Effects of magnesium reduction in artificial low-salinity water on the growth of Pacific white shrimp Litopenaeus vannamei in a biofloc system

Intensive shrimp production in salinized waters proposes to supply fresh seafood away from the coast. However, the high prices of specific salts, such as magnesium chloride, excessively increase costs, impairing financial returns. A 77-day study was conducted to evaluate the effects of reductions in magnesium [Mg2+] concentrations and magnesium: calcium ratio (Mg2+: Ca2+) of artificial seawater at salinity 5 g L−1 on the zootechnical performance of shrimp in the nursery and grow-out phases in a biofloc system. The experiment was composed of five treatments and three repetitions each, with the Mg2+: Ca2+ ratios of 3:1, 2:1, 1:1, 0.5:1, and 0.1:1. Both phases were carried out in 40 L tanks. In the nursery, post-larvae (0.017 ± 0.001 g) were stocked at a density of 1500 m−3 and reared for six weeks until they reached approximately 1.2 g. Grow-out was carried out at a density of 500 m−3 for five weeks. Water quality parameters, major ion composition, and zootechnical performance were evaluated during the experiment. The evolution of the concentrations of total ammoniacal nitrogen, nitrite‑nitrogen, and nitrate‑nitrogen in the 0.1:1 treatment showed different behaviors compared to the other treatments throughout the experimental period. At the end of the nursery and grow-out phases, Mg2+ concentrations in the 0.1:1 treatment were 26.6 ± 2.1 mg L−1 and 7.1 ± 1.0 mg L−1, respectively. In the nursery, final weight, food conversion ratio (FCR), and yield were affected in the 0.5:1 and 0.1:1 treatments compared to the control. In grow-out, FCR, yield, and survival were affected at the 0.1:1 ratio. In addition to low [Mg2+], high nitrite and nitrate concentrations were responsible for the low growth and survival at the 0.1:1 ratio. The study showed that it is possible to reduce [Mg2+] and the Mg2+: Ca2+ to approximately 30 mg L−1 and 1:1, respectively, in a biofloc system without affecting shrimp performance.