- a b s t r a c t The effects of sub-lethal CO2(aq) concentrations were tested for the first time on gilthead seabream (Sparus aurata) juveniles (4-25 g; 64 growth days) and adult (∼300-400 g; 71 d) 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 5.2-56.3 mg CO2/L) the specific growth rate, mortality rate, and physi- cal fish disorders were monitored. In the full scale experiment, two groups of fish, originally from the same batch, were exposed for 197 d to controlled (by NaOH dosage) and uncontrolled pH conditions, result- ing in exposure of the fish to significantly different CO2(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 20 mg CO2/L (compared to a pre- viously published curve). Mortality rate was considerable only at the highest CO2 concentration (∼56 mg CO2/L). Physical irregularities were not observed, apart from abnormally high absence of swim bladder at the highest CO2(aq) treatment. The (statistically significant) results from the full-scale RAS operation showed that growing gilthead seabream for 197 d at roughly constant and relatively low (∼16 mg/L) CO2(aq) concentration resulted in fish with ∼10% larger mean weight relative to the fish grown in ponds in which CO2(aq) was not controlled and its concentration fluctuated daily between 19 and 37 mg/L. © 2013 Elsevier B.V. All rights reserved.