The effects of lime in aquaculture.

(Baonghean) - First of all, lime helps to reduce soil and water acidity, eliminate unwanted fish, pests, algae, and even pathogens in the pond. Lime helps decompose the sediment at the bottom of the pond, improving the pond bottom due to mineralization, and also improving water quality, thus enriching the natural food source for shrimp and fish. For farmed shrimp, lime also helps to harden the shell after molting. However, excessive lime application should be avoided as it can harm the environment, fish, and farmed shrimp.

Inspecting the fish farming environment in Dien Hong commune, Dien Chau district.

- If renovating a fish pond, use powdered lime CaCO3 or slaked lime Ca(OH)2, at a dosage of 10-15kg/100m2.

- If used to stabilize pH, use powdered lime (CaCO3) in the following dosages:

+ For commercial fish and shrimp ponds: use a dosage of 1-2 kg/100m2, mix with water and spread into the pond.

For cage fish farming, use a dosage of 2-4 kg/10m³ of water, hanging small bags at the beginning of the water flow.

- To adjust the clarity of pond water, use lime (CaCO3) at a dosage of 1-2 kg/100m2, dissolve it in water and spread it throughout the pond; the clarity will return to normal.

- If used for disease prevention in shrimp and fish, agricultural lime (CaCO3) should be applied to the pond every 10-15 days at a dosage of 1-2 kg/100m2 (for cage or raft farming, hang lime bags at a dosage of 2-4 kg/10m3 of water). This is the most effective and inexpensive method of disease prevention for fish and shrimp.

In aquaculture, water pH fluctuates frequently in a diurnal cycle, with the amplitude increasing gradually from the beginning to the end of the farming season. Additionally, water pH can be low due to acidic soil, especially a sudden drop in pH after rain. Excessive pH fluctuations directly affect the cultured organisms, specifically slowing growth and potentially leading to high mortality rates. Very high pH (at midday) or very low pH (in the early morning) also indirectly increases the levels of toxic gases NH3 and H2S. High pH increases NH3 levels, while low pH increases H2S levels, which are toxic to fish and shrimp. Therefore, farmers often use lime to maintain stable pH, promoting good growth, high survival rates, and high productivity. Cases where ponds require lime application include:

- The pond has a nutritional imbalance with excessive organic matter and sediment at the bottom. This nutritional imbalance is compounded by the presence of alum in the water.

- The pond water has low alkalinity.

- The water has a high CO2 content.

The types of lime commonly used in aquaculture include four types: agricultural lime or limestone (CaCO3), dolomite or black limestone (CaMg(CO3)2), slaked lime (Ca(OH)2), and quicklime (CaO). The higher the amount of impurities in the lime, the lower its neutralization efficiency. Therefore, when using lime, it is important to choose fine lime (particle size smaller than 0.25 mm) with few impurities to achieve the highest neutralization efficiency.

Additionally, liming ponds can reduce turbidity caused by silt. The amount of lime needed for pond water is usually determined based on total alkalinity. The appropriate total alkalinity is greater than 40 mg CaCO3/L for freshwater aquaculture ponds and greater than 80 mg CaCO3/L for saltwater and brackish water aquaculture ponds. To ensure the alkalinity reaches the acceptable level in the pond, the water alkalinity should be checked again 2-3 weeks after liming. If the alkalinity is not yet at the required level, additional lime should be applied.

Le Hang (Agricultural Extension Center)