CHEMICAL & PHYSICAL FACTORS AFFECTING WATER QUALITY IN FISH CULTURE
Water quality determines not only how well fish will grow in an aquaculture operation, but whether or not they survive. Fish influence water quality through processes like nitrogen metabolism and respiration. Knowledge of testing procedures and interpretation of results are important to the fish farmer. Some water quality factors are more likely to be involved with fish losses such as dissolved oxygen, temperature, and ammonia. Others, such as pH, alkalinity, hardness and clarity affect fish, but usually are not directly toxic. Each water quality factor interacts with and influences other parameters, sometimes in complex ways. What may be toxic and cause mortalities in one situation, can be harmless in another. The importance of each factor, the determination method and frequency of monitoring depends upon the type and rearing intensity of the production system used.
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| How to control water quality in Fish culture |
Water Quality Management (Physical Factors)
• Successful pond culture operations mainly depend on maintenance of a healthy aquatic environment and production of sufficient fish food organisms in ponds.
• Physical, chemical and biological factors play an important role in governing the production of fish food organisms and fish production in the pond.
• It helps in the survival and growth of the fish. Hence, fish farmers should take a lot of care to maintain hygienic conditions in the pond, so that they get more profits.
• If the water quality is maintained with utmost care, the farmers need not spend much money for treatment of the diseases. If the water quality is maintained, the fishes also have a good taste.
• Water quality is influenced by physical, chemical and biological factors of the water.
Physical Factors
a. Water Depth
• Depth determines the temperature, circulation pattern of water and the extent of photosynthetic activity.
• In shallow ponds, sunlight penetrates up to the pond bottom and facilitates an increase in the productivity.
• A depth of 1-2 meters is considered optimal for biological productivity of a pond. If the depth is very less, water gets overheated and thus has an adverse effect on the survival of the fish.
• In arid and semi-arid areas, water depth should be more than 2 meters.
• The excess water from pond can be removed through pumping or through the use of outlet in the embankment.
• If the water depth is reduced then from a nearby source it should be filling up.
b. Water Temperature
• Temperature affects fish migration, reproduction and distribution.
• Fish pose well defined limits of temperature tolerance with the optimal being 20-32°C.
• Indian major carps can thrive well in the temperature range of 18-38°C.
• Wide fluctuations of water temperatures affect the survival of fish.
• In very low or very high temperatures, the fishes are strained and spend more energy; ultimately growth of the fish is affected.
• If the water temperature changes to a remarkable level then supplying feed and fertilizer to the pond should immediately be stopped.
• Replenishment of water from a nearby source, harvesting the table size fish, is some of the corrective measures to be taken for it.
c. Turbidity
• Water turbidity is mainly due to suspended inorganic substances like clay, silt, phytoplankton, zooplankton and sand grains.
• Ponds with a clay bottom are likely to have high turbidity.
• Turbidity reduces sunlight penetration and photosynthesis and hence acts as a Limiting Factor.
• If the turbidity is due to more suspended particles, they absorb nutrients in their ionic form, making them unavailable for plankton production.
• High turbidity reduces the dissolved oxygen in the pond water.
• Turbidity is measured with the Secchi Disc.
• If the secchi disc disappears at 30-50 cm, the water is productive in nature. If it is not visible at a depth less than 25 cm it indicates the blooming of algae, if it is more than 50 cm, the plankton produced is less in the pond water.
• In less turbid waters, the aquatic weeds growth is more.
• In highly turbid waters, the sand grains accumulate in the gills of the fish and prawns, causing suffocation and excessive secretion of mucous and finally leads to fish death.
• High turbidity can be reduced by adding lime and alum.
• If the water is more turbid, it should be stored in sedimentation tanks and then used for fish culture tank.
• If the turbidity is more due to phytoplankton, water from the pond should be changed.
• Fertilizers have no effect in high turbid waters; hence fertilization of the pond should be stopped.
d. Light
• Availability of light energy to a fish pond greatly influences its productivity and photosynthesis.
• In shallow ponds, light penetrates to the bottom and is responsible for lush growth of aquatic weeds.
• In high turbid waters, the light will not penetrate to the bottom. Due to this, the vegetation at the bottom will decay and produce harmful gasses, which affect the fish and prawn life.
e. Water Color
• Water gets its color due to phytoplankton, zooplankton, sand particles, organic particles and metallic ions in the pond water.
• Water used for fish culture should be clear, either colorless or light green or blue in color.
• Water color of golden or yellow brown indicates the abundant diatoms.
• Water becomes greenish in color when phytoplankton is more, develops a brown color due to zooplankton and mud color due to more sand grains.
• Water with black, blackish green, dark brown, red, yellow colors are not good for culture. These colors are due to the presence of more phytoplankton, bad pond bottom and acids in the water.
• The red color of water is due to the presence of high levels of iron and death of phytoplankton (phytoerythrin released).
