ABSTRACT
The effects of changes in environmental temperature and oxygen tension on gill ventilation, arterial 
, pH and [HCO3−] were evaluated in carp (Cyprinus carpio L.). Gill ventilation was measured continuously in specimens acclimated to 10 or 20°C, combining the method of electromagnetic flow determination with the application of a rubber mask technique. After establishing control values in airequilibrated water the environmental water 
was reduced from about 150 mmHg (20 kPa) during control conditions to 110 or 75 mmHg (14.7 or 10 kPa), respectively. Measurements of blood gases and acid-base parameters were performed repeatedly before, and 1 and 4 h after, initiation of hypoxia. Regardless of temperature, these moderately hypoxic conditions caused considerable and lasting increases in gill ventilation of about 70% 
or 180% 
, relative to the respective normoxic control values of about 50ml kg−1 min-1 at 10°C and 230 ml kg−1 min−1 at 20°C. These increases in ventilation reduced 
substantially, resulting in a rise in pHa by about 0.1 units at 
of 110 mmHg (14.7 kPa) and by about 0.2 units at 
of 75mmHg (10 kPa). Arterial 
was low under normoxic conditions at both temperatures (≈15 mmHg, ≈2kPa). During hypoxia, 
was marginally reduced, whereas the arterial O2 content and saturation remained at normoxic levels, mainly because of the increase in the blood O2-affinity induced by respiratory alkalosis. This lack of any clear relationship between arterial O2 content and ventilatory response to moderate hypoxia contrasts with previously reported data for trout, and supports the hypothesis that a change in 
is an adequate stimulus for the adjustment of ventilation in carp.
The considerable ventilatory response, together with small and inconsistent reductions in arterial 
, may also represent an expression of the action of waterfacing oxygen receptors on the regulation of breathing. A striking feature of the regulation of ventilation in carp compared with that in air-breathing lower vertebrates is that the hypoxic response is maintained at low temperatures, possibly indicating a relatively small safety margin for complete tissue oxygen supply in fish.
