Research work on eel culture in the new water recycling system started in 1978. The team on eel culture research obtained good results in some experiments, such as medium-sized eel rearing and market-sized eel rearing. In these cases, fish density in the tanks was low. However, elver rearing experiments failed both in 1978 and 1979 in the recycling tanks due to mechanical breakdowns of the recycling plant. The density of elvers in the tanks was very high (15 kg per tank in 1.2 m3 of water volume) and water in the tanks was not aerated. Therefore the breakdown of the water supply even for a short time caused severe oxygen deficiencies and as a result the elvers died.
It is suggested that the stocking rate of elvers must be lower than 5 kg/m3 of water volume. The water of the recycling system is presently aerated in the aeration tank of the system located in front of the experimental fish tanks. To improve the oxygen concentration in the tanks and to combat emergency situations, it would be advisable to aerate each tank separately.
Construction of the experimental eel culture pond system, which was designed in collaboration with the consultant and the personnel of the Institute by a local design bureau, was started late in 1978. The design of the ponds is shown schematically in Fig. 1 and 2. A part of the pond system was constructed in May 1980 and an outdoor elver rearing experiment started on 29 May using two ponds 150 m2 each. About 30 kg (80 000–100 000 pieces of elvers were stocked in each pond. This stocking rate is not high in traditional eel culture ponds; however, as the water supply system and the aeration of the ponds were not completed, there were many problems with low oxygen concentrations in the water. A tentative water supply pipeline and a diesel-driven, blower-type aerator were set up; and, just before the termination of the consultancy period, the final water supply system was completed. An aeration system must be installed for each pond as soon as possible. As mentioned in section 3.2, the water supply of the experimental pond system of the Institute comes from the Körös river. This water is rich in zooplankton, and therefore a high zooplankton density was also observed in the pond water one week after the feeding of elvers started.
The pond system was designed to use the flow-through system, but as the construction of the water supply pipeline was delayed, this was not possible. The stagnant water method (which is traditional in Japan) was thus adopted and in order to develop a bloom of phytoplankton, Flibol (a selective insecticide) was used to kill the zooplankton. As a result, in two days the phytoplankton population was increased to a density which produced sufficient oxygen for elvers in daytime and the water colour became green or greenish-brown. Oxygen was enough to saturate the water about 100 percent in daytime and sometimes over 200 percent in the afternoon of a sunny day. The elvers showed very good appetites. Water was supplied after sunset until early in the morning and the aerator was operated in the daytime on cloudy days and early in the morning when the oxygen content became low.
The former recommended composition of the eel diet (Arai, 1977) was slightly modified as shown in Table 2, because the suggested binder was not available (see section 3.1). After six weeks of the feeding experiment, it was proved that the No. 1 diet is very suitable for elvers.
The research team of the Institute gained experience in techniques such as the control of the water quality, feeding and sorting during the consultancy period.
