Management of fish water quality to include colour, pH, dissolved oxygen content, turbidity, total alkalinity and hardness as well as pollutants is important because they affect survival, growth and reproduction of fish.
Quality of water must be checked weekly, monthly or bimonthly depending on prevailing conditions. However, colour of water should be checked daily.
Fish Water Quality Management
Good water is characterized by the following:
It should be greenish or bluish in colour due to phytoplankton. Yellow or brown colour may indicate acid water.
It should be about neutral or slightly alkaline. Best pH range for fish production is 6.5 – 9.0. Acidic and alkaline death point for pond fish is pH 4 and 11 respectively.
It must contain enough dissolve oxygen above 4mg/l. At values of 3mg/l or less growth will slow down, fish may become susceptible to diseases.
It must not be muddy or turbid. Secchi disc visibility less than 25cm is not desirable (adequate range = 40-90cm).
When Secchi disc is unavailable, the palm should be visible when hand is dipped in water except turbidity is caused by plankton bloom.
It must not have offensive odour
It must be free of pollutants e.g. oil films, detergents, heavy metals etc.
Control of pH Levels in Fish Ponds
When pH level is consistently low agriculture lime should be applied to raise the pH at the rate of 250-500kg/ha. Acidic waters are common in ponds constructed in acid sulphate soils (found in coastal mangrove swamps).
Such pond dykes should be limed at the rate of 1-2kg/m (length) and the pond water limed periodically (30kg/ha/month) in addition to the normal one dosage application before stocking. High Ph levels can be controlled by application of filler alum (aluminum sulphate at dosages of 10-20mg/h).
Dissolved oxygen (DO): This is probably the most critical factor in fish culture.
Causes of low (DO) in ponds includes;
Heavy plankton blooms caused by excess manuring of fish ponds.
Accumulation of uneaten feeds in ponds caused by over–feeding (high feeding rates) or bad feeding methods.
Overcrowding caused by high stocking densities.
Reduction in rates of photosynthesis caused by extended periods of cloudy weather or high turbidity of water.
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Indicator of Low Dissolved Oxygen in Fish Ponds
Swimming of fish to water surface to gasp for air. It is serous when this involves bottom fishes and it occurs all over the pond especially around midnight.
Lack of response to noise i.e. escapes into deeper waters when scared by noise e.g. clapping of hands.
Control of Low Dissolved Oxygen in Fish Ponds
Fertilization should be suspended.
Feeding rate should be reduced or feeding stopped.
Water should be changed and fresh oxygenated water added to the pond.
Water should be aerated either by use of aerators or agitators especially if pond water cannot be changed due to shortage.
Turbidity-cause
Suspended silt or clay particles. Clay turbidity restricts light penetration and may limit growth of plankton. It may also clog and block gill systems of fish and affect rate of reproduction by damaging fish eggs and destroying breeding grounds. Habitats of benthic organisms are also damaged.
High plankton density. Turbidity arising from planktonic organisms is desirable so long as it is not in excess. Excess plankton bloom may limit light penetration due to algal scum on surface and may deplete dissolved oxygen due to respiration
Control of Turbidity
Simply by soaking dry vegetation in pond.
By use of chemicals such as lime, filter alum to precipitate suspended solids.
Fertilization should be suspended if turbidity is caused by plankton blooms.
Alkalinity and Water Hardness
Desirable level of total alkalinity and total hardness in ponds ranges between 20-250mg/l.
If these variables are low (less than 20mg/l) formation and growth of plankton may slow due to low supply of carbon dioxide. Low levels of total alkalinity and hardness are increased by liming.
Pollutants
Their presence in ponds can result in extreme mortality of fish. Control of pollution includes:
Location of fish ponds away from heavy industrial centres, oil fields and chemically treated agricultural farms
Diverting or re-channeling run-off water from agriculture farms away from ponds
Ensuring that source of water for fish culture is of high quality
Each pond should have its own inlet and outlet
Pollution caused by waste products of fish can be controlled by changing the water in the pond
General Water Management in Fish Ponds
Low oxygen can kill fish. The decay of excess feed and organic fertilizer consumes oxygen from pond water. Fish will die of asphyxiation if too much oxygen is consumed.
Ponds receiving large applications of fertilizer and/or feed must be closely monitored to determine if oxygen levels in the pond are satisfactory for fish. Low oxygen occurs most frequently just before sunrise.
Farmers should visit their ponds early in the morning to see if fish are suffering from low oxygen. Fish will come to the water surface seeking higher oxygen levels from water in contact with the atmosphere. The fish appear to be “drinking” the surface water (Figure 1).
Almost all fish in the pond will be evenly dispersed over the pond surface and gasping for air. When scared, they will make a splash and dive for deeper water, but will quickly return to the surface. This behavior is sometimes confused with feeding.
However, feeding fish will not return to the surface quickly if scared. Fish suffering from low oxygen will usually not eat. Take immediate action to remedy low oxygen using the following steps.
How to Correct Low Oxygen in Fish Ponds
Add fresh water to the pond to replace water with low oxygen until fish stop gasping at the surface. More oxygen may be added to the fresh water by letting it run over a terraced structure before it enters the pond (Figure 2).
Stop fertilization and feeding for several days. Observe fish behavior closely during this time. If low oxygen has been corrected, fish will resume their normal feeding habits. When plankton abundance decreases and a submerged object is visible at a depth of 30 cm, fertilization may be resumed.
If low oxygen becomes a chronic problem, reduce the amount of fertilizer and/or feed placed in the pond.
In summary, good water quality is imperative for the success of a fish culture. Management of fish water quality to include colour, pH, dissolved oxygen content, turbidity, total alkalinity and hardness as well as pollutants is important because they affect survival, growth and reproduction of fish.
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