SUMMARYEuropean eels were exposed for 6 weeks to water CO2 partial pressures (PCO2) from ambient (approx. 0.8 mmHg), through 15±1 mmHg and 30±1 mmHg to 45±1 mmHg in water with a total hardness of 240 mg l–1 as CaCO3, pH 8.2, at 23±1°C. Arterial plasma PCO2 equilibrated at approximately 2 mmHg above water PCO2 in all groups, and plasma bicarbonate accumulated up to 72 mmol l–1 in the group at a water PCO2 of 45 mmHg. This was associated with an equimolar loss of plasma Cl–, which declined to 71 mmol l–1 at the highest water PCO2. Despite this, extracellular acid–base compensation was incomplete; all hypercapnic groups tolerated chronic extracellular acidoses and reductions in arterial blood O2 content (CaO2), of progressive severity with increasing PCO2. All hypercapnic eels, however, regulated the intracellular pH of heart and white muscle to the same levels as normocapnic animals. Hypercapnia had no effect on such indicators of stress as plasma catecholamine or cortisol levels, plasma osmolality or standard metabolic rate. Furthermore, although CaO2 was reduced by approximately 50% at the highest PCO2, there was no effect of hypercapnia on the eels' tolerance of hypoxia, aerobic metabolic scope or sustained swimming performance. The results indicate that, at the levels tested, chronic hypercapnia was not a physiological stress for the eel, which can tolerate extracellular acidosis and extremely low Cl–levels while compensating tissue intracellular pH, and which can meet the O2 requirements of routine and active metabolism despite profound hypoxaemia.

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The feasibility of using a differential pressure sensor connected to an acoustic telemetry device to monitor opercular activity as a correlate of oxygen consumption was investigated. Four starry flounders Platichthys stellatus were fitted with a miniature differential pressure sensor mounted close to the operculum. A cannula was connected to the sensor and inserted under the operculum, inside the branchial cavity. Measurements of oxygen consumption and opercular activity were carried out over a broad range of metabolic activity, from the post-surgery stress (high metabolic rate) to routine metabolic rate the following day. Relationships between differential pressure changes (rate and amplitude) were highly correlated with oxygen consumption (r 2 ¼ 0Á74 and 0Á60 respectively). The results indicate that monitoring opercular activity offers an alternative method for measuring aerobic metabolism in free-swimming fishes in nature.