaller amounts of some binding ingredients are necessary to make a good consistency. Alfa-potato starch, which is one of the most expensive ingredients in commercial eel feed, can then be substituted with low-grade wheat flour as a carbohydrate source.
As a diet for eels, pelleted dry feeds can also be used. However, the eel is not as well adapted to taking pellet diets as are salmonids or other fishes. In order to use pelleted dry diets, eel should be trained to eat suitable sized dry pellets from the elver stage, and sorting is needed more often than when using moist diet feed.
3. Feeding practices
All species of eel are wild carnivorous fish and the life cycle of eel is not yet controlled as is that of the rainbow trout or some other fishes. Glass eels or elvers require to be domesticated to eat artificial feed or trash fish. For successful culture, it is better to start adaptation to artificial feed as early as possible. The starter diet is thus very important for elver rearing. Traditionally live tubificid (a worm of the Tubificidae family), fresh fish meal or short-necked clam meat are used as starter feeds for glass eels or elvers. Tubificids seem to be the most favourable feed for elvers. After stocking the glass eels in a nursery pond, they should be fed live tubificids from a feeding basket hanging close to the water surface at the fixed feeding place in the pond. For the first 2–3 days, it may be better to use chopped tubificid. If enough tubificids cannot be obtained in winter or early spring, beef liver can be used as a substitute. After the removal of the hard connective tissues from the liver, it must be finely ground with a meat grinder and mixed thoroughly. Then the ground feed must be packed in a suitable container and kept in a freezer at a temperature -20°C until used. Recently, a packed and frozen artificial feed for glass eels has been developed by a Japanese feed company, as a substitute for live tubificid; this has been used by many farmers because of the shortage of tubificid during the winter season, and because of convenience. As these substitutes of live tubificid will pollute the pond water, more care over water quality is needed. After about one week of being fed tubificid, most elvers will be adapted to feeding from the basket.
b) Acclimatization to formulated diets
After glass eel is completely accustomed to eating tubificid from the basket, the diet must be changed gradually from starter to formulated diet feed or fresh fish, meat or minced beef liver. At first, a mixed diet is prepared by mixing 5–7 parts by weight of formulated diet feed with 5–3 parts by weight of tubificid. After kneading the mixed diet feed, this is given to the glass eels in the basket. If the formulated diet feed contains alfa-potato starch as binder, it should be prepared just before each feeding time. The amount of tubificid decreases gradually, and formulated diet feed increases correspondingly. In several days, the eels will be accustomed to eating only formulated diet feed. After domestication, the diet feed should be given at a rate corresponding only to the amount that they can eat completely in about 10–20 minutes. At this stage, the eels should be fed two to four times daily, but when the eels become bigger and all the eels are trained to eat sufficiently in one feeding, the frequency decreases to once daily. In commercial-sized eel culture, feed is normally given once in the morning. Just after feeding time, the remaining diet feed on the basket and at the bottom of the ond should be taken out, even though a small number of fish continue to eat. In eel culture techniques, this nursery stage is a most important step because the digestive ability of eel will be strengthened by eating sufficient amounts of diet feed at this elver stage.
c) Feeding rate
The daily feeding rate of formulated feed at a water temperature of 25°C is 6 to 8 percent of the total wet body weight for glass eel and small-sized eels, and 2 to 3 percent for bigger sizes. For fresh fish feed, the feeding rate at 25°C is 20 to 30 percent of the total wet weight of glass eels and small-sized eels, and about 10 percent for bigger eels. Since these figures are standard feeding rates, changes to the daily feeding rate should be made by observing eels and checking water conditions, such as temperature and oxygen content. Good results are not usually obtained by overfeeding larger eels, especially during the summer. Fish can eat a lot of feed during the warm season, but should be fed rather less than satiation for best food conversion efficiency, which for dry feeds should be 2–2.5:1 (weight of food fed: weight of fish growth).
d) Size differences
Many workers have reported that eels raised in experimental tanks showed great variations in their growth. It has also been known from experience in eel culture that growth varies greatly. Because of this characteristic, the eel farmer has to carry out several sorting or grading operations during the rearing of eels. This usually requires several types of pond, suitable for different growth stages of the eels. In commercial-sized eel ponds, bigger sizes of eel are sometimes harvested by a thinned-out operation using a net of a suitable size.
e) Size difference with sex
The typical adult size of male eels is 40–50 cm in length and about 120–500 g in weight, but the female reaches 100 cm in length and 1–2 kg in weight. Size differences seem to be greater in European eel than in the Japanese eel. Commercial sizes of eel are 120–250 kg in Japan and over 500 g in Europe, though an intermediate-size market for 100–150 g eels is also developing. This means in practice that the European market consumes mainly females and the Japanese market consumes both males and females. If farmers wish to culture large-sized eels for the European market, they must select mainly females and therefore need more elvers for commercial success with eel culture. Growth rate differences are also significant: female eels can reach a weight of 0.5–1 kg in two to four years feeding under normal conditions, while males may take more than ten years to grow to 500 g.