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Water is essential for the life of fish. It is the medium that must supply or support all their needs, including breathing, eating, reproducing and growing. In a manual WaterFAO Training Series, 4 you learned: where water comes from and where it goes; which water to use to fill your ponds; and how much water you will require. Here you will learn a bit more about water itselfand about the main physical and chemical characteristics that are important for the production of fish in earthen ponds see Sections 2.
Once you know your water well, you will understand more easily how to control its quality and, if necessary, improve it through good pond management practices see Sections 2. 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. Some of these substances are of particular importance for successful fish farming. You will therefore learn more about: the particles suspended in pond water see Section 2. Pond water changes with climate and season Changing water composition 8. The characteristics of pond water depend both on the water that has been used to fill the pond and on the characteristics of the soil.
However, within the pond water, some major chemical processes take place:. The greater the quantity of plants, animals and bacteria in the water, the more these processes change the water composition. In heavily stocked ponds, therefore, these changes are greater and need more careful management. All of these chemical processes are influenced by the water temperature : the warmer the water, the more these processes increase, and the more quickly the water composition can change. To manage and control the water compositionyou need to sample and measure the composition particularly of the more important characteristics.
In the next sections you will also learn more about the four water characteristics that are of particular importance for fish pond management:. As we will see later, the composition of the water can vary according to: when you sample the water i. These locations are commonly called stations ; how you sample the water i. There are three ways in which water is usually sampled. This method is best for getting immediate information and often lets you measure the water exactly where it lies in the pond.
This method may be necessary if you have to add chemicals to the water to test it. It is more difficult to get water from a precise location this way. Although the methods and the equipment used can give more precisethe water may have to be specially preserved to make sure it does not change during transport to the laboratory. Whichever method is used, you should: make sure all your equipment is clean; rinse out all buckets, bottles and instruments to be used for sampling with the water to be sampled; try not to disturb the water while you are sampling it; and note when, where and how you have made the sample and taken the measurement.
Obtaining a good water sample using a sample bottle 4. This procedure is the best for testing for dissolved oxygen using chemical methods see Section 2. It may also be used for other analyses. Use a narrow-mouthed glass bottle of known volume, for example ml or ml. This sample bottle should first be washed out and rinsed with the pond water, then carefully filled, avoiding splashing or bubbling. Then with the bottle held below the water surface, push the stopper in.
Take care to avoid trapping air bubbles in the neck of the bottle. This method is good for surface and shallow waters. To obtain a sample from deeper waterhowever, you need to transform the sample bottle into a water sampler. Note : instead of only one piece of string, you can also use two, one attached to the bottle and the other attached to the top of the stopper.
It should now start filling with water, while air bubbles appear on the surface of the pond. Water may be acid, alkaline or neutral. Depending on this, water will react in different ways with the 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 values greater than 7 indicate alkalinity. Obtain a water sample, using one of the methods described in the section. You can use methods and tools to measure the pH of water similar to those you used to measure the pH of soil Soil, 6, Section 4. The colour of the paper changes, and this new colour is compared to a colour chartwhich gives the pH value according to the colour obtained. You can buy litmus paper cheaply from some chemists. The pH value is directly read from the meter after placing the glass electrodes in a water sample.
Such electrodes are very fragile and should be well protected when being transported. They should be accurately calibrated in buffer solutions of known pH, at regular intervals. Note : because of the variation of the pH in fish ponds during the day see belowyou should measure pH at a regular timepreferably at sunrise. It is even better to measure the pH at regular intervals of two to three hours from sunrise to sunset, which will give you a good idea of the pH variations during daytime.
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 young fish are more sensitive than adults.
Waters ranging in pH from 6. Most cultured fish will die in waters with:. Fish reproduction can be greatly affected even at pH below 5. The original pH of the water may be affected by the pH of the soil see Sections 4. However, the pH of pond water varies throughout the day mostly as a result of photosynthesisand through the night through respiration. More and more carbon dioxide is removed from the water by the plants causing the pH to increase. Less and less carbon dioxide is removed from the water; as respiration adds more carbon dioxide to the water, pH starts to decrease. More and more carbon dioxide is produced, and pH keeps decreasing until sunrise, when it reaches its minimum.
This pH fluctuation varies in intensity. The more productive the pond, the richer its water will be in minute plant organisms phytoplanktonthe plant and animal respiration will be greater, and the daily fluctuation in pH will be stronger. The pH values of 9. As you will learn later in this manual, pond water with a pH unfavourable for good fish production can be corrected. As you learned earlier, pond water contains suspended particles of different kinds. Water turbidity is caused by the presence of these suspended particles in varying quantities:.
It may occur when the water supply is turbid or a bottom feeding fish, such as the common carp, stirs up the bottom mud. Its origin is usually the water supply, although it can also be caused by an excess of organic matter entering the pond. Mineral and humic turbidity reduce the amount of light that penetrates the water. In highly turbid waters, light penetrates only a short distance, and photosynthesis is reduced. Oxygen production during the daytime is relatively small.
Both the growth of the fish and of their natural food organisms can be badly affected. In addition, a high mineral turbidity can affect fish directly by injuring their breathing organs, reducing their growth rate or preventing their reproduction. In the same way, it can harm the minute animals called cladoceres and copepods zooplanktonwhich are very important food for young fish see Section Turbidity of pond water varies from almost zero to highly turbid, depending on the amount of suspended particles.
The method used for its measurement varies according to the kind of turbidity present. If it is a mineral turbidity brownish wateryou will need the help of a laboratory to determine the weight of material suspended in a given volume of water. When taking samples, be careful not to disturb the water too much, as you can increase the TSS very easily. Also, do not take the water only from the surface, as it is often much less turbid. If it is a plankton turbidity greenish wateryou can estimate the level yourself using the two simple methods described below. They will also give you an estimate of the potential fertility of your pond, from which you can decide on the kind of management practice to be applied see also Section 6.
Measuring plankton turbidity with your arm 7. This is a very simple method which does not require any special equipment. Proceed as follows. The Secchi disc is a very simple tool which can be used to give a better estimate of turbidity.More fish in the pond
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1. GENERAL BACKGROUND