Subhendu Datta

Senior Scientist

Salt Lake City, Kolkata

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Water quality of any water body plays a very significant role in fish production. Analysis of water is extremely important for understanding the water fertility. Productivity of a water body is directly dependent on the physical and chemical parameters of water.

Colour Water colour of a pond indicates the density of planktons. Phytoplankton or algae are green in colour. Brownish colouration without any foul smell is an indication of good growth of zooplankton. Phyto and zooplanktons serve as natural food for fishes. Clear and transparent water indicates very poor growth plankton and the water may be acidic. Brownish green to greenish brown colour of water is good for aquaculture (Pl. see the water colour in Fig. 2). Temperature: Fish production is influenced by temperature. Metabolic activity is doubled with every 100C rise in temperature. Optimum temperature for IMC culture: 15 -380C, for breeding 20 - 300C.

Collect water in a bucket few meters away from the dyke and 1.5-2 ft below the water surface. Record the temperature with a centigrade thermometer (Fig. 1).

Transparency:

It is the measure of light penetration in water and this is inversely proportional to the quantity of dispersed suspended particles in water i.e. turbidity. Ideal range of transparency: 20 - 40 cm. When transparency is more than 60 cm and turbidity is less than 30 ppm, poor growth plankton may occur and productivity will also be low. The observation should not be made early in the morning or

Fig. 1

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late in the afternoon and record the observation through a shaded area of water surface.

It can be measured well with Secchi disc, which is a metallic plate of 20 cm diameter with four alternate black and white quadrants on the upper surface and a hook at the center to tie a graduated rope.

Let down the disc from the water surface (Fig 2) and determine the point of disappearance as the disk is lowered (d1) (Fig. 3).

Allow it to drop a little further, and then determine the point of reappearance as the disk is raised (d2).

The mean of these two readings [(d1+d2)/2] is taken as the Secchi disk transparency.

Alternatively Fix a bright new pin at '0' (Zero) point on a meter scale (Fig. 4)

Slowly dip it in water till the pin just disappears from the sight and record the reading of scale at water surface (Fig. 5).

Fig. 2 Fig. 3 Fig. 4 Fig. 5

pH:

It is an indication of acid or alkaline character and a vital index of productivity of water body. Below 7 is acidic while above 7 is basic. Optimum range for pH: 7.0-8.5.

A rough estimation can be made in a simple manner: Dip pH paper in water sample (Fig. 6). Compare colour so developed with standard colour

chart provided with the pH paper (Fig. 6).

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Fig. 6 Fig. 7 Fig. 8 Fig. 9 For accurate estimation, follow this method: Use Lovibond comparator (Fig. 7), which opens like a book (Fig. 8), having two sample tubes and one standard disc (Fig. 9). The disc can be changed.

Take 10 ml of sample water in left side tube of the comparator

Add 0.2 ml Universal indicator (Fig. 9) in the right side tube and fill the water up to 10 ml mark.

Shake gently and compare the colour by using universal disc

After ascertaining the approximate pH value, use suitable indicators to determine the exact pH.

Bromothymol blue for pH range of 6.0–7.6, phenol red for 6.8–8.4 and thymol blue for 8.0–9.6 should be used as indicators.

After adding 0.5 ml of the required indictor, fill the water sample up p 10 ml mark

Stir the sample, match the colour against appropriate standard colour disc and read the values (Fig. 10).

pH values can be measured directly with the help of a pH pen (Fig. 11) or pH meter (Fig. 12) by dipping the electrode into the pond water.

Fig. 10 Fig. 11 Fig. 12

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Dissolved Oxygen (O2):

Oxygen in a water body is derived from atmosphere and through photosynthesis of aquatic plants. Dissolved oxygen is a crucial factor for the survival and growth of fishes. Ideal range for dissolved oxygen is: 6 -10 mg/L. Below 3 mg/L is critical for IMC. Reagents (Fig. 13):

Winkler’s A solution: Dissolve 36g manganese sulfate monohydrate (MnSO4.H2O) [or 40g MnSO4.2H2O or 48g

