The white leg shrimp, Litopenaeus vannamei is an economically important exotic candidate species being cultured in India with high marketable value and minimum challenges during culture. Water quality parameters play an important role in Vannamei shrimp culture. The poor water quality is the leading cause of disease, stop feeding high FCR, retardation of growth, mortality, crop failure & economical losses. The maintenance of good water quality is essential for both survival and optimum growth of culture organisms. The levels of metabolites in pond water that can have an adverse effect on growth are generally an order of magnitude lower then those tolerated by fishes/prawns/ shrimps for survival. Good water quality is characterized by adequate oxygen and limited levels of metabolites.
The following water quality parameters are considered to be the most important in shrimp culture.
Dissolved oxygen pH Ammonia Temperature Alkalinity Hardness Hydrogen sulfide Salinity & Turbidity.
The pH alkalinity and Hardness are having greater effect to growth and survivals of shrimp as well as shrimp culture pond environment. Carbon dioxide, pH, Alkalinity and hardness are interrelated and can have profound effects on pond productivity, the level of stress and shrimp health, oxygen availability and the toxicity of ammonia as well as that of certain metals. Carbon dioxide and pH fluctuate or cycle daily. Alkalinity and hardness are relatively stable but can change over time, usually weeks to months, depending on the pH or mineral content of watershed and bottom soils.
As water pH is one of the most critical chemical parameters for shrimp farming. pH is the measure of hydrogen ion concentration. The optimum water pH range in the shrimp pond is 7.8-8.5. It is essential to stabilize the pH with in this range. The pH value in the water is normally lowest in the early morning and highest in the afternoon. For the best water quality, the maximum diurnal pH fluctuation should not exceed 0.5. It is important to maintain a stable pH at a safe range because it affects the metabolism and other physiological processes of culture organisms.
The pH will vary in pond environment depending on a number of factors
- Acid sulfate soil (acidic alum soil, acidic soil), acidic source of water
- Rate of rainfalls in pond areas
- Poorly buffered water
- Stocking density of shrimps
- Feeding & rate of sludge formation in pond bottom.
- Presence of micro/ macro organisms.
- Existence of phytoplankton in pond water. Rate of carbon dioxide production in pond water
- Quantity of respire by aquatic species in the pond water.
pH levels of the pond water will change depending on the aquatic life within the pond. Carbon dioxide produced by aquatic organisms when they respire has an acidic reaction in the water. The pH in ponds will rise during the day as phytoplankton and other aquatic plants remove CO2 from the water during photosynthesis. The pH decreases at night because of respiration and production of CO2 by all organisms. The fluctuation of pH levels within the pond. Waters of moderate alkalinity are more buffered and the degree of pH fluctuation is lower.
Affect of Sub-optimal pH:
Sub-optimal pH has a number of adverse affects on vannamei shrimps. If pH changes significantly, it can make shrimp shocked, weakened and stop eating. In high or low pH extends for a long time, it will make shrimp grow slowly, stunting growth and susceptible to diseases. It can cause stress, less survivals, low production and leads to poor growth. Signs of sub +optimal pH include increase mucus on the gill surfaces, black gill disease, damage to the eye lens, abnormal swimming behavior, loose shell, soft shell, irregularity in molt, poor phytoplankton and zooplankton growth. The mortality may occur for culture shrimps in below pH and above pH 10 range respectively. The low pH levels will cause the shell of shrimp to become soft. This is due to the shell of the shrimp being composed of calcium carbonate which reacts with acid.
High pH may also increase the toxicity of other substances. For example, the toxicity of ammonia is ten times more severe at a pH of 8 than it is at pH 7. It is directly toxic to aquatic life when it appears in alkaline conditions. Low concentrations of ammonia are generally permitted for discharge.