Cardiovascular and respiratory changes during heat stress in rainbow trout (Salmo gairdneri)

Trout were subjected to a steady increase in water temperature (1·5 °C/h) from 15 °C until death occurred, while several respiratory and cardiovascular parameters were monitored. Oxygen consumption increased during the warming (Q10 = 2·35 between 16 and 20 °C). At the higher temperatures the increase was more marked (Q10= 4·96 between 20 and 26 °C). Ventilatory frequency increased during the rising temperature with a general levelling off observed above 23 °C. The amplitude of the pressure changes in the buccal and opercular cavities increased more than did the ventilatory frequency. Further analysis of the differential pressure across the gills suggests that the adjustment of respiratory pumping to the increased oxygen demand is predominantly in the volume pumped per stroke (cycle). Heart rate rose steadily with the increasing temperature until about 24– 25 °C, when a bradycardia usually became evident. Synchrony between the heart beat and the respiratory pumps was observed in some preparations at the higher temperatures. Blood pressure increases during the warming were more marked in the ventral aorta than in the dorsal aorta. At the highest temperatures, abnormal cardiac cycles were frequently observed. Arterial oxygen content declined slightly during warming and venous oxygen content dropped to zero above 23 °C. It is suggested that cardiovascular adjustments may be a limiting factor in this type of stress.