Water quality is the most important factor affecting fish health and performance in aquaculture production systems. Good water quality refers to what the fish wants and not what we think the fish wants. This means that we must understand the water quality requirements of the fish under culture very well. Fish live and are totally dependent on the water they live in for all their needs.
Different fish species have different and specific range of water quality aspects (temperature, pH, oxygen concentration, salinity, hardness, etc.) within which they can survive, grow and reproduce.
Within these tolerance limits, each species has its own optimum range, that is, the range within which it performs best. It is therefore very important for fish producers to ensure that the physical and chemical conditions of the water remain, as much as possible, within the optimum range of the fish under culture all the time.
Outside these optimum ranges, fish will exhibit poor growth, erratic behaviour, and disease symptoms or parasite infestations. Under extreme cases, or where the poor conditions remain for prolonged periods of time, fish mortality may occur. Pond water contains two major groups of substances:
- Suspended particles made of non-living particles and very small plants and animals, the plankton.
- Dissolved substances made of gases, minerals and organic compounds.
The composition of pond water changes continuously, depending on climatic and seasonal changes, and on how a pond is used. It is the aim of good management to control the composition to yield the best conditions for the fish. For producers to be able to maintain ideal pond water quality conditions, they must understand the physical and chemical components contributing to good or bad water quality.
Physical aspects of water quality
Fish are "cold-blooded" and therefore assume the temperature of the water they live in. Water temperature is therefore the most important physical factor for fish survival and growth. Body temperature, and thus the water temperature, has an effect on level of activity, behaviour, feeding, growth, and reproduction of the fish.
Each species has its tolerance limits and optimum range. When water temperatures are outside the optimum range, fish body temperature will either be too high or too low and fish growth will be affected or the fish will even die.
Fine solid particles suspended lead to a turbidity. Turbid water can be said to be "cloudy". Turbidity can result from suspended solids (clay) or plankton. Clay turbidity in pond water (muddy water) can be harmful to fish and limit pond productivity. Clay turbidity in pond can be controlled by
- Treating affected ponds with animal manures at rates of 2.4 T/ha every three weeks or agricultural limestone, using recommended rates to improve soil pH and water alkalinity
- Avoiding stocking species that stir up pond bottom mud eg the common carp
- Designing water supply system such that muddy water can be diverted away from ponds
Plankton are small often microscopic aquatic plants (phytoplankton) and animals (zooplankton) found suspended in the water column. Phytoplankton form the base of the food chain while zooplankton form the second link in the chain in aquatic systems such as ponds.
In addition to their role as food for fish in ponds, phytoplankton produce large amounts of oxygen for the pond during the day by photosynthesis providing dissolved oxygen (DO) in ponds.
Low phytoplankton density in ponds means less food and DO for the fish. On the other hand, too much (algal bloom) lead to minimised sunlight penetration causing algal deaths. Less phytoplankton and decomposing plankton also lead to less food and DO for the fish. Good water quality, in relation to plankton therefore means water with just right bloom. Visibility in a pond with the right plankton density should be about 30 cm.
How to measure turbidity
A simple method of measuring turbidity it to stretch one arm, and immerse it vertically into the water until the hand disappears from sight.
Note the water level along your arm:
- If it is well below your elbow, plankton turbidity is very high
- If it reaches to about your elbow, plankton turbidity is high
- If it reaches well above your elbow, plankton turbidity is low
Suspended fish wastes are generally not a problem in semi-intensive aquaculture but in intensive systems, especially recirculation systems, they may be a major cause of poor water quality:
- 1 kg of fish waste per kg of fish produced
- Up to 70 percent of the nitrogen load in the system
- Build-up of ammonia and nitrite
- Reduction of dissolved oxygen
Chemical aspects of water quality
- Dissolved gases: oxygen, carbon dioxide, nitrogen, ammonia
- Essential nutrients: N, P, K
Soil pH and acidity
Pond water may be acidic, alkaline or neutral. Depending on this, water will react in different ways with substances dissolved in it. It will also affect in different ways the plants and animals living in the water.
The measure of the alkalinity or acidity of water is expressed by its pH value. The pH value ranges from 0 to 14, with pH 7 indicating that the water is neutral. Values smaller than 7 indicate acidity and greater than 7, alkalinity. Fish production can be greatly affected by excessively low or high pH.
Extreme pH values can even kill your fish. The growth of natural food organisms may also be greatly reduced. The critical pH values vary according to the fish species, the size of individual fish and other environmental conditions.
For example, fish are more susceptible to extreme pH during their reproductive seasons, and eggs and juveniles are more sensitive than adults. Waters ranging in pH from 6.5 to 8.5 (at sunrise) are generally the most suitable for pond fish production. Most cultured fish will die in waters with pH below 4.5 and 10 or above. Fish reproduction and general performance can be greatly affected at pH below 6.5 and above 8.5.
How to correct the pH of your pond water
Pond water with pH unfavourable for fish production can be corrected by:
- If the pH is below 6.5 (at sunrise), use lime and alkaline fertilisers
- If the pH is above 8.5 at sunrise, you can use acid fertilisers
Ensuring that soil pH and acidity are within acceptable limits is a necessary part of managing the alkalinity, hardness, and pH of the water, which were discussed above. The key is to keep soil pH at 6.5 or above, which will usually maintain water pH, hardness, and alkalinity at desirable levels.
