Prolonged SDA and reduced digestive efficiency under elevated CO2 may explain reduced growth in Atlantic cod (Gadus morhua)

a b s t r a c t Land-based aquaculture systems expose fish to elevated dissolved CO2 levels, a factor that is corre- lated with reduced growth, feed conversion efficiency and body condition index. The physiological basis underlying the pathological effects of environmental hypercapnia is poorly understood, in particular for marine fish species. We investigated whether changes in energy expenditure and the specific dynamic action (SDA) of digestion and assimilation could account for the lower growth of adult Atlantic cod (Gadus morhua) under environmental hypercapnia. Fish acclimated to a partial pressure of 800 atm CO2 (0.6 mmHg, 1.5 mg/L) and 9200 atm CO2 (7 mmHg, 18.7 mg/L) exhibited no difference in main- tenance metabolic rates, which concurs with previous research for this species and other fish species. At 9200 atm CO2 Atlantic cod had a significantly diminished (14%) maximum aerobic capacity. While hypercapnia did not affect the amount of oxygen required for the SDA process, it did prolong the SDA duration by 23%. The longer SDA process time may offer an explanation for the observation of lower feed intake, growth and condition factor in long-term hypercapnia studies. Comparison of aerobic scope and cardiac performance during digestion suggested that reduced oxygen delivery capacity under hypercap- nia could be one mechanism by which CO2 prolongs SDA, although our results could not definitively demonstrate this effect.