Long-term carbon dioxide experiments with salmonids

Oxygen is generally the first limiting factor for the water flow requirement in a land based aquaculture system, while carbon dioxide and pH are secondary limiting factors. This means that if oxygen is added to the inlet water or directly to the water in a fish tank carbon dioxide becomes the limiting factor. pH is easy to regulate by addition of bicarbonate, while carbon dioxide always will be elevated in both single pass oxygenated systems and in recirculation systems. Carbon dioxide has both direct physiological effects on the fish, as well as indirect effects by changing the pH and thereby the chemistry of metals in the water. During the last 20 years several long-term carbon dioxide experiments have been performed at Bergen University College. In the present paper general methods and findings from the research at Bergen University College are described. These findings are compared with studies performed elsewhere. Increased plasma PCO2 increases the plasma bicarbonate concentration and reduces the plasma chloride concentration. In a long-term carbon dioxide experiment on Atlantic salmon postsmolts about 94% of the total variation in plasma carbon dioxide partial pressure was explained by the partial pressure in the water in a simple linear regression model, and 72% of the total variation in plasma chloride was explained by the plasma bicarbonate concentration. There was a reduction in about 1 mM Cl- for every 1 mM increase in plasma bicarbonate. For carbon dioxide the safe criterion used for the Norwegian production of Atlantic salmon smolts is 15 mg L-1. In low alkalinity fresh water carbon dioxide has adverse effects on fish in combination with labile Al/pH at concentrations around 8-10 mg L-1 CO2, and the smolts are not acclimated to such conditions. When toxic AI is not present in the water 8-10 mg L-1 carbon dioxide may have slight but significant effects on condition factor during the first month, but the smolts seem to be acclimated to these conditions after 2 months. However, when the concentration of carbon dioxide is 17-19 mg L-1 (5-6 mm Hg) in low alkalinity water, condition factor or specific growth rate is reduced after 2 months exposure. In high alkalinity fresh water, carbon dioxide had only minor effects on growth up to 24 mg L-1, however, further research may be needed to validate this and to study how the safe threshold level for carbon dioxide is influenced by temperature and salinity. (C) 2012 Elsevier B.V. All rights reserved.