MAINTAINING MINERAL BALANCE IN SHRIMP CULTURE SYSTEMS
The Pacific white shrimp, Litopenaeus vannamei is reared in various regions in Asia including India. Though L. vannamei is a euryhaline crustacean, tolerant to salinities ranging from 0.5 to 40 ppt, the availability of various minerals in water in the right proportions is important to enhance productivity. The right balance of minerals helps in reducing the energy spent on maintaining osmotic equilibrium. The leading mineral products are positioned and sold for shrimp culture waters, irrespective of salinity or mineral composition of culture waters.
Objective of the study was to understand the mineral distribution across shrimp culture ponds and to develop suitable mineral supplements to increase the productivity in shrimp culture. Water samples were collected from 46 different locations including high saline and low saline culture systems. The salinity was measured using salinometer and the samples were analyzed for various minerals including sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), sulfate (SO4), phosphorus (P), chlorine (Cl) and iron (Fe) using standard methods.
To achieve optimum performance at any given salinity, the proportion of minerals should be similar to the corresponding levels in seawater. Seawater with a salinity of 35 ppt has 0.38 ppt of potassium and hence similar optimal potassium concentration in water of 4ppt salinity should be 0.043 ppt. With this information, optimal levels of each mineral were calculated for each water sample based on its salinity and compared to the measured mineral levels in the same sample. It was found that magnesium was highly deficient (lower than the optimal requirement by 100-200 mg/L) in culture ponds above 15 ppt salinity, however the magnesium levels in low saline waters (<15 ppt) met the requirement. Potassium deficiency was found across all salinities (lower than optimal level by 33-53 mg/L). Levels of calcium and sulfate were found to be excess than required across all salinity ranges in culture waters; the reason may be due to practice of liming and gypsum application during pond preparation. Sodium and chlorine levels correlated well with salinity values in all the samples. Phosphorus and iron were found to be negligible in most of the culture waters. Based on these insights, two variants of mineral formulations were prepared for maintaining the mineral balance in low saline (<15 ppt) and high saline (>15 ppt) culture systems to achieve optimum performance in shrimp culture.