Evidence for size-dependent cardiac oxygen supply in a fish

Gill oxygen uptake and cardiac oxygen supply have been proposed as key parameters governing size-dependent environmental resilience in fish. We investigated the allometry of arterial (PaO₂) and venous (PvO₂) oxygen partial pressures in rainbow trout (Oncorhynchus mykiss) across a large body mass range (282–3834 g, n=40) during acute warming (18–25°C), with or without hyperoxia (150% air saturation), and following exhaustive exercise. PaO₂ scaling exponents (b) were not significantly negative during warming, indicating no size-dependent limitation in gill oxygen uptake of resting fish. However, following exhaustive exercise at 25°C, PaO₂ declined significantly with body mass (b=-0.21), suggesting oxygen uptake may become limiting in larger individuals under combined thermal and physical stress. PvO₂ exhibited significant negative scaling with body mass at both 18°C and 25°C (b=-0.34 and -0.37), suggesting lower cardiac oxygen availability in larger fish. Hyperoxia mitigated the mass effect on PvO₂ — disproportionately benefiting larger fish — but loss of equilibrium occurred in several fish following exhaustive exercise regardless of oxygen treatment. The negative scaling of venous oxygenation with size highlights a novel mechanism that may constrain oxygen transport in large fish. Our results emphasize the need to account for size-dependent physiological vulnerability in conservation planning under climate change.