Long-term separate and combined effects of environmental hypercapnia and hyperoxia in Atlantic salmon (Salmo salar L.) smolts

Atlantic salmon (Salmo salar L.) parr (mean start weight 50 g) were reared in freshwater (FW) and exposed to three levels of oxygen saturation measured in effluent water; control group (93% O2, LO2), medium (111% O2, MO2) and high (123% O2, HO2). Further three groups were exposed to similar water oxygen levels in combination with elevated carbon dioxide levels (17–18 mg L–1 CO2), named LO2–CO2, MO2–CO2 and HO2–CO2, respectively. The experiment was run in duplicate tanks for 42 days, and the fish were subsequently transferred to the same seawater (SW) regime for 45 days for an assessment of post-smolt growth. As a consequence of the CO2 addition, tank pH levels in the FW period were reduced from 6.7 to 5.9 for the hypercapnia groups compared to for the normcapnia groups. Water temperature in FW ranged between 6.4 and 9.0 °C. Citrate was added to the water to complex labile aluminium. In the CO2 groups observed ventilation frequencies were significantly increased compared to the control (p