Water Quality Management (Chemical Factors)
pH:
• Water is slightly alkaline in condition, with the optimal range of 6.5 – 8.
• Less than 5 and more than 10 pH are lethal to fish.
• The pH of pond water undergoes a diurnal change; it is alkaline during the day time and slightly acidic just before day break.
• The fluctuations of pH are similar to dissolved oxygen. pH fluctuations are more in phytoplankton and weed infested waters and water with less hardness.
• The difference in pH from morning to evening should not be more than 0.5. When pH increases, ammonia and nitrites become toxic, when it is reverse H2S becomes more toxic.
• pH below 6.5 and above 8.5 is responsible for reduction of growth and resistance of parasitic infection increases in acidic waters.
• Whenever pH falls, lime should be added to the pond water.
• When pH is high, lime and urea should not be used to reduce pH. This is because NH3 becomes toxic at high pH. It is always better to add new water to maintain an optimal pH. Alum or aluminium sulphate can be used to reduce the PH and turbidity.
• Alum removes phenolphthalein alkalinity; 1 ppm alum reduces 1 ppm phenolphthalein alkalinity.
Dissolved Oxygen:
• Dissolved oxygen is one of the most important chemical parameters, which has a great influence on the survival and growth of fishes and prawns.
• The pond water gets oxygen mainly through interaction of atmospheric air on the surface water of the pond and by photosynthesis.
• It is produced only during day time and reaches a maximum at 3 PM, then gradually decreases up to early morning.
• During night period, it decreases and it reaches a minimum during the early hours of the day. It is due to nil production of dissolved oxygen at night and instead, consumption of oxygen by plankton, weeds, fishes and prawns will be there.
• During overcast days, the production of dissolved oxygen during the day is less and during the subsequent nights it decreases drastically.
• When water temperature rises, oxygen is released into atmosphere. When salinity increases it is dissolved in water.
• The optimum dissolved oxygen is 5-8 ppm. If less than 5 ppm the growth rate of the fish and prawns decreases and are prone to get diseases.
• Less than 1ppm of dissolved oxygen results in death and more than 15 ppm results in gas bubble disease in fishes and prawns.
• Whenever the animals are under stress due to less dissolved oxygen the food consumption temporarily decreases.
• When oxygen decreases, fishes come to the surface and engulf the air.
• Precautionary measures should take at nights, especially during the early hours to increase oxygen levels.
• If it is very less, the water surface should be disturbed by beating water with bamboo poles or by running boats or by using aerators.
Alkalinity:
• Alkalinity is caused by carbonates and bicarbonates or hydroxides of Ca, Mg, Na, K, NH4 and Fe.
• Alkalinity is less in acidic soils and in ponds with more organic load.
• Alkalinity is more in clay soil ponds and is increased if water is not exchanged.
• The optimal level of total alkalinity is 40 – 150 ppm.
• Alkalinity has direct effect on the production of plankton.
Hardness:
• Hardness is caused by Ca and Mg.
• Water with less than 40 ppm is soft and more than 40 ppm is hard.
• The pond water with a hardness of 15 ppm or more is satisfactory for growth of fishes and do not require additional lime.
• If water has less than 11 ppm hardness, it requires liming for higher production.
• If it is less than 5 ppm, the growth rate is affected and causes eventual death of the fish.
Carbon Dioxide:
• CO2 is produced during respiration and consumed during photosynthesis.
• CO2 is less during daytime and more at nights.
• The optimal level of CO2 is 5 ppm.
• At high CO2 levels, pH decreases and water becomes acidic.
• CO2 is accumulated in the blood of the animals. Then, the animals become sluggish, loss of resistance occurs, they cannot utilize dissolved oxygen and they ultimately die.
• Whenever CO2 increases lime should be added to the pond.
• 1 ppm of lime reduces 0.9 ppm of CO2.
Dissolved Ammonia and Its Compounds:
• NH3 is found in excreta and is also released due to decomposition of organic matter and also increases with unfed feed due to high protein levels and death of phytoplankton.
• It is an important compound influencing the growth of phytoplankton in the aquatic ecosystem.
• The optimal limit of NH3 is 0.3-1.3 ppm and less than 0.1 ppm is unproductive.
• Whenever NH3 increases pH also increases, but dissolved oxygen decreases.
• CO2 reduces the toxic effect of NH3.
• NH3 accumulates in the blood and oxygen transport in the blood reduces. Gills become black, biochemical tissue is damaged and gaseous exchange is affected.
• NH3 levels can be reduced with good management like no excess feed, optimal stocking and water exchange.
• Lime should not be added when NH3 is high.
• Optimal level of nitrites is 3.5 ppm.
Hydrogen Sulphide (H2S) :
• H2S is produced in anaerobic conditions by the action of micro organisms on sulphur compounds.
• H2S is toxic to fish and prawn and is responsible for respiratory problems.
• It should be less than 0.05 ppm in pond water.
• When H2S increases, lime should be added.