Partial seawater inclusion to improve Litopenaeus vannamei performance in low salinity biofloc systems

Abstract Litopenaeus vannamei production in oligohaline waters (0.5–5 g L−1) is a potential alternative for the continuous expansion of shrimp farming. However, ionic imbalance may affect physiological processes and productivity. In this work, natural seawater inoculation was used as strategy for correction of ionic composition in a biofloc-dominated oligohaline system and its effects on the water quality and nursery performance were evaluated. A 27-day trial was conducted using an experimental biofloc-dominated oligohaline water, salt formulation was based on the mean concentrations of the major ions present in natural oligohaline waters found in Northeastern Brazil. The trial was composed of four treatments of natural seawater inoculation (3, 6, 10 and 13%) and a control, without inoculation (0%); all with four replicates. Post-larvae (PL) of 22 days (0.057 ± 0.002 g) were stocked at 1000 PL m−3 in 20,150 L indoor circular tanks. Shrimp were fed 40% crude protein commercial diets twice a day at rations adjusted weekly. Water temperature, oxygen saturation, pH, total ammonia nitrogen, nitrite, nitrate, phosphate, alkalinity, hardness, salinity, conductivity, total suspended solids and settleable solids were monitored. Concentrations of the major ions were measured at the Days 0, 14 and 27. Shrimp sampling was performed weekly and the final zootechnical performance was evaluated by final weight, survival, specific growth rate (SGR), weekly growth rate (WGR), yield and food conversion ratio. Salinity, conductivity and hardness significantly increased according to the increase of seawater inoculation, whereas the other parameters did not differ; all of them were within the range recommended for shrimp culture at low salinity, except nitrate for 0%. Concentrations of Cl−, Na+, Mg2+ and K+ and Na+:K+ ratio significantly increased according to the increase of seawater inoculation. Final weight, survival, SGR, WGR and yield were significantly lower in the 0%, but no differences were found among the treatments (from 3% to 13%). The results found in this work indicate that a seawater inoculation at 3% was an efficient strategy to mitigate negative effects of the ionic imbalance on growth performance of PL reared in biofloc-dominated oligohaline systems.