The effects of carbon dioxide on growth performance, welfare, and health of Atlantic salmon post-smolt (Salmo salar) in recirculating aquaculture systems

High carbon dioxide (CO₂) concentrations negatively impact fish, which makes data on its tolerance especially relevant for production systems that can accumulate CO₂ such as recirculating aquaculture system (RAS). The current study evaluates the effect of CO₂ on the growth performance, welfare, and health of Atlantic salmon post-smolts in RAS. This study consisted of two phases. The first was a CO₂ exposure phase, where eighteen tanks were used with six treatments in triplicate: 5, 12, 19, 26, 33 and 40 mg/L of CO₂ during 12 weeks in a 12 ppt salinity RAS (hereafter RAS phase). In the second phase, PIT-tagged fish were transferred to a 34 ppt salinity single flow-through tank at CO₂ < 5 mg/L (hereafter seawater phase) for an additional 6-week experimental period mimicking a seawater phase. Overall, mortality of fish exposed to CO₂ was low and not related to treatments. The mean final body weight was significantly higher in the 5 mg/L treatment compared to CO₂ treatments ≥12 mg/L at the end of RAS phase and to CO₂ treatments ≥33 mg/L at the end of seawater phase. Moreover, regressions showed that growth significantly decreased linearly with increasing CO₂ in the water. Eye cataracts and visible external damage on skin, operculum, and fins were inexistent and similar among CO₂ treatments. Kidneys showed no signs of mineral deposits in any of the structures of the tissue. However, skin analysis showed that fish exposed to high CO₂ concentrations had a significantly thinner dermis layer (both at the end of RAS and seawater phase) and a significantly thinner epidermis layer and lower mucus cells count (at the end of seawater phase). In conclusion, Atlantic salmon post-smolts cultured in brackish water RAS showed a maximum growth performance at CO₂ concentrations below 12 mg/L. Except skin, no major effects of health and welfare were observed, including cataracts and nephrocalcinosis. Further studies should evaluate the molecular and physiological responses to both short-term and long-term carbon dioxide exposure.