Effect of ionic adjustment frequency in low-salinity water on zootechnical performance, water quality and mineral composition of Litopenaeus vannamei in a synbiotic nursery system

Abstract

Ions are important for shrimp metabolic processes and can affect their growth. However, little is known about the need to supplement the major ions to maintain an adequate Ca:Mg:K ratio for shrimp as well as how often these ratios should be adjusted during a culture cycle using low-salinity water so that shrimp growth is not affected. The aim of this study was to evaluate the effect of ionic adjustment frequencies in low-salinity water in a synbiotic nursery system and verify its effects on Litopenaeus vannamei growth, water quality, and mineral composition of microbial flocs and shrimp. The experiment was carried out for 40 days, with a density of 2000 post-larvae m−3. The following treatments were tested, in triplicate: SW (seawater - salinity 31 g L−1); SWD (seawater diluted to a salinity of 2.3 g L−1); 1IA (ionic adjustment to achieve Ca:Mg:K ratio of 1:3:1 only on day 1 - salinity 2.3 g L−1); 2IA (ionic adjustment to achieve Ca:Mg:K ratio of 1:3:1 on day 1 and day 20 - salinity 2.3 g L−1); and 3IA (ionic adjustment to achieve Ca:Mg:K ratio of 1:3:1 on day 1, day 10, and day 20 - salinity 2.3 g L−1). Shrimp were fed four times a day with a 45% protein diet. The synbiotic fertilization protocol used a powder blend of Bacillus subtilis, B. licheniformis, Pseudomonas sp., and instant dry biological yeast (Saccharomyces cerevisiae) every 3 days. Nitrogen compounds remained within the recommended levels (TAN = 0.66 mg L−1, NO2−-N = 0.52 mg L−1, NO3−-N = 1.91 mg L−1) for L. vannamei reared in low-salinity water. Mean weight (1.14 to 1.18 g), survival (88.61 to 94.44%) and yield (2.10 to 2.15 kg m−3) were not significantly different in low-salinity treatments but were higher than those found in SW treatment (mean weight = 1.02 g; survival = 84.72%; and yield = 1.73 kg m−3). Mineral concentration in microbial floc and shrimp biomass did not differ among the frequencies tested, only differing when compared to SW treatment. To evaluate the juvenile's resistance, an ammonia stress test was performed during 96 h, obtaining 100% survival for all treatments. According to the results, L. vannamei juveniles can be produced in diluted seawater (salinity 2.3 g L−1, Ca2+ = 38.0 mg L−1, Mg2+ = 96.0 mg L−1, K+ = 48.3 mg L−1, total alkalinity = 92.33 mg CaCO3 L−1, total hardness = 490.13 mg CaCO3 L−1, Ca:Mg:K ratio 1:2.5:1.3) without ionic adjustments and without water exchange for 40 days in the nursery phase using a synbiotic system without affecting production performance.