THE EFFECT OF CARBON DIOXIDE EXPOSURE ON ATLANTIC SALMON REARED IN BRACKISH WATER RAS

Design and operation of recirculating aquaculture systems (RAS) for Atlantic salmon (Salmo salar) production is largely a function of the CO₂ tolerance of these fish. In this study, Atlantic salmon post-smolts were exposed to six CO₂ concentrations (5, 12, 26, 33 and 40 mg/L) for 12-weeks (RAS phase) followed by non-CO₂ exposure period of 6-weeks (Seawater phase). A total of 900 fish (± 71 g) were randomly distributed over eighteen tanks (V=0.5 m³) connected to a RAS. Fish weight, length and welfare score together with blood and tissue samples for physiology (plasma ions), histology (kidney and skin) and molecular (gills) analysis were taken during RAS and Seawater phases.

The current study brings a new insight on the growth of Atlantic salmon post-smolts in brackish water RAS and shows that maximum growth performance is obtained in CO₂ concentrations below 12 mg/L. Skin health appeared to be compromised in fish exposed to very high CO₂ concentrations (40 mg/L). In contrast, no major effects on external welfare indicators, cataracts and nephrocalcinosis were observed. Moreover, several osmoregulation and acid-base balance parameters showed a positive (pH, K⁺, HCO₃^- and PCO₂) or negative (Na⁺, Cl^-) linear relationship with environmental CO₂ concentrations (5 - 40 mg/L range), whereas others (haematocrit, Ca²⁺, Mg²⁺, urea and glucose) were unaltered. These physiological alterations were maintained as long as CO₂ exposure persisted but returned to normal concentrations when CO₂ exposure ended. In gills, CO₂ exposure down-regulated a number of  genes with various functions, which changed to up-regulation when the CO₂ exposure ended. The data from this study adds to recent findings that CO₂ concentrations below the previously thought threshold of 15 mg/L still have an impact on Atlantic salmon, and this finding may be relevant to achieve better design and dimensioning of production systems where CO₂ may accumulate in the water.

Funded by CtrlAQUA SFI (NRC nr 237856/O30) and the partners of the center