MnSO4.4H2O or 50g MnSO4.5H2O] in 100 ml of distilled water. Winkler’s B solution: Dissolve 50g Sodium hydroxide and 15g potassium iodide in distilled water and make the total volume 100 ml. N/80 Sodium thiosulphate solution: Dissolve 1.55g of sodium thiosulphate (Na2S2O3.5H2O) in distilled water and make the volume 500 ml. Add 5 drops of chloroform as preservative. Prepare this reagent fresh before the estimation. 1% Starch solution: Dissolve 1 gm of starch powder in 100 ml warm (80-900C) distilled water. Cool it and stir it with 3g of boric acid. Concentrated Sulphuric Acid (36N)

Fig. 13 Fig. 14 Fig. 15 Procedure:

Water collection method: Collect water in 250 ml water sample bottles without air bubbles before sunrise from 1.5 - 2 ft below the water surface of pond. For this go inside the pond, dip the closed bottle with both the

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hands, remove the stopper with left hand, and wait till the bubbling on the water surface subsides (Fig. 14). Put the stopper with left hand below the water surface (Fig. 15) when the bottle is filled with water. Then bring the bottle to the dyke (Fig. 16). Immediately after the collection, add 2 ml each of Winkler’s A and Winkler’s B solution one after another with the help of two separate pipettes well below surface of water sample in the bottle (Fig. 17). Shake thoroughly (Fig. 18) and allow the precipitate to settle down for 15 minutes (Fig. 19).

Fig. 16 Fig. 17 Fig. 18 Fig. 19

Indication of dissolved oxygen from the colour of precipitate: Colour Dissolved oxygen quality Whitish Poor (< 2 ppm) Light brown Medium (2- 6 ppm) Brown - dark brown High (> 6 ppm)

Fig. 20 Fig. 21 Fig. 22

After 15 minutes (Fig. 20), add 2 ml concentrated sulphuric acid (Fig. 21) and shake upside down to dissolve the precipitate (Fig. 22).

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Take 100 ml of this solution (Fig. 23) and titrate with N/80 sodium thiosulphate (Fig. 24). When deep yellow colour of the solution changes to pale yellow (Fig. 25), add few drops of starch solution (Fig. 26). Titrate until colour of solution changed from blue (Fig. 27) to Colourless (Fig. 28).

Fig. 23 Fig. 24 Fig. 25 Fig. 26 Dissolved oxygen (mg/L) = ml of N/80 sodium thiosulphate used for titration.

Dissolved oxygen can also be measured directly with the help of dissolved oxygen meter (Fig. 29) by dipping the dissolved oxygen probe into the water.

Fig. 27 Fig. 28 Fig. 29 Free Carbon dioxide (CO2):

Carbon dioxide in water regulates photosynthesis process, so this is essential. Free CO2 should be <10 mg/L, > 30 mg/L is not good for IMC.

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Reagents (Fig. 30): Phenolphthalein indicator solution N/88 Sodium hydroxide solution: Dissolve 2.3 g of

sodium hydroxide in distilled water and make the total volume 500 ml with shaking. Take 100 ml from this solution and make the volume 1 L with distilled water to get N/88 NaOH solution.

Fig. 30 Fig. 31 Fig. 32 Fig. 33

Procedure: Take 100 ml of water sample in 250 ml flask Add 10 drops of phenolphthalein indicator solution (Fig.

31). If pink colour appears, then CO2 is absent If not, then titrate against N /88 NaOH (Fig. 32) until

permanent pink colour appears (Fig. 33). Record the volume of NaOH used.

Dissolved CO2 (mg/L): ml of N/88 NaOH used X 10 Total Alkalinity: Alkalinity is due to Carbonates, bicarbonates and hydroxide of calcium and magnesium. Carbonates and bicarbonates in water act as buffer in keeping the pH of water almost constant. Ideal range: 100 – 250 mg/L CaCO3.

Reagents (Fig. 34): Phenolphthalein indicator solution Methyl orange Indicator solution

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N/50 H2SO4 solution: Slowly add 3 ml concentrated sulfuric acid in distilled water, stir the solution and make the volume 100 ml. Then take 20 ml from this solution and make the volume 1L with distilled water to get N/50 H2SO4 solution

Fig. 34 Fig. 35 Fig. 36

Procedure: Take 50ml water sample in 250 ml conical flask Add 5-6 drops of phenolphthalein indicator (Fig. 35). If pink colour appears then titrate against N/50 H2SO4

until it becomes colourless. Record N/50 H2SO4 used (P) Add 5-6 drops of methyl orange indicator in the same

solution (Fig. 35). Sample becomes yellow in colour (Fig. 37).

