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 the farmer thinks the fish wants. This means that farmers 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.
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:
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.
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:
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.
Plankton are small often microscopic aquatic plants (phytoplankton) and animals (zooplankton) found suspended in the water. 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. [/p/]
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:
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.
Pond water with pH unfavourable for fish production can be corrected by:
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
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.
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:
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).
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.
The DO content of pond water can be increased in several ways:
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:
It is desirable to maintain both alkalinity and hardness at 40 - 70 mg Calcium carbonate per litre. This can be done by:
Un-ionized 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:
Substances toxic to fish and other organisms (herbicides, insecticides, and other chemicals) should be kept out of the ponds. Ponds should be protected by: