The effects of acute and long-term exposure to CO2 on the respiratory physiology and production performance of Atlantic salmon (Salmo salar) in freshwater

A high-level of free CO2 is a prevalent feature of intensive RAS and chronic exposure is common for most species during the production process. Currently, standard operating procedures, regulations and safe levels of CO2 are based on values that do not necessarily represent a point at, up to which, production and fish performance are unaffected. The high solubility of CO2 in water and the large input from fish respiration also means that current practices for the removal of CO2 are often inadequate for the scale of production. High CO2 levels can be addressed by the control of alkalinity, thereby creating a scenario where the majority of CO2 exists as carbonate and bicarbonate. Any acute reduction in pH can shift the equilibrium towards a large and sudden conversion of inorganic carbon to free CO2, which can have a detrimental effect on fish. The current investigations aimed to determine the effects of both; acute increases in dissolved CO2 on the physiological capacity of Atlantic salmon, as well the effects of chronic exposure to different CO2 concentrations on production in freshwater. Results show that acute exposure (up to 40 mg L-1) significantly reduces aerobic capacity and the rate of recovery from stress. They also show that these effects are driven primarily by CO2 exposure, and to a much lesser extent by the associated reduction in pH. Growth and feed conversion experiments during chronic exposure suggest that there is no CO2 concentration where production performance is unaffected.