Relevance of alkalinity, pH and carbon dioxide in the survival and growth of Litopenaeus vannamei (Boone 1931) in a biofloc system
The cultivation of aquatic organisms in systems without water renewal that use high stocking densities, strong aeration and predominantly aerobic and heterotrophic biota, forming microbial flakes is called biofloc technology (BFT). These systems are considered biosafety and allow to increase the productivity of cultivation due to the nutritional supplement provided by bioflocs. As there is no renewal of water during cultivation and the water can be reused for several production cycles, there is a reduction in the levels of alkalinity and pH, and there may also be an accumulation of carbon dioxide (CO2), resulting from respiratory processes shrimp and bioflocs. Changes in the quality of farming water generate stress and reduce the growth and responsiveness of the immune system,affecting the survival of the shrimp. This work seeks to improve the cultivation techniques of Litopenaeus vannamei in biofloc systems without water renewal, understanding and correcting the concentrations of carbon dioxide, alkalinity and pH. To this end, five experiments were carried out at the marine aquaculture station at the Federal University of Rio Grande (EMA-FURG-Brazil) and one at the Universidad Nacional Autónoma de México (UMDI-Sisal-UNAM): 1. Determination of the lethal level of acid pH and basic (pH50-96h) and evaluation of the sublethal effect of pH on oxidative stress parameters in juveniles of L. vannamei; 2. Determination of the lethal concentration and safety level of carbon dioxide for juveniles of L. vannamei; 3.Verification of CO2 concentrations in different crop densities and suspended solids when the aeration system fails and hydrogen peroxide (H2O2) is applied as an oxygen source in the BFT system; 4. Evaluation of the effect of CO2 on oxygen consumption of L. vannamei juveniles grown in BFT system; 5. Evaluation of the effect of different concentrations of alkalinity on water quality parameters, on the formation of bioflocs and on the zootechnical performance of L. vannamei grown in BFT system; 6. Determination of the best dosage of calcium hydroxide (Ca (OH) 2) in the parameters of water quality and zootechnical performance of L. vannamei grown in BFT system. In the experimental conditions described in chapter 1, the pH50 96 h acid and basic pH were 4.04 and 9.58 for juveniles of L. vannamei.Regarding the oxidative stress test, we found that shrimp exposed to both pH 4.5 and pH 9.5 showed an imbalance in the activity of antioxidant enzymes. In Chapter 2, we found the LC50 values and 95% confidence limits at 24, 48, 72 and 96 h were 130.05 (104.2-162.1), 77.2 (73.8-80.02 ), 69.65 (65.47-74.32), 59.12 (53.08-66.07) mg/L of CO2, respectively. Thus, the safety level calculated for L. vannamei was 5.9 mg/L CO2. In chapter 3 we verified that the CO2 concentrations increased with the increments of biomass and total suspended solids over time, reaching sublethal CO2 values for the shrimp. In chapter 4 there was an increase in oxygen consumption of juveniles at concentrations of up to 60 mgCO2/L and a reduction in oxygen consumption at concentrations of 95,150 and 300 mgCO2/L followed by symptoms of anesthesia and mortality. In Chapter 5, it was possible to form bioflocs with alkalinity values above 70 mgCaCO3/L and pH above 7.38. However, the best nitrification rates and best zootechnical performance occurred in treatments with higher alkalinity. In chapter 6 for the correction of alkalinity, pH and CO2, dosages of 0.05 g/L of calcium hydroxide can be applied or daily applications between 10 and 20% of the amount of feed offered to the shrimp. The results obtained may help the daily life of shrimp producers in BFT systems.