津市米津地区の異常酸性養鰻池の改良に対する施肥の効果

Abstract

Acid eel-culture ponds with pH lower than 5 are very rare, but the authors favourably found one of such a kind of pond near the seashore in the southern vicinity of Tsu City, Mie prefecture, Middle Japan. It hasα-mesohaline brackishwater and the pH-value of 4.2, (area 25 a, depth 90 cm, Cl 2.12.4‰). Its acidity is possibly due to sulphuric acid which was derived from sulphide in the bottom mud.1) The biological and chemical features of the acid eel-culture pondThe pond-water is of the pH4.24.6. Although a green alga Chlamydomonas acidophila enormously increased, the transparency about 30 cm, and the oxygen dissolved in the water was not so rich (4.5 cc/L, 61%). The diurnal changes of dissolved oxygen and pH are extremely slight ; and are rich in both NH4- and NO3N, but a little in PO4-P. The plankton is quite monotonous, being composed of Chlamydomonas acidophila and an oligochaeta, Nais pardalis, though their quantities are large. The fry of Mugil cephalus L. transplanted there from the sea were killed entirely. No feeding activity of eels in culture was observed at all.2) The effect of limingTo neutralize the acid pond-water, 160kg of lime in total were sprinkled there on April 19th, 20th, and 23rd. After this operation, the feeding activity of eels became remarkably buoyant, but the green alga Chlamydomonas acidophila, as it was expected, suddenly diminished and the transparency accordingly rose, while the dissolved oxygen decreased. It is worthy of notice that an aquatic plant Potamogeton pectinalis began to grow over the bottom of the pond. Then the feeding activity of eels became dull day by day.3) The effect of fertilizationTo increase the phytoplankton and to control Potamogeton in the pond, fertilization was planned. It was carried out three times according to the following program : 1st fertilization May 17 phosphate of lime 14 kg2nd fertilization May 22 phosphate of lime 28 kg dry poultry manure 150 kg3rd fertilization July 16 phosphate of lime 35 kg urea 34 kgThe effects of the first and the second fertilization were not satisfactory, Potamogeton grow thicker, no “water-bloom” being observed (Figs.13). However, the third fertilization was quite effective after Potamogeton was removed as much as possible (Figs.13); a striking “water-bloom” occurred and the feeding activity of eels was rapidly improved. The quality of the pond-water, the zooplankton and phytoplankton have been restored as in a pond of normal conditions. Thus, the acid pond has been almost perfectly improved through the liming and three times of fertilization.