Survival and osmoregulation of juvenile of hybrid grouper (Epinephelus fuscoguttatus × Epinephelus lanceolatus) during acclimation in calcium-supplemented freshwater

The aim of this research was to evaluate the effect of calcium concentration in water to the osmoregulation and survival of hybrid grouper (Epinephelus fuscoguttatus × Epinephelus lanceolatus) acclimated in freshwater environment. There was no mortality observed in all groups during acclimation to the freshwater up to 3 ‰. Once the fish reached the freshwater (0 ‰), rapid mortality was occurred. Lower salinity induced the lower serum osmolality. Lowest serum osmolality value was observed in the positive control group at 6 h post-transfer (HPT) to freshwater. The values of muscle water content were not significantly different among all groups in all salinities. The highest Na⁺–K⁺-ATPase (NKA) activities were observed in the 500 and 1000 mg/L CaCl₂ groups at D₁₀ 1 and 6 HPT, respectively, whereas lowest NKA activities were recorded in the positive control and 100 mg/L CaCl₂ groups at D₁₀ 6 HPT. This study shows that the hybrid grouper demonstrates excellent tolerance in environment up to 3 ‰.