Water

Water quality management

Fish are in equilibrium with potential disease organisms and their environment. Changes in this equilibrium, viz. deterioration in water quality (environment) can result in fish becoming "stressed" and vulnerable to diseases. It is, therefore, important to know about water quality parameters and their management, which have influence on growth and survival of aquatic organisms.

Dissolved oxygen

The optimum dissolved oxygen (DO) content of pond waters is in the range of 5 mgllitre to saturation level for good growth of fish. Aeration is a proven technique for improving dissolved oxygen availability in ponds. Paddle-wheel and aspirator aerators are the most common types of aerators used for pond aeration. In heavily aerated ponds where aerators are positioned around the edges to create circular water flow, strong water currents can cause severe erosion of pond bottom. Therefore, placement of aerators in a pond is also an important aspect that needs attention.

Temperature

Temperature sets the pace of fish metabolism by controlling molecular dynamics (diffusibility, solubility, fluidity) and biochemical reaction rates. The optimum temperature range for several 'coldwater' and 'warmwater' fishes are 14°-18°C and 24°-30°C, respectively. Water temperature can be adjusted to optimum levels in controlled systems such as hatcheries. It is difficult to adjust water temperature in large water-bodies. Operation of aerators during calm and warm afternoons helps to break thermal stratification of ponds by mixing warm surface water with cool subsurfacewater.Plantingoftreesonpond bankstogive shadewillreduce stratification but at the same time, reduces the beneficial effects of wind mixing and restricts sunlight needed for photosynthesis, which can reduce productivity of the pond.

Turbidity

It is the result of several factors including suspended soil particles, planktonic organisms and humic substances produced through decomposition of organic matter. Turbidity is measured by Secchi disc visibility. Optimum Secchi disc visibility of fishponds is considered to be 40-60 cm. Turbidity resulting from plankton is generally desirable. However, heavy blooms limit heat and light penetration, thus reducing the effective volume of the productive zone. Turbidity due to suspended soil particles can be controlled by application of organic manure at 500-1,000 kglha, gypsum @ 250­500 kglha or alum @ 25-50 kglha.

Ammonia

Fish are very sensitive to unionized ammonia (NH3) and the optimum range is 0.02-0.05 mg/litre in the pond water. Normally in the case of high dissolved oxygen and high carbon dioxide concentrations, the toxicity of ammonia to fish is reduced. Aeration can also reduce ammonia toxicity. Healthy phytoplankton populations remove ammonia from water. Addition of salt @1,200-1,800 kglha can reduce the toxicity of ammonia in water. Formalin may also be used to reduce ammonia. Biological filters may be used to treat water for converting ammonia to nitrite and then to harmless nitrate through nitrification process.

Nitrite

Under normal conditions, the nitrite concentration of fish ponds is negligible, as the ponds are kept well-oxygenated. In hatcheries, control may be accomplished by installing biological filters and addition of chloride ions (through addition of salt). Effective removal of organic wastes, adequate aeration and correct application of fertilizers are the methods to prevent accumulation of nitrite to toxic levels in pond culture.

Hydrogen sulphide

Freshwater fish ponds should be free from hydrogen sulphide. At concentration of 0.01 mg/litre of hydrogen sulphide, fish lose their equilibrium and subjected to sub­lethal stress. Frequent exchange of water can prevent building up of hydrogen sulphide. Further, increasing water pH through liming can also reduce the hydrogen sulphide toxicity.

pH

pH is a measure of hydrogen ion concentration in water and indicate how much water is acidic or basic. Water pH affects metabolism and physiological processes of fish. pH also exerts considerable influence pH on toxicity of ammonia and hydrogen - sulphide as well as solubility of nutrients and thereby water fertility. The generalized effects of pH on fish are presented in Table. The best way to counter water pH problem is application of lime for increasing soil pH to greater than pH 6, and total alkalinity and total hardness to greater than 40 mgllitre as calcium carbonate.

Calcium carbonate (calcite) CaC03, dolomite-CaMg(C03)2' calcium hydroxide (slaked lime)-Ca(OH)2 and calcium oxide (quick lime)-CaO are the different lime materials used. Liming should be carried out a few weeks before addition of fertilizers and stocking of fish. Agricultural gypsum (CaSO4) is applied to correct the total hardness without affecting total alkalinity. It may also be applied to correct alkaline pH.

Effect of pH on fish

Total alkalinity

Pond waters with a low alkalinity (less than 20 mgllitre) as CaC03, have a very low buffering capacity and consequently are much vulnerable to fluctuations in pH, for example, during rainfall and phytoplankton blooms. Such fluctuations may be directly harmful to fish populations. Ponds with alkalinity greater than 300 mgllitre may also be unproductive because of limitation to carbon dioxide availability at such high concentrations. The ideal range of total alkalinity for freshwater fish is 60-300 mg! litre as CaC03. Low alkalinity ponds can be treated with lime.

Total hardness

Total hardness for fresh water fish ponds should be greater than 40 mgllitre as CaC03. This concentration of hardness helps to protect fish against harmful effects of pH fluctuation and metal ions. Ponds with low hardness can be treated with lime.

Carbondioxide

Freshwater fish ponds should contain a low concentration of free CO2 «8 mg! litre). Aeration and increasing the pH of water by hydrated lime (calcium hydroxide) can control high carbon dioxide concentration. Experiments have showed that 1.0 mgllitre of hydrated lime can remove 1.68 mgllitre of free CO2,

Source

(Hand book of Fisheries and Aquaculture. 2006. Indian Council of Agricultural Research. New Delhi).