The Importance of the Chemistry of the Water to the Production of Carp in Ponds

Summary

1.	Carp culture can be divided into three systems, the European, based on the common carp, the Chinese based on Chinese carps, mainly herbivorous or plankton feeders and the Indian based on Indian carps which are mainly plankton or bottom feeders.
2.	Oxygen deficiency is the major cause of fish death in the tropics, the main cause being decaying organic matter on the pond bottom. Excessive plankton growth or excessive weed growth due to over-fertilization may also produce oxygen deficiencies at night. In general oxygen should not be below 3 p.p.m., but Indian carps can survive 1.5 p.p.m. Supersaturation with oxygen can lead to gas disease in fry.
3.	pH in itself has little meaning, but a single pH taken before dawn may indicate the unsuitability of the water but not the suitability. Toxic lower levels vary with the authority and dissolved iron raises the toxic level. The alkaline danger point also varies, but fish may survive at high pH due to photosynthesis. The optimum level is probably about 6.5–9.0; the narrow range of 7.0–8.0 is not attainable in the tropics.
4.	Alkalinity figures are unreliable because sulphates, phosphates and iron can interfere. It is not known whether calcium from calcium sulphate and calcium phosphate can be brought into circulation with the high metabolic rates of algae and bacteria in the tropics. The alkalinity figures for the most productive ponds in Malacca are below the optimum given for Europe.
5.	Calcium enters into the buffer system, promotes bacterial activity, is a constituent of fish and acts antagonistically towards toxic metals. It is rarely limiting for plant growth as ponds are usually limed to neutrality. High levels tend to be accompanied by blue-green algal blooms. Excessive liming may lower productivity by precipitating phosphate, while excessive slaked lime may lead to depletion of dissolved carbon dioxide.
6.	Magnesium is essential for plant life, but since lime is added to ponds usually enough magnesium is added as well. Toxic effects only occur with really high levels, and such levels are rare in ponds.
7.	Potassium is essential for plant growth and is rarely limiting. In Malacca it was found unnecessary, and ponds were slightly less productive with its addition. It was found sufficient potassium occurred for the fish. The use of N-P-K mixture is based on experiments where the absence of potassium has not been tested. Most fish culturists find the addition of potassium unnecessary.
8.	Phosphate is the most important fertilizer and this alone in Malacca sustains the best crops of fish. Dissolved phosphate levels are not indicative of the fertility of the water, since phytoplankton can take up more than its immediate needs.
9.	Nitrogen is not needed in Malacca and its use gives doubtful results in Europe. Fixation of nitrogen by blue-green algae or bacteria can be adequate for the pond needs. Nitrogenous fertilizer is used in Israel, being most needed in spring and autumn. Aqueous ammonia is used in Israel and this also controls Prymnesium parvum. Where N-P-K is used there has been no real test eliminating nitrogen. There seems to be little difference between the crop from nitrate and from ammonium salts, and urea is no better than either. Urea produced a lower residual cropping effect in Malacca. Excessive application of nitrogen favours blooms of blue-green alga.
10.	Iron is rarely limiting except in marl areas or heavily limed ponds. High levels of iron are toxic and floes due to liming can clog the gills of fish. Iron floes can absorb phosphate. Dissolved iron can raise the low toxic level of pH.
11.	Aluminium is rarely considered, but in certain areas may reach toxic levels. Aluminium floes can clog the gills of fish, and they may also absorb phosphate from the water.
12.	Sulphate is rarely limiting in ponds. Excessive sulphate, particularly as sulphuric acid is lethal to fish. It may also inhibit plankton growth. Heavy liming is needed to neutralise high sulphate levels.
13.	Animal manures and sewage are often used in ponds. In Malacca the effect of cowdung was found to be due to its phosphate content. Heavy doses of animal manure or sewage lead to semipermanent algal scums, either blue-greens such as Microcystis aeruginosa or Eugleninae. Various organic compounds in the water can be utilized by algae. Flagellates need vitamins, whereas many other species do not. Zooplankton may need vitamins.
14.	Molybdenum, copper, manganese, vanadium and cobalt are probably needed for all algae. They are unlikely to be limiting except in special areas where cobalt and copper are known to be needed for land plants.
15.	Primary, Secondary & Tertiary Production are discussed in relation to the use of inorganic or organic fertilizers. Plankton feeding fish can only digest certain algae but not Microcystis or Euglenineae. Microcystis aeruginosa may be toxic to fish.

In sewage lakes zooplankton can.only account for a small part of the primary production. Both crustacea and rotifera appear to be selective in feeding, size and shape of the algae being important. Mucilage tends to prevent digestion. Microcystis aeruginosa blooms, even decaying appear to be toxic to crustacea, and other blue-green blooms are also reported as toxic.

Knowledge of the biology and distribution of bottom fauna is limited. They appear to be selective in feeding, but bacteria may form a part of the food.

The fish or zooplankton may be living on saprophytic organisms in ponds fertilized with animal manure or sewage. The energy system is highly inefficient and utilizes little solar energy. The use of inorganic fertilizers is more efficient.