Increasing CO2 concentration within one measurement temperature resulted in a significant Bohr Effect, shifting OECs right and decreasing nH slope values. For example, 17 °C-acclimated Pacific bluefin tuna nH values measured at 20 °C were 2.04 (±0.15), 1.37 (±0.04), and 1.02 (±0.18) at 0%, 0.5% and 1.5% CO2, respectively