Comparative evaluation of calcium and magnesium concentrations in growth and survival of pearlspot ( Etroplus suratensis ) reared in the constituted saltwater and freshwater

The present study evaluated the growth and survival of pearlspot, Etroplus suratensis, under two rearing environments, freshwater (0 ppt) and constituted saltwater (10 ppt), which differed in calcium and magnesium concentrations due to the addition of raw sea salt in the constituted saltwater. A 60-day experiment (initial weight: 29 ± 2.5 g and stocking density: 20 fish/900 L) was conducted in duplicate in 1200-L fiber reinforcement tanks, each equipped with a biofilter. Water quality parameters and growth performance metrics of pearlspot were assessed in both treatments. Constituted saltwater exhibited substantially higher concentrations of calcium (38 mg/L), magnesium (285 mg/L) and total hardness (336 mg/L as CaCO3) compared to freshwater (calcium: 9 mg/L; magnesium: 42 mg/L; total hardness: 50 mg/L as CaCO3). Fish reared in constituted saltwater showed a significantly higher final mean weight (p < .05), daily weight gain (p < .01), percentage weight gain (p < .001) and condition factor (p < .001). Survival was also significantly higher (p < .05) in constituted saltwater, whereas mortality increased in freshwater, likely due to osmoregulatory stress. These findings highlight the importance of calcium and magnesium concentration in rearing water and support the use of raw sea salt to constitute saltwater at 10 ppt as a suitable medium for rearing E. suratensis in recirculatory aquaculture-based hatchery and nursery systems (RAS).