Blood oxygenation and cardiorespiratory function in steelhead trout (Oncorhynchus mykiss) challenged with an acute temperature increase and zatebradine-induced bradycardia

To explore whether temperature-dependent increases in cardiac output (Q) are mediated solely through heart rate (f_H) in fish to ensure adequate/efficient blood oxygenation, we injected steelhead trout with saline (control) or zatebradine hydrochloride (1.0 mg kg⁻¹), and measured blood oxygen status, cardiorespiratory variables and cardiorespiratory synchrony during a critical thermal maximum (CT_Max) test. The increasing temperature regimen itself (from 12 °C to CT_Max) resulted in large decreases in arterial oxygen partial pressure (P_aO₂) and content (C_aO₂) (by ∼35% and 25%, respectively). Further, there was little evidence of cardiorespiratory synchrony at 12 °C, and the number of fish that showed synchrony at high temperatures only increased marginally (to 3 out of 7) despite the large decrease in P_aO₂. These results: (1) indicate that in some situations (e.g. when ventilation is exclusively/predominantly dependent on buccal–opercular pumping) the upper thermal tolerance of fish may be constrained by both cardiovascular and ventilatory performance; and (2) question the importance of cardiorespiratory synchrony (ventilation–perfusion matching) for gas exchange in salmonids, and fishes, in general.