Effects of salinity on growth characteristics and osmoregulation of juvenile cobia,Rachycentron canadum (Linnaeus 1766), reared in potassium‐amended inland saline groundwater

The suitability of inland saline groundwater as a medium to culture juvenile cobia, Rachycentron canadum, was assessed. In the first experiment, juvenile cobia stocked in raw (unamended) saline groundwater at salinities of 5, 10, and 15 g/L exhibited complete mortality after 108, 176, and 195 hr, respectively. The second experiment evaluated the rearing of juvenile cobia (mean weight ~9.23 ± 0.12 g) in potassium (K⁺)‐amended saline groundwater (100% K⁺ fortified) and reconstituted seawater at salinities of 5, 10, and 15 g/L to assess growth and osmoregulation in distinct culture media. Following 60 days of culture, all fish survived the experimental period. Final mean bodyweight of cobia reared in K⁺‐amended saline groundwater (103.2–115.8 g) and seawater (111.2–113.8 g) of different salinities did not vary significantly (p > .05). No differences (p > .05) were observed in specific growth rate, weight gain (%), and feed conversion ratio between treatment groups. Serum osmolality increased with salinity and was significantly higher (p < .05) for fish in K⁺‐amended saline groundwater (353–361 mOsmol/Kg) than in reconstituted seawater (319–332 mOsmol/Kg), although differences were not observed between salinities by water type. Cobia stocked in saline groundwater of different salinities were osmoregulating normally, and the higher values observed may be because of variations in ionic composition and other interfering ions in saline groundwater. Trial results suggest that juvenile cobia can achieve optimal growth in K⁺‐amended saline groundwater of low and intermediate salinities.