High levels of dissolved carbon dioxide (CO2 ) is a daily occurring phenomenon in earthen ponds. Hypercapnic conditions lead to a respiratory acidosis, which fish buffer by an accumulation of bicarbonate . The degree of hypercapnia that fish are able to recover appears to be species specific. We investigated the effects of three levels of dissolved CO2 (10, 30, and 60 mg L-1) on the standard (SMR) and maximum metabolic rate (MMR) of Nile tilapia, following acute (1h) and prolonged (24h) exposure. Acute hypercapnic exposure resulted in significant decreases metabolic scope , that persisted for 24h for the highest exposure groups (Fig. 1) .
We evaluated the effects of daily variations in dissolved CO2 and O2 , individually and in combination, on the appetite, growth, and feed utilization in Nile tilapia using groups reared under normoxic - normocapnic conditions (control, C), diurnal hypoxia (HO), diurnal hypercapnia (HC), or combined diurnal hypoxia and hypercapnia (HO × HC) in a digestibility system. We show that hypercapnia and hypoxia exerted strong individual effects on appetite, specific growth rate, and feed conversion (Fig. 2) , but also that simultaneous hypoxia and hypercapnia amplified these effects. Simultaneous exposure to hypoxia and hypercapnia resulted in a day-long loss of appetite, and reduced specific growth rates by >60%. Surprisingly, the digestibility of dry matter, protein, and lipid was either unaffected or even slightly improved in groups exposed to single or combined diurnal variation in dissolved oxygen and carbon dioxide. It is unknown whether this is the result of the change in feed intake or represents an adaptive mechanism to satisfy an increase energy demand caused by environmental stress. Overall, we conclude that although Nile tilapia is considered resilient to environmental stress, feeding and feed utilization are strongly influenced by daily fluctuations in dissolved gases , particularly CO2.