Elevated CO2 negatively affects marine fish. In a recirculating aquaculture systems, we exposed juvenile Scophthalmus maximus, a CO2¬-sensitive, high-value species , to CO2 at 0 mg/L (control), or at 8, 16, 24, or 32 mg/L, for 7, 14, 30, or 60 d. Cumulative survival decreased significantly with increasing CO2 , to 68% at 32 mg/L. Weight gain, specific growth rate, and feed conversion rate differed significantly between the control and maximum concentration. CO2 caused histopathological damage. Plasma glutamate pyruvate transaminase and glutamate oxalate transaminase were significantly and substantially elevated at 24 and 32 mg/L CO¬2, relative to the control. At 32 mg/L, hemoglobin was significantly reduced, and methemoglobin significantly elevated , indicating reduced oxygen-carrying capacity. GHR, IGF-1, IGF-1R, and THR expression were substantially lower at 32 mg/L than in the control. For Scophthalmus maximus , these findings indicate that elevated CO2 retards growth, impairs health, and causes metabolic disorders, possibly by impairing liver function.