Increased heart rate and blood oxygen extraction are key to the ability of schoolmaster snapper (Lutjanus apodus) to swim maximally at high temperatures.

Consistent with data on other tropical fish species: we previously showed that schoolmaster snapper (Lutjanus apodus) exhibited a maximum metabolic rate (MMR) during a critical thermal maximum while swimming (CTSMax) test that was 60% higher than during a standard critical thermal maximum (CTMax) test; and based on this data suggested that this elevated MMR contributed to swimming fish fatiguing at a temperature only 1.4°C lower than their CTMax (37.5 versus 38.9°C, respectively). However, the mechanism(s) underlying the greater MMR and realistic aerobic scope (ASR) in fish given a CTSMax test remained unclear. To answer this question, CTMax, Ucrit (critical swimming speed) and CTSMax tests were performed on snapper surgically implanted with Transonic® blood flow probes, and we measured heart rate (fH), stroke volume (SV), cardiac output (Q̇), oxygen consumption (ṀO2) and blood oxygen extraction (EO2; ṀO2/Q̇). The ∼1.75-fold higher MMR observed snapper when given CTSmax versus CTmax tests was not explained by a greater contribution of SV to Q̇. Instead, the higher MMR was primarily driven by a ∼40 beat min-1 (25%) higher maximum fH and a ∼1.55-fold greater increase in EO2. This elevated EO2 may reflect enhanced oxygen uptake at the gills (i.e. higher arterial oxygen content) due to the fish engaging in ram ventilation while swimming at elevated temperatures and/or improved blood oxygen extraction. These findings highlight the critical role of EO2 in setting tolerance limits under combined thermal and activity-related stressors, and caution against relying solely on fH variables (i.e. fH,Max) as an indicator of thermal tolerance.