Cardiorespiratory responses of skipjack tuna (Katsuwonus pelamis), yellowfin tuna (Thunnus albacares), and bigeye tuna (Thunnus obesus) to acute reductions of ambient oxygen

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Cardiorespiratory responses to acute reductions of ambient oxygen were measured in skipjack tuna (Katsuwonus pelamis), yellowfin tuna (Thunnus albacares), and bigeye tuna (Thunnus obesus). Prevented from swimming by a spinal injection of lidocaine, fish were placed in seawater flowing at a velocity equivalent to their normal swimming speed. Ventilation volume [Formula: see text], heart rate, cardiac output, mouth gape, and inspired water and exhalant water oxygen partial pressures ([Formula: see text] and [Formula: see text], respectively) were simultaneously measured during periods of full oxygen saturation (normoxia) and brief (ca. 3–4 min) periods of reduced oxygen (hypoxia). During hypoxia, [Formula: see text] ranged from 140 to 50 mmHg. [Formula: see text] during normoxia was significantly different in skipjack, yellowfin, and bigeye tunas (6.7, 3.9, and 1.5 L∙min−1∙kg−1, respectively) and paralleled differences in oxygen consumption (740, 455, and 322 mg O2∙kg−1∙h−1). All three species were sensitive to [Formula: see text], and mild hypoxia [Formula: see text] elicited significant cardiorespiratory adjustments, including increased mouth gape and [Formula: see text] and reduced heart rate. Cardiac output was maintained until [Formula: see text] reached 95 mmHg, but at lower oxygen levels it too began to decrease. Therefore, the three tuna species studied appear as sensitive to hypoxia as other marine teleosts and show cardiorespiratory adjustments at [Formula: see text] values well above those eliciting swimming speed changes.