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

Abstract High carbon dioxide (CO 2 ) concentrations negatively impact fish, which makes data on its tolerance especially relevant for production systems that can accumulate CO 2 such as recirculating aquaculture system (RAS). The current study evaluates the effect of CO 2 on the growth performance, welfare, and health of Atlantic salmon post-smolts in RAS. This study consisted of two phases. The first was a CO 2 exposure phase, where eighteen tanks were used with six treatments in triplicate: 5, 12, 19, 26, 33 and 40 mg/L of CO 2 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 CO 2 2 was low and not related to treatments. The mean final body weight was significantly higher in the 5 mg/L treatment compared to CO 2 treatments ≥12 mg/L at the end of RAS phase and to CO 2 treatments ≥33 mg/L at the end of seawater phase. Moreover, regressions showed that growth significantly decreased linearly with increasing CO 2 in the water. Eye cataracts and visible external damage on skin, operculum, and fins were inexistent and similar among CO 2 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 CO 2 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 CO 2 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.