The effect of dissolved carbon dioxide (CO₂) on white grouper (Epinephelus aeneus) performance, swimbladder inflation and skeletal deformities

The effect of three seawater CO₂ concentrations (0.8 ± 0.18, 5.64 ± 0.64 and 28.58 ± 3 mg l^{− 1}), was tested on white grouper (Epinephelus aeneus) post-larvae reared at two water temperatures (23 and 27 °C) in a 30 aquarium (17 l each) experimental system. Forty-five dph fish were stocked per aquarium allowing the testing of the 6 CO₂ temperature treatment combinations in replicates of 5 aquaria treatment^{− 1} for 60 days. The final weight of fish reared under the high CO₂ treatment was 25% and 10% (P < 0.05) lower than that of fish reared under the control conditions at 23° and 27 °C, respectively. A similar pattern in total length (TL) was demonstrated where fish exposed to the high CO₂ treatment grew 10 and 7.4% more slowly (P < 0.05) than the control at 23° and 27 °C, respectively. A large percentage (78.8 ± 6.1%) of fish reared at the control CO₂ concentration (0.8 ± 0.18 mg l^{− 1}) and exposed to 27 °C had inflated swimbladders compared to only 17.6 ± 9.2% in fish exposed to the high CO₂ concentration (28.58 ± 3 mg l^{− 1}) at 23 °C. Following the 60 days of the CO₂ and temperature exposure period (45–105 dph), 87–100% of the fish from the six treatment combinations, independent of temperature and CO₂ exposure history, had inflated swimbladders at 150 dph. It was shown that, during the recovery phase (125 dph) at 23 °C, the ratio between the swimbladder volume and body weight (mm³ g^{− 1}) in fish exposed to the high CO₂ concentration was 4.65 ± 0.79 mm³ g^{− 1}, which was significantly smaller (P < 0.05) than 10.12 ± 0.72 mm³ g^{− 1}in the control CO₂ treatment. In general, the swimbladder volume was directly correlated with the rearing temperature and was up to 50% larger in fish reared at 27 °C compared to those from 23 °C independent of CO₂ concentration. The relative presence of skeletal deformities in fish with non-inflated swimbladders was 6.49%, which was significantly (P < 0.05) higher than the 0.93% in fish with a normally inflated swimbladder. In addition, fish with an uninflated swimbladder demonstrated a significantly (P < 0.05) more severe lordosis deformity with a vertebra angle of 47.5 ± 15° compared to cohorts with an inflated swimbladder that exhibited an angle of 27 ± 6°. In conclusion, there was a clear effect of high aqueous levels of CO₂ and environmental temperature on growth and skeletal deformity which, in turn, affected juvenile quality in the white grouper.