Allometry describes how the characteristics of living organisms change with body size. Although a decline in fish size is acknowledged as one of the widespread responses to global warming, relatively few studies have investigated the allometry of thermal limits, and even fewer have attempted to quantify the cardiorespiratory responses that may underlie any size-dependent thermal tolerance.

We measured the allometry of arterial and venous oxygen tension (PaO2 and PvO2) of 65 rainbow trout ranging from 200 to 3800 g across stepwise, acute temperature increases (18, 21.5, and 25°C) and under both normoxia and hyperoxia (150% O2). Additionally, we measured the aerobic metabolic rates of 130 rainbow trout ranging from 10 to 350 g under similar temperature and oxygen regimes.

While PaO2 did not scale with body mass throughout the temperature ramp, oxygen supplementation via hyperoxia lifted overall arterial oxygen tension. Aligning with previous work on salmonids, PvO2 declined with increasing fish size in normoxia, suggesting the potential for cardiac impairments in large individuals. While the mass-dependent decline in PvO2 was attenuated in hyperoxia, thermal limitations were observed without clear correlation to oxygen supply capacity. These findings have implications for the global discussion around climate warming and fish oxygen limitations and highlight the need for a better empirical understanding of size-dependent thermal tolerance.