Four years ago an eel diet containing local ingredients was formulated by the consultant (Arai, 1977), and it was used until the first week of June 1980. Composition of this diet was slightly modified. Alginic acid was used as a binder instead of carboxymethyl-cellulose (CMC); the amount was reduced from 5 percent to 2 percent, and about half of the starch was substituted with wheat flour. The composition of the basal eel diet is shown in Table 1. The amino acid balance of the diet protein was checked using the data of the standard ingredient component. The amino acid pattern was adjusted to the amino acid requirement pattern of Japanese eel. Elvers were trained to consume starter food (such as Tubifex) at first, then a mixture of the formulated diet and Tubifex. Thereafter Tubifex was substituted with minced beef liver. Later the amount of beef liver was reduced gradually. From July, 2 percent of soy oil was added to the diet. During the consultancy period elvers fed with this diet have shown a good growth and healthy condition, However, this diet contains a high amount of imported fish meal and the cost is high. The primary objective of the research work on diets is to develop a practical and economical diet for adult eel. Experiments on the possibility of substituting the basal diet partly with locally available raw ingredients (such as sheep liver, sheep spleen, by-product of fish canning factory and trash fish) are programmed. It is planned to use the water recycling system and feed replicate lots of 50–60 fish each, initial average weight 1–2 g, for 8–12 weeks on diets containing 20–40 percent of local raw ingredients (Table 2). These raw ingredients must be fresh and must be stored in a refrigerator at about -20°C.
After the removal of the hard connective tissues from the sheep liver or spleen, these materials must be finely ground with a meat mincer and mixed thoroughly. The the ground material must be packed in a suitable container and the containers must be placed in a refrigerator at about -20°C until used.
Carp viscera with liver (but without heads and gills) or trash fish (without heads and gills) must be finely minced with a meat mincer before pasteurization. When thawed, the minced fish must be heated up to about 82°C as rapidly as possible for pasteurization. Pasteurization time at 82°C must be at least 5 min, and the material must be kept about 60°C at least for 30 min. The temperature of the pasteurized fish must drop to less than 38°C before packing it in the container and the containers must be placed in the refrigerator within 1 h after filling. If the pasteurized product is used without packing, it must be cooled to 15°C or less, before usage and must be used within 12 hours.
The starter diet is very important for elver rearing and Tubifex seems to be the most important starter diet for elvers in Hungary. However, it is difficult to obtain a large amount of Tubifex in winter or early spring. Substantial quantities of Tubifex will be needed for large-scale elver rearing in 1981, therefore it is recommended to culture the required amount of Tubifex for the next year in some of the Institute's ponds during the late summer or autumn.
The water supply of the FCRI's experimental fish ponds is from the Körös river. Water is pumped from the river to a buffer reservoir of the FCRI, and then supplied by gravity to the experimental ponds. This river water seems to be only slightly polluted at present; however, it contains many species of zooplankton and also phytoplankton. Zooplankton-rich surface waters are not suitable for intensive eel culture, because they contain pathogens and parasites. Therefore, more attention should be given to the abundant resources of shallow underground waters available in a layer of 8–10 m. Since the temperature and oxygen content of underground water are lower than required, before use the water should be exposed to the air for a while. It is recommended that this underground water be used for eel culture, especially for elver rearing, since it is free from pathogens and parasites.
The importance of pond aeration is well recognized in increasing the productivity of fish culture. Pond aeration as an emergency measure for supplying oxygen is one of the most important facilities of intensive eel culture, Training eels to feed from special feeding areas is an important technique in eel culture, and this will not be possible under low stocking densities. Therefore, aeration and water supply facilities are indispensable for controlling the water quality. Several types of aeration devices have been developed and used in fish culture. The stirring wheel type of aerator is used commonly in large stagnant eel ponds, and a bubbling aeration system in small ponds in Japan. It was recommended that such aerators be tried at the Institute. A model of stirring wheel-type aerator was fabricated by the Institute staff just before the end of the consultancy period.
As a result of the feeding trials of elvers at the experimental outdoor ponds during the consultancy period, it is concluded that the following water quality parameters should be monitored;
Water temperature in the morning, at noon and in the evening.
Levels of dissolved oxygen and pH three times daily. These will show the abundance of phytoplankton.
Levels of ammonia nitrogen (NH3 N), nitrite nitrogen (NO2 N), nitrate nitrogen (NO3 N), total organic nitrogen, alkalinity and phosphate (PO4 P) at weekly intervals.
Microscopic observation of the pond water every alternate day to check the zooplankton population.
If an increase in zooplankton is observed, Flibol should be applied and the pond water mixed well by the aerator. Flibol is effective in destroying zooplankton (except rotifers) at a concentration of 1 ppm.
Detailed nutritional requirements of the European eel are not known. The qualitative amino acid requirement of the European eel, has already been studied (Arai et al., 1972) and the basal purified diet proposed can be used for studying other nutrient requirements with slight modifications.
The troughs in the recycling system are not suitable for eel feeding experiments because eels can easily escape from them. It is suggested that tanks specially designed to prevent the escape of eel be used instead (Arai et al., 1971). The tank (20 × 30 × 50 cm) can be used safely for 12 weeks' feeding experiment using 1–2 g eels, 50–60 fish in the tank, and beginning with aeration.
In spite of the difficulties arising from the fact that the facilities have not been completed totally, project personnel learned general techniques of the practical eel culture in outdoor ponds. It was proved that the traditional Japanese stagnant water eel culture method can also be used very efficiently in this country. It is recommended that one of the staff members responsible for eel culture research in the Institute be sent to Japan to study Japanese traditional eel culture techniques.
The utilization of geothermal energy for fish culture was recommended in an earlier consultancy (Arai, 1977). High temperature artesian water has been efficiently utilized as heat energy source in the Institute's water recycling plant.
Another site with excellent water resources was also studied at Hortobagy-Tuka. One artesian well here can supply 3 500 t of water with a temperature of 27°C per day and another can supply 1 000 t of water per day with a temperature of 51°C. Analytical data of the artesian waters show that the water quality is satisfactory for fish culture. There are 20 ponds (30 × 10 × 1.2 m) with vertical concrete walls and a supply pipeline and water drainage line. These facilities were originally constructed for other purposes, therefore the ponds must be adapted for fish culture. The water of all the ponds can be changed once in 48 h; so it is possible to culture fish using a semi-flow-through method, and even in winter the water temperature can be kept about 17°C.
These valuable water resources are available in many parts of this country; therefore, it is recommended to use these waters for eel culture, especially for elver rearing. Elvers of European eel can be stocked in these warmwater facilities from January: The eels can be fed throughout the year and market-sized eel (around 500 g) can be produced within 12–18 months.
These warmwater fishponds can be also used for wintering. Before putting them into operation, however, the pond system must be revised and an aeration system installed. It is also essential to arrange for the ponds to be looked after by skilled workers almost throughout day and night.