The effect of calcium hydroxide, carbonate and sodium bicarbonate on water quality and zootechnical performance of shrimp Litopenaeus vannamei grown with biofloc technology (BFT)
The shrimp Litopenaeus vannamei is the most cultivated in super-intensive systems in the middle of microbial flakes (BFT) and without water renewal. In these biofloc systems, the natural tendency is for the carbonate material to decrease during cultivation, due to the nitrification processes that reduce alkalinity in the form of carbonates and bicarbonates. In addition, the pH may decrease due to the reduction of alkalinity and the accumulation of dissolved carbon dioxide from breathing. The objective of the study was to evaluate the effect of the application of calcium hydroxide, carbonate and sodium bicarbonate on the water quality of L. vannamei cultivation in BFT system. For this purpose, 600 juveniles (6g) were stored in 12 tanks with 150L of useful volume (ρ = 333/m3). The prawns were fed twice a day,with commercial feed (Guabi®) following the feeding table for 60 days of experiment. Four treatments were evaluated with three repetitions each: T1- Na2CO3 (pH correction above 7.5); T2- Ca (OH) 2 (correction of alkalinity above 100mg/L of CaCO3 and pH above 7.5); T3- NaHCO3 (correction of alkalinity above 100 mg/L of CaCO3); and T4- Control (without correction of pH and alkalinity). To correct pH, alkalinity and both, 0.06 g/L sodium carbonate, 0.20 g/L sodium bicarbonate and 0.15 g/L hydrated lime were used, respectively. The results of physical, chemical and biological parameters showed a significant difference (p <0.05) between treatments. Control animals showed lower zootechnical performance (p <0.05) than other treatments.The applications of sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) resulted in favorable water quality conditions for the growth of cultured bioflocs and shrimp, however the costs of these compounds make them unattractive for application on a commercial scale. Hydrated lime, on the other hand, presented the best cost-benefit ratio. Finally, this study makes it clear that the levels of alkalinity and pH decrease throughout the cultivation and that the levels of dissolved CO2 increase in super-intensive systems, with bioflocs, without water renewal. In addition, the quality of the farming water and the zootechnical performance of the shrimp are negatively affected when the alkalinity levels remain for long periods below 100 mg CaCO3/L and the pH below 7. Therefore,correction of alkalinity and pH is necessary through the application of carbonate materials or calcium hydroxide.