Titrate against N/50 H2SO4 until permanent orange colour appears (Fig. 38).

Record the end point (T) from burette.

Total alkalinity : Total ml of N/50 H2SO4 used (T) x 20 (mg/L CaCO3)

Fig. 37 Fig. 38

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Four other measurements of alkalinity can also be obtained from the above readings e.g.

Phenolphthalein alkalinity (mg/L CaCO3): P X 20 Carbonate alkalinity (mg/L CaCO3): 2P X 20 Methyl orange alkalinity (mg/L CaCO3): (T-P) X 20 Bicarbonate alkalinity (mg/L CaCO3): (T-2P) X 20

Total Hardness: Hardness is the total soluble Ca and Mg Salts (in same cases Fe salts). It includes sulphates and chlorides along with carbonate, bicarbonate and hydroxide salts. Soft water: 0-100 mg/L, Hard water: 100 - 300 mg/L, Very hard water: > 300 mg/L CaCO3. Total hardness should be 50-300 mg/L. Very hard water causes osmoregulatory stress to fish. Hardness below 50 mg/L reduces growth of plankton. Reagents (Fig. 39):

0.01M EDTA solution: Dissolve 3.72 g disodium salt of EDTA in distilled water and make the volume 1L by shaking.

Eriochrome Black T (EBT) Indicator: Add 0.2 g dry Eriochrome powder to 50 ml of triethyl amine. Add 5 ml absolute alcohol and shake it.

Ammonium chloride-Ammonium hydroxide buffer solution: Dissolve 17.5 g ammonium chloride (NH4Cl) in 142 ml of ammonia solution (NH4OH). Add distilled water to make the volume 250 ml.

Procedure:

Take 50ml water sample in a 250 ml conical flask Add 2 ml buffer solution (Fig. 40) and 5 drops of EBT

indicator (Fig. 41). Wine-red colour will appear (Fig. 42). Titrate against

0.01M EDTA (Fig. 43) till the colour changes into bright greenish blue (Fig. 44).

Total hardness (mg/L CaCO3): ml of 0.01M EDTA used in the titration X 20

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Fig. 39 Fig. 40 Fig. 41

Fig. 42 Fig. 43 Fig. 44

Salinity: Salinity is the relative concentration of dissolved salts, usually sodium chloride, in given water. It is reported as ppt or g/L and can be calculated from the value of chlorinity. Classification of water on the basis of salinity: Freshwater: 0-0.5 ppt, Brackish water: 0.5- 30 ppt, Marine water: > 30 ppt. Reagents (Fig. 45):

0.1 N Silver nitrate (AgNO3): Dissolve 4.25 g silver nitrate in distilled water and make the total volume 250 ml by shaking.

5% Potassium chromate (K2CrO4) indicator: Dissolve 2.5 g potassium chromate in distilled water and make the total volume 50 ml by shaking.

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Fig. 45 Fig. 46 Fig. 47 Procedure: Salinity in estuarine and coastal water can be estimated by this method.

Take 10 ml of water sample in a 100 ml conical flask. Add 3-4 drops of potassium nitrate indicator solution to

it. (Fig. 46). Titrate against 0.1N Silver nitrate till the yellow colour

changes to brick red (Fig. 47 to 50).

Fig. 48 Fig. 49 Fig. 50 Fig. 51 Salinity (in ppt or g/L) = 0.03 + 1.805 x chlorinity

Chorinity (ppt or g/L) = Volume of 0.1 N silver nitrate consumed x 0.355 Salinity can also be measured directly with salinometer (Fig. 51). Many water parameters can also be measured with water analysis kit. Kit consists of reagents, apparatus and a printed manual where procedure of analysis is described.

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Single parameter (Fig. 51: Kit for alkalinity test; Fig. 52: Kit for hardness test) and multi parameters (Fig. 53) kits are available.

Apart from chemical and physical factors, biological conditions like presence of predators, parasites and weeds can alter production.