How to keep soil pH at the right level
- Drying the pond for at least two weeks after each harvest before refilling and restocking.
- Applying lime (preferably agricultural limestone) to the pond after each harvest. Normally lime should be applied to the pond bottom before it is refilled, but if necessary, it can be applied to the water surface after filling the pond. Only recommended liming materials and application rates should be used.
Pond water pH varies over the course of a 24-hour day. This variation is related to the light intensity which is important in photosynthetic activity of phytoplankton.
- pH is lowest at sunrise and as photosynthesis increases as the light intensity increases, more and more carbon dioxide is removed from the water by the plants causing the pH to increase
- A peak pH value is reached in late afternoon.
- As the light intensity starts decreasing, which reduces photosynthesis less and less carbon dioxide is removed from the water; as respiration adds more carbon dioxide to the water, pH starts to decrease.
- At sunset, photosynthesis stops, but respiration continues for the rest of the night. More and more carbon dioxide is produced, and pH keeps decreasing until sunrise, when it reaches its minimum.
Dissolved oxygen in fish ponds
The most important gas dissolved in water is oxygen. Dissolved oxygen (DO) is essential for respiration and decomposition.
Dissolved oxygen in water comes from atmospheric oxygen and photosynthesis. The atmospheric oxygen diffuses and dissolves into the water. But the diffusion and its subsequent dissolves into water is a slow process. The major source of dissolved oxygen in ponds is photosynthesis. However this process depends on the amount of light available to the aquatic plants in water (Phytoplanktons). Therefore:
- Oxygen production decreases during cloudy days
- It stops at night
- It decreases in increase in water depth the rate of the decrease depending on the water turbidity
How to measure dissolved oxygen (DO)
DO can be measured by chemical or by electrical methods. Chemical methods rely on the use of kits which can be bought from shops dealing with laboratory equipment. They contain chemicals and equipment necessary to determine the DO content with sufficient accuracy for pond management purposes.
Electrical methods use an oxygen meter, this too can be bought from laboratory equipment shops but it is expensive. Using this equipment, DO can be measured directly from the pond at any depth. DO and water temperature should be measured at the same time so as to be able to relate the DO to the temperature. DO is expressed as mg of oxygen/litre of water (mg/l).
Fluctuating oxygen levels from sunrise to sunset
- Photosynthesis increases the DO level
- DO production is higher on clear sky days than on cloudy days
- The higher the phytoplankton population, the higher the DO production.
- Photosynthesis does not take place
- Respiration and decomposition which are the main activities taking place, reduces the DO content until sunrise
- The higher the plankton population and dead matter, the faster the DO will fall
There may be very little oxygen left by morning and fish may suffocate if corrective measures are not taken. In over fertilised ponds, where there is very high plankton density and high turbidity, the DO content of the bottom water may become anoxic (without oxygen) even during the day.
The fish will concentrate at the surface of the pond to survive. This will be much worse at night.
Where DO test equipment is not available, signs indicating reduced DO in pond water include
- Fish not feeding well or even stopping feeding
- Fish coming to the water surface to breathe from the better oxygenated surface water (this is called piping).
The DO content of pond water can be increased in several ways
- Through design and management
- Through structures that cause water to splash eg by use of cascades along the inlet canal and raised inlet pipes before the water gets into the ponds
- By use of mechanical aerators for the emergency aeration of pond water
A simple way to ensure a good supply of atmospheric oxygen to fish ponds is in the design of the pond. The ponds should be designed such that they take maximum advantage of the winds. The ponds should be designed so that the lengths are parallel to the direction of the prevailing winds.
Proper pond management can also improve the DO content of the water. The following measures can be taken before any emergency happens:
- Flashing the pond by removing the less oxygenated bottom water and replacing it with better oxygenated water
- Use of water aerators eg mushroom blowers and paddle wheels
Alkalinity and hardness
It is desirable to maintain both alkalinity and hardness at 40 - 70 mg calcium carbonate per litre. This can be done by:
- Where water is "soft" or acidic and soils are acid, apply lime (agricultural limestone) to the pond soil at recommended rates before to filling the pond
- Lime may also be added after filling by spreading it uniformly over the water surface.
- In areas where soils are alkaline and hardness and alkalinity are high, application of lime is not required.
- Note that proper management of hardness and alkalinity will usually eliminate the need to worry about pH.
Un-ionised ammonia (NH3) concentrations in pond water should be kept below 0.5 mg/l. Concentrations of this form of ammonia, which is toxic to fish, are influenced by DO, pH, and alkalinity, therefore it is important to manage this by:
- Maintain water alkalinity at 40 mg calcium carbonate per litre or above
- Keeping pH near neutral, and at least below 9.0
- Keeping DO concentrations high
Substances toxic to fish and other organisms (herbicides, insecticides, and other chemicals) should be kept out of the ponds. Ponds should be protected by
- Not using insecticides, herbicides, or other chemicals (except for recommended inorganic fertilisers) in or near your pond
- Keeping agricultural runoff from the ponds
- Avoiding spraying agricultural crops near ponds on windy days
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