The cardiorespiratory responses were examined in yellowtail, Seriola quinqueradiata exposed to two levels of hypercapnia (seawater equilibrated with a gas mixture containing 1% CO2 (water PCO2 = 7 mmHg) or 5% CO2 (38 mmHg)) for 72 hr at 20°C. Mortality was 100% within 8 hr at 5% CO2, while no fish died at 1% CO2. No cardiovascular variables (cardiac output, Q̇; heart rate, HR; stroke volume, SV and arterial blood pressure, BP) significantly changed from pre-exposure values during exposure to 1% CO2. Arterial CO2 partial pressure (PaCO2) significantly increased (P < 0.05), reaching a new steady-state level after 3 hr. Arterial blood pH (pHa) decreased initially (P < 0.05), but was subsequently restored by elevation of plasma bicarbonate ([HCO3–]). Arterial O2 partial pressure (PaO2), oxygen content (CaO2), and hematocrit (Hct) were maintained throughout the exposure period. In contrast, exposure to 5% CO2 dramatically reduced Q̇ (P < 0.05) through decreasing SV (P < 0.05), although HR did not change. BP was transiently elevated (P < 0.05), followed by a precipitous fall before death. The pHa was restored incompletely despite a significant increase in [HCO3–]. PaO2 decreased only shortly before death, whereas CaO2 kept elevated due to a large increase in Hct (P < 0.05). We tentatively conclude that cardiac failure is a primary physiological disorder that would lead to death of fish subjected to high environmental CO2 pressures.
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