Effects of sub-lethal CO_2(aq) concentrations on the performance of intensively reared gilthead seabream (Sparus aurata) in brackish water: Flow-through experiments and full-scale RAS results

The effects of sub-lethal CO_2(aq) concentrations were tested for the first time on gilthead seabream (Sparus aurata) juveniles (4-25g; 64 growth days) and adult (~300-400g; 71d) fish, both in fully controlled pilot tests and the latter also as part of full-scale RAS (recirculating aquaculture system) operation. In the pilot experiments (concentration range 5.2-56.3mg CO₂/L) the specific growth rate, mortality rate, and physical fish disorders were monitored. In the full scale experiment, two groups of fish, originally from the same batch, were exposed for 197d to controlled (by NaOH dosage) and uncontrolled pH conditions, resulting in exposure of the fish to significantly different CO_2(aq) concentrations. The pilot results showed, as expected, that the seabream fish grew faster at the lower CO₂ concentrations and that the growth rate of both juveniles and adult fish was only minimally inhibited up to roughly 20mg CO₂/L (compared to a previously published curve). Mortality rate was considerable only at the highest CO₂ concentration (~56mg CO₂/L). Physical irregularities were not observed, apart from abnormally high absence of swim bladder at the highest CO_2(aq) treatment. The (statistically significant) results from the full-scale RAS operation showed that growing gilthead seabream for 197d at roughly constant and relatively low (~16mg/L) CO_2(aq) concentration resulted in fish with ~10% larger mean weight relative to the fish grown in ponds in which CO_2(aq) was not controlled and its concentration fluctuated daily between 19 and 37mg/L.