In freshwater fish, cortisol increases the number and activity of chloride cells (ionocytes/mitochondria-rich cells) in the gills. These specialised cells actively absorb ions (Na⁺, Cl⁻) from the dilute freshwater environment against the concentration gradient, using energy (Na⁺/K⁺-ATPase). This is essential for osmoregulation in freshwater fish, which constantly lose ions and gain water. Cortisol also stimulates gill Na⁺/K⁺-ATPase activity directly.