Understanding how gas regulation impacts behavioural and physiological processes in phocid seals is essential to understanding their foraging ecology. The accumulation of circulating CO₂ across a series of dives is thought to prolong surface recovery, thereby reducing foraging efficiency. This can be empirically tested by experimentally altering circulating gas tensions in diving seals and quantifying the effect on net rate of energy gain. In the current study, six grey seals (Halichoerus grypus