4. RESULTSOur main problem was to f

4. RESULTS

Our main problem was to find the dose of sewage water which does not cause any harm to fish, providing, at the same time, a proper grade or purification. Extreme doses of sewage water were used in 1975 at a uniform stocking structure. We found that, compared with the control, a load of 5 m3/ha does not significantly change the water quality and fish yield. At the highest loading of 250 m3/ha/day, however, a mass mortality occurred due to the early morning oxygen depletion in the ponds. The best result was achieved at a dose of 50 m3/ha/day, with a yield of 1.2 t/fish/ha.

In 1976 we optimized the stocking structure at a load of 50 m3/ha/day considering the feeding activity of the species, as well as the quality and quantity of natural nutrient sources of the pond. Considering the results of experiments carried out in 1976, we established that the rate of common carp could not be more than 10 percent in the stocking structure if we wanted to yield market size fish and properly purified water at the same time. Grass carp should have a share of 5-7 percent due to the huge mass of macrovegetation. The greatest problem was the rate of bighead which was the most aggressive species of the stocking structure. Its optimal rate was established as 22-25 percent. Silver carp grazing on zooplankton and phytoplankton had a great significance in the structure. The best purification grade and highest yield was achieved with the stocking structures, where the share of silver carp was 50-65 percent. The yields, as the function of stocking structure and number, ranged between 1.3-1.6 t/ha, which can, however, be enhanced by increasing sewage water loading, maintaining at the same time the water quality. In 1977 experiments were conducted using the experience of 1976, to find the optimal dose of sewage water loading.

The best result, 1.8-2.0 t/ha, was obtained at a water disposal of 100 m3/ha/day. The water quality obtained this way met all the requirements. In the case of sewage water loading higher than 100 m3/ha/day, the quality of water was significantly impaired and the yield decreased.