Sublethal effects and safe levels of carbon dioxide in seawater for Atlantic salmon postsmolts (Salmo salar L.): ion regulation and growth

Atlantic salmon (Salmo salar L.) postsmolts (0.17–0.26 kg) were exposed to four different levels of carbon dioxide partial pressure for 43 days in an open flow system: 0.6 mm Hg (control), 4.9 mm Hg (low), 12 mm Hg (medium), and 20 mm Hg (high). The water temperature was 15–16°C and the salinity 34‰. In the low carbon dioxide group (P_CO2=4.9 mm Hg; 10.6 mg/l), no significant differences were found in blood parameters (haematocrit, plasma chloride and plasma sodium) or in growth parameters (weight, length and condition factor) when compared to the control group. After 43 days, the mean plasma chloride concentration for the medium group (P_CO2=12 mm Hg; 26 mg/l) was significantly reduced, while weight and condition factor were slightly, although not significantly, lowered. For the high carbon dioxide group (P_CO2=20 mm Hg; 44 mg/l) plasma sodium and plasma pH were significantly increased and plasma chloride, oxygen consumption, weight, length and condition factor were significantly reduced at the end of exposure. There was no mortality in the control group or in the low carbon dioxide group. The mortalities in the medium and high carbon dioxide groups were 1.1 and 4.3%, respectively. Nephrocalsinosis was not observed in any of the groups. The results of the present investigation indicate that the CO₂ concentration of the low group may represent a safe level for Atlantic salmon postsmolts when the temperature is 15–16°C and the oxygen level is 6–7 mg/l. Further studies are required.