Monitoring water quality
Regular monitoring of the water quality is important to ensure ideal living conditions for the fish.
Turbidity of the pond water – This gives information about its nutrient status. Turbidity is measured using a Secchi disk. The disk is made of metal or plastic (if it is plastic, weight under the disk is needed to make it sink in the water) and can be white or black and white. On the cord, which holds the disk, there are nodes every 5 cm on the first 50 cm and every 10 cm on the next 50 cm. The disk is immersed slowly into the water and the number of immerged nodes is then counted. The depth, at which the disk disappears when we look at it from the water surface, is called the Secchi depth. An ideal value is between 25 and 30 cm. If visibility is as deep as 40 cm, 50 cm or deeper, algae growth is poor and should be enhanced by adding organic manure. If visibility is below 25 cm, algae growth is too strong and can result in low oxygen content in the water during the night. In this case, feeding and manuring must be stopped for some days until the Secchi depth has reached 30 cm again.
Oxygen – Depletion of oxygen is the most common reason for losses in the fish pond. If algae growth in the pond is very strong, the oxygen content during the day is high, since algae produce oxygen through photosynthesis. But during the night, the algae competes with the fish for oxygen and its content decreases to a minimum level in the very early morning before sunrise.
If the growth of the algae is ideal (Secchi depth of 25 to 30 cm) and if there are not too many fish in the pond and the fish are small, oxygen saturation of the pond water should be sufficient. Competition for oxygen between fish and all other organisms in the pond can become a serious problem, however, if the stocking density is too high and the fish get bigger.
If the farmer has the means to control the oxygen concentration in the water, he should do it twice or three times a day, when the fish show occasional gulping behaviour. Gulping usually appears when oxygen content falls below 1 to 2 grams per litre. Oxygen should be measured early in the morning before sunrise and half an hour after feeding in the afternoon or evening.
pH-value – The pH value gives information on the acidity of the water. A pH of 7 means that the water is neutral and does not have acid or alkaline characteristics. This value is ideal for fish and algae. The pH can be determined with colour tests, available as liquid tests or as a blotting paper test. Values below pH 5 can be corrected with the careful addition of lime-milk and through a reduction in the stocking density. Reducing stocking density also helps if the pH is very high (above 9).
Toxic substances – Many chemicals used in animal husbandry and crop production on non-organic farms are poisonous to fish. Such chemicals and synthetic medicines should never be used in proximity of the pond. Prevention is crucial in this regard. Any existing or potential source of water pollution must be thoroughly investigated. If contamination is suspected, analyses of the water and the fish must be done by sending samples to specialized laboratories.
FiBL (2011): African Organic Agriculture Training Manual. Version 1.0 June 2011. Edited by Gilles Weidmann and Lukas Kilcher. Research Institute of Organic Agriculture FiBL, Frick