Fig. 51 Fig. 52 Fig. 53

Management measures to maintain the water quality Clour and Transparency:

Too much growth of phytoplankton is not good as it indicates algal bloom which in turn is an indication of over-fertilization.

Algal bloom can be restricted by application of Takazine-50 (Cymazine) @ 2-4 kg/acre.

If the pond water is covered by floating weeds, Wolfia sp, Lemna minor, Lemna major or Spirodella for one week, algal growth is checked due to lack of sunlight.

Turbidity due to suspended clay or silt (Fig. 54) particles may be settled by application of alum @ 10-30 mg/L or hydrated lime.

Deep brown colouration with foul smell is an indication of excessive accumulation of undecomposed organic matter in the sediment. This favours production toxic gases like

Fig. 54

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hydrogen sulfide, methane, ammonia etc. which are harmful to fishes.

During culture period at every one month interval apply lime @ 20-40 Kg/bigha followed by repeated netting to rack the bottom.

After harvesting of fish dewater the pond. Remove undecomposed top layer of sediment if possible or dry the bottom in sunlight till cracks appear on soil surface. Plough it and apply lime @ 500 Kg/ha. Plough again after lime application. After 3-4 days fill with water (max. 2-3 ft). After 2-3 days increase water level up to 4-6 ft. Apply raw cow dung in emulsified form @ 2 tons/ha for growth of plankton. Pond will be ready for stocking after 4 days.

pH, alkalinity, hardness:

pH of pond water undergoes diurnal fluctuation. It is lowest (acidic) at dawn and highest (alkaline) at mid-afternoon.

Due to intense photosynthesis at noon utilization of carbon dioxide is highest. Photosynthesis stops after the sunset. During night, due to respiration of plants and animals, CO2 is produced, its concentration increases with the progress of night and maximum at dawn. Carbon dioxide reacts with water and produces carbonic acid and makes the water acidic. Therefore, the fluctuation of pH is wide when population of aquatic weed and algae is beyond the normal level of the pond.

Acidic water can be treated with lime (CaCO3).

pH Dose of lime 7.0 - 8.5 100 Kg/ha limestone 6.0 - 7.0 250 Kg/ha limestone 5.0 - 6.0 500 Kg/ha limestone 4.0 - 5.0 750 Kg/ha limestone < 4.0 750 Kg/ha slaked lime

Alkaline water can be treated with raw cow dung (20 tons/ha), farm yard manure or acidic fertilizers (e.g.

Fig. 55. Application of Lime

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Urea, Single super phosphate, Ammonium chloride, Ammonium sulphate) three weeks before the stocking.

Dose of quick lime (CaO) and slaked lime Ca(OH)2 should be 1/2 and 3/4 to the dose of limestone (CaCO3) respectively.

Water becomes harder during summer season due to accumulation of salts and gets softer during rainy season due to dilution with rain water. Rain water is a cheap source of soft water.

Addition of lime, gypsum makes water harder. Boiling of water, mixing with rain water, use of water softeners and water filters makes the water soft.

Dissolved oxygen and free carbon dioxide:

When heavy infestation of aquatic weed and dense algal bloom causes a marked diurnal fluctuations and dangerous oxygen deficiency.

During day time, because of photosynthesis, water is super-saturated with O2. During night, photosynthesis stops but respiration of all plants and animals continues. Due to this consumption of O2 continues and utilization of CO2 stops. Therefore, concentration of O2 decreases while concentration of CO2 increases, which is severe during late night hours.

On cloudy days, the photosynthesis may be reduced due to lack of sunlight prolonging the night deficit in the O2 budget.

When there is a continuous cloudy days, most of the O2 fluctuation are below the critical level for fish survival (< 3 mg/L) and mass mortality of fish may occur at dawn.

To Mitigate the oxygen deficiency following steps can be taken:

Beating the water by long bamboo stick on all sides of ponds

Repeated netting Use of aerator (Fig. 56) Introduce fresh oxygenated water from other areas

to pond.

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Install one or two water pumps in the bank of the pond and night long continuous recirculation of pond water (Fig. 57).

Application of chemicals like lime (@ 60 – 70 Kg/ha), KMnO4 (@ 4 Kg/ha) has very limited impacts at the time of emergency.

Fig. 56. Use of different kinds of aerator in aquaculture

Fig. 57