Water quality refers to the physical, chemical and biological characteristics of water with respect to its suitability for a particular purpose. It is a critical factor in all aquatic life. Any characteristic of water that affects survival, reproduction, growth or management of fish is a water variable.
Water quality is determined by such factors as dissolved oxygen (DO), carbon dioxide (CO₂), turbidity, water pH, temperature, alkalinity, hardness, unionised ammonia, nitrite, nitrate, primary productivity and plankton population. In this article, we shall focus on temperature, water pH, DO, and ammonia. Dissolved oxygen is a measure of how much oxygen is dissolved in the water, that is the amount of oxygen available to the living aquatic organisms which includes your fish, the phyto and zoo plankton in your pond or cage system. DO is important for the process of respiration which generates energy from the breakdown of food. The preferred DO for optimum fish growth should be above 5 mg/l although tilapia species can tolerate slightly lower levels.
Certain factors will affect the level of DO in a pond/cage. These include stocking density, and clogging of nets in a cage system. The higher the stocking density, the less DO will be available to the fish. Always ensure that you maintain an optimal stocking density. A high stocking density will result in rapid oxygen depletion, and this will lead to stress or even death in severe cases.
Water temperature is also an important parameter of water quality. Fish are poikilothermic, which means the ambient environmental temperature determines their body temperatures. Temperature has a direct effect on metabolism, thereby influencing growth and development. Tilapia has an optimum temperature for normal development, reproduction and growth, which is in the range of 28 to 32 °C.
Fish excrete ammonia and lesser amounts of urea into water as waste. Two forms of ammonia occur in aquaculture systems, ionised and unionised. The unionised form of ammonia (NH₃) is extremely toxic while the ionised form (NH4+) is not. High concentrations of NH₃ in water make it difficult for fish to eliminate NH₃ from their bodies thereby causing stress. Ammonia can have sublethal effects such as reduced growth, poor feed conversion and reduced disease resistance. High NH₃ levels can result from overfeeding and overcrowding. In order to avoid these high levels, ensure that there is a good water exchange and stick to the recommended fish stocking densities. pH describes the quantity of hydrogen ions in the water. This will determine whether the water is acidic or basic. Water pH has an influence on the growth of the fish. Extreme pH levels mean that fish will have a reduced feed consumption, consequently leading to slow growth, and their immunity becomes compromised. The desirable pH range for optimal growth and reproduction is between 6,5 and 9,0.
In order to maintain good water quality, one can take the following actions: water exchange for ponds and ensuring that nets are not clogged in cage systems in order to facilitate free flow of water, reduce overfeeding, reduce overcrowding and take water readings for DO, temperature, pH and ammonia levels. This will help to detect problems earlier and rectify the situation beforehand.
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