Water quality is crucial for the farming of freshwater fish (aquaculture) in South Africa as it ensures a healthy water environment for the fish. Good water quality influences the growth of fish (good quality water = faster growth), the occurrence of diseases as well as the taste and colour of the fish.
Water quality is determined by four main factors: Water temperature Oxygen levels in the water Ammonia levels in the water Water pH (the level of acidity or alkalinity) Most water quality problems with freshwater fish farming are due to oxygen depletion, high ammonia levels or high pH levels.
These problems are very closely linked to the levels of microscopic algae in the water. The growth of these algae depends on the amount of nutrients - nitrogen (N) and phosphorus (P) - in the water. These nutrients enter the dam via: The source water (inflow of water into the dam). The inflow of sediments during heavy rainfall events (runoff). Fish feed during the production cycle.
Water Temperature in Fish Farming
Trout generally grow optimally between 14 and 18°C (16°C optimum water temperature) but warm species like sharptooth catfish grow well in warmer water (20 - 30°C) and tilapia prefer even higher water temperatures of 24 - 34°C. Trout are sensitive to warm water.
Temperatures over 20°C will cause trout to stop growing and will create other production problems, such as in creased risk of diseases and oxygen problems, explains Henk Stander principal technical manager of Aquaculture: Department of Animal Sciences, Stellenbosch University.
These problems (increased disease risk and oxygen reduction) are mainly caused by too many microscopic algae in warmer water.
As soon as temperatures reach 21°C and above, special care must be taken to prevent fish deaths and the on-set of algae taint. Algal taints are caused by the increase in blue-green algae numbers triggered by higher temperatures and nutrient availability.
The algae release chemical compounds such as the earthy-smelling substance, geosmin. This substance is absorbed by fish and changes the taste of the fish meat.
Oxygen Levels in Fish Farming
The oxygen levels of water in freshwater fish farming must be 5 mg/L and above at all times. If oxygen levels in the water are too low, the fish will not be able to breathe well and will be stressed. Oxygen problems are mainly caused by microscopic algae.
These small algae photosynthesize and produce oxygen. With normal levels of these algae, there will be no problems. If levels of these algae increase, the levels of oxygen during night time can become lower than 5 mg/L and there will be oxygen problems in the water the following morning.
Secondly, if large amounts of small algae die suddenly (due to cloudy weather conditions or spraying of algaecides), aerobic bacteria will grow massively to help decompose the dead algae. These aerobic bacteria consume oxygen which will decrease oxygen levels in the dams rapidly and cause stress to the fish.
Ammonia Levels in Fresh Water
Ammonia is a component of nitrogen (N) in water. Unless there is a direct inflow of ammonia or the water is in very anoxic conditions (not enough oxygen), most nitrogen in water will be in the form of nitrate, which is not harmful to fish.
Ammonia occurs in a toxic (free ammonia) and non-toxic (ionised ammonia) form but the toxic form is usually less than 10% of the total ammonia concentration.
In warmer water (+20°C) with a high pH (8.5), toxic ammonia levels will increase. Ammonia levels will mostly not create problems in dams unless they are very shallow (5 m or less) and with a low annual turnover rate of less than one. (‘Annual turnover’ is the rate at which a dam’s full water volume is replenished by rain or inflow water).
The pH of Dam Water
The pH of water tells us if a dam is more acidic or alkaline. A pH of 7 is neutral. Most freshwater fish species can tolerate pH levels of 5 to 9. The most important factor that will increase pH levels is the growth of small algae.
An abundance of these algae can increase the pH levels in especially small farm dams to levels higher than pH 9 (especially in the afternoon).
This is due to the removal of carbon dioxide during the photosynthesis process. This high pH will put fish under stress. To adjust/improve these levels, a water change is needed, add freshwater if possible. Otherwise, add barley straw.
By Marinda Louw