Air-breathing behaviour of the African catfish (Clarias gariepinus) in response to different hypoxia, hypercapnia and temperature regimes

The African catfish (Clarias gariepinus) is an extensively researched species in various aquaculture studies, but knowledge about its air-breathing behaviour is generally limited. This study aimed to evaluate changes in air-breathing behaviour in response to environmental stressors by investigating the air-breathing frequency (fAB) under varying O₂ tensions, dissolved carbon dioxide (CO₂) levels, and temperatures. Varying O₂ tensions had a significant influence on the air-breathing behaviour of C. gariepinus. Under normoxic O₂ conditions, (15.9 kPa O₂), f_AB was 0.47 +/- 0.11 breaths min^-1. Under moderate (7.9 kPa) and severe O₂ tensions (0.8 kPa), mean f_AB were significantly elevated to 1.40 +/- 0.25 and 1.23 breaths min^-1, respectively. Increasing water temperatures also significantly increased f_AB up to temperatures greatly exceeding thermal optima where there were significant declines in f_AB. Subjecting fish to thermally stratified water columns also resulted in significant adjustments in air-breathing behaviour. Under isothermal conditions at 26 degrees C, C. gariepinus performed 0.43 +/- 0.07 air-breaths min^-1. Fish sequentially increased f_AB as the temperature of the top water layer increased. C. gariepinus responded to the elevated CO₂ levels by sequentially upregulating f_AB. There was a threefold increase in f_AB at 40 mgL^-1 CO₂ relative to normocapnic control conditions. This study has demonstrated that the air-breathing behaviour of C. gariepinus is complex and largely driven by fluctuations in dissolved gas concentrations and water temperature. These insights are especially relevant in the context of the increments in surface water temperatures, which typically exacerbate hypoxic conditions in many aquatic ecosystems.