The effects of magnesium concentration in low-salinity water on growth of Pacific white shrimp (Litopenaeus vannamei)

The current study evaluates the effect of different Mg levels in low-salinity (3 g/L) water on growth, survival, haemolymph osmolality, cationic composition in haemolymph, carapace and whole body of Pacific white shrimp,Litopenaeus vannamei. A low-cost salt mixture (LCSM) comprised of Na, K, Ca and Mg concentrations of 298, 9, 17 and 39 mg/L, respectively, in a 1-g/L solution was modified by decreasing the Mg level of LCSM to allow different Mg levels in the culture medium (100, 78, 55, 30, 17, 13 and 12 mg/L). Reconstituted sea salt (RSS) was used as the control treatment with Mg level of 129 mg/L. A 42-day growth trial (initial weight: 0.11 +/- 0.01 g and stocking density: 20 shrimp/tank) was carried out in 150-L plastic tanks, each equipped with a fluidized bed biofilter. As Mg(2+)level was reduced, there was a subsequent reduction (p < 0.05) in shrimp performance reflected in final weight, weight gain, haemolymph osmolality, osmoregulatory capacity, Mg(2+)concentration in haemolymph, carapace and whole body of shrimp paralleled by an increase in feed conversion ratio (FCR). Low haemolymph osmolality and Mg(2+)concentrations (p < 0.05) in shrimp haemolymph were found to be indicative of stress, which was ultimately reflected as reduced growth and increased FCR in low Mg(2+)concentrations at 3 g/L salinity.