Accumulation of CO₂ limits energy gain in freely diving grey seals

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 present study, six grey seals (Halichoerus grypus) voluntarily dove in a simulated foraging setup, swimming from a breathing chamber to and from an underwater feeder. During surface intervals, seals were exposed to ambient, hypercapnic (high CO₂), hypoxic (low O₂) or hyperoxic (high O₂) respiratory gas conditions. The effect of gas condition on net rate of energy gain, dive behaviour, respirometry-derived energy expenditure, post-dive circulating lactate concentration, and digestion (indicated by circulating triglyceride concentrations) was quantified. Net rate of energy gain significantly decreased under hypercapnia, likely owing to extended surface recovery durations, rather than underlying changes in energy expenditure or other post-dive metabolic processes. Extended surface durations reflected the slower rate of CO₂ elimination relative to O₂ uptake. Our findings show that the accumulation of CO₂ is a significant limiting factor to net rate of energy gain in grey seals. Furthermore, we provide evidence of both digestion and anaerobic metabolism during diving, which contrasts with previously hypothesised optimal foraging strategies. Phocid seals are therefore not limited by digestive activity or the accumulation of lactate during short foraging bouts.