By Dr Mustapha ABA, Aquaculture Researcher, Fish Nutrition, Morocco.
In general, aquaculturists seek to maximize fish production in their ponds by increasing stocking density, feeding rates and aeration power, while striving to maintain adequate oxygen levels by providing aeration, control of other important water quality parameters, whose pH is invariably neglected. Aquaculturists are not aware of the extent of economic losses in fish and shrimp farming, which could be avoided by controlling pH in water, associated with the adoption of management practices to maintain this parameter at levels that are safe for fish.
Variations in water pH throughout the day
In a practical way, the pH of the water reflects the concentration of hydrogen (H+) or hydroxyl (OH-) ions in the water: H20 = H+ + OH-. It can then be said that in neutral water (pH = 7.0) the concentrations of H+ and OH- are equivalent. If a water contains more H+ ions than OH- ions, it will be acidic (pH < 7.0). Otherwise, it will be alkaline (pH > 7.0).
Like oxygen concentration, the pH of fish and shrimp pond water also varies throughout the day. The pH of the water increases from dawn to mid-afternoon (16:00 h), as microalgae (phytoplankton) remove carbon dioxide from the water during photosynthesis. The decrease in carbon dioxide concentration throughout the day reduces the concentration of H+ ions and increases that of OH- ions, making the water more alkaline. The more phytoplankton there is in the nurseries, the more the pH varies throughout the day.
pH values above 9.0 (often between 9.5 and 10) are common in growing waters containing large quantities of phytoplankton (low transparency green water). Water has a buffer mechanism formed by carbonate, bicarbonate and hydroxyl ions (total alkalinity of water) and calcium and magnesium ions (total hardness of water). The high pH and oxygen over-saturation of these tanks allow fish to avoid prolonged exposure to surface water. This may explain the reduction in dietary activity and food consumption of fish during peak hours of photosynthesis (usually between 11 am and 4 pm) in very green ponds. Extreme pH values mean that fish and shrimp have a reduced food consumption, with a loss for growth and food conversion. In addition, aquatic animals exposed periodically to extreme pH values may have compromised immunity and become more susceptible to disease.
The pH of the water influences the toxic potential of ammonia.
Ammonia (NH3) is a toxic gas generally present in growing water. Ammonia generated by fish comes from the catabolism of amino acids (proteins) assimilated during digestion. Fish and shrimp excrete ammonia mainly through the gills. A large part of the ammonia in livestock water is produced by the microbial decomposition of organic matter (dead algae, faeces, food residues and organic fertilizers). Some inorganic fertilizers applied to farm water can also be direct sources of ammonia (ammonium sulphate, ammonium nitrate, ammonium phosphate, etc.).
The use of analysis kits allows the quantification of the total ammonia concentration in water. Total ammonia includes gaseous ammonia (NH3: toxic form) and ammonium ion (NH4+: slightly toxic form). Often, the daily amount of ammonia supplied to nurseries is quickly used as a source of nitrogen by algae and bacteria, so that the total ammonia concentration in the water is zero or very close to zero. However, when the ammonia supply exceeds the treatment and assimilation capacity of algae and bacteria, which is the case, for example, in fish farms where the diet is excessive, the ammonia concentration in the water begins to increase. For example, the total ammonia concentration in the water of fish and shrimp farm ponds can range from zero to values close to 10 ppm or 10 mg/litre (1 mg/l is equivalent to 1 ppm or “part per million”). But total ammonia values between 4 and 6 mg/litre are generally recorded in ponds at high feeding rates. Several factors determine the total ammonia concentration in nurseries. The most important are the daily feeding rate, feed quality and water renewal rate. In farms with low water turnover, feed flow adjustments and the use of high quality feed are important practices for controlling ammonia inputs.
The pH of the water determines the amount of total ammonia in toxic (NH3) and non-toxic (NH4+) forms. The increase in pH transforms the ammonium ion (NH4+) into ammonia gas (NH3), which increases the concentration of toxic ammonia in the water. The pH is therefore a determining factor in the risk of ammonia poisoning of fish and shrimp. And this risk is higher in ponds where the pH of the water is high.
The following table summarizes the effects of pH variation on warm water fish such as Tilapia and Catfish, two species used in aquaculture and valued by African consumers.
|The Effects of pH on Warm-Water Pond Fish|
|pH||Effects on fish|
|4||Acid death point|
|4 to 5||No reproduction|
|4 to 6.5||Slow growth|
|6.5 to 9||Desireable ranges for fish reproduction|
|9 to 10||Slow growth|
|≥11||Alkaline death point|