Maintenance and monitoring

To achieve a high production of fish in the pond, regular maintenance and monitoring is vital. Daily management includes:

1. Checking the water quality (oxygen, pH, colour, transparency, temperature, etc.)

2. Checking the pond for possible water leaks

3. Cleaning the screen of the water inlet and outlet

4. Observing the fish while they feed: Do they eat normally? Are they active? If not, and if they are gasping for air at the surface, the oxygen level in the water is too low. Stop feeding and fertilising and let water flow through the pond until the fish behave normally again. Otherwise, look for symptoms that could indicate a disease.

5. Watching out for predators, or signs of predators such as footprints, and taking precautions if necessary

6. Removing aquatic weeds growing in the pond

Water quality is a vital factor for good health and growth in fish. Some of the most important water characteristics are described below.

Oxygen

Oxygen is a gas that is produced by all plants in the pond (therefore also by phytoplankton) with the help of sunlight. The more sunlight falls on the pond and the larger the quantity of phytoplankton, the higher the oxygen-production will be. The oxygen produced partly dissolves in the water and the rest escapes to the air.

If fish are gulping for oxygen at the water surface, you can solve this problem by flowing extra freshwater through the pond. Stirring up the water in the pond also helps to increase the amount of dissolved oxygen. Do not feed and fertilise the pond at this moment because this is often one of the reasons for the oxygen shortage. Over-stocking of fish in the pond could be another possible cause of oxygen shortage problems. This can cause oxygen stress for the fish, which can result in disease outbreaks and mortality.

Water acidity, alkalinity and hardness

Water suitable for fish farming should have a certain degree of acidity, indicated by the water pH-value. This should preferably range between 6.7 and 8.6 . Values above or below this range inhibit good fish growth and reproduction. Phytoplankton require a pH of about 7 and zooplankton (tiny animals in the pond water on which the fish feed) a slightly lower pH of 6.5. The effect of pH on fish growth Sometimes the pH of the pond water can change quickly.

For example, heavy rain may carry acid substances, dissolved from the soil into runoff water, into the pond. In this way, the pond water gets more acid and thus the pH-value decreases.

The best way to increase the pH-value of the water again to neutral (about 7) is to add lime to the pond .

So, the aim of adding lime is to increase either water alkalinity, water hardness or pond water pH (to about 7). Ponds that have just been built need a different treatment than ponds that have already been limed before.

Newly built ponds

These should be treated with 20 to 150 kg agricultural lime per 100 m2 (Appendix 2). This is mixed with the upper (5 cm) layer of the pond bottom. The pond is subsequently filled with water to a depth of 30 cm. Within one week the pH of the pond water should have reached 7 and you can start fertilising.

Ponds limed before

These should be treated with 10 to 15 kg quicklime per 100 m2 , added to the damp pond bottom to get rid of fish pathogens, fish parasites and fish predators. After a period of 7 to 14 days the ponds should be refilled with water. After filling the pond to a depth of 30 cm, the pH of the water can be adjusted by adding agricultural lime .

Toxic substances

Toxic substances in the water supply of the pond can decrease fish production seriously, so it is wise to investigate any existing or potential sources of water pollution in the vicinity of the pond. Many chemicals used in animal husbandry and crop cultivation are poisonous to fish. Therefore, chemicals should never be used in the area around the pond, especially avoid spraying on windy days.