Effects of thermal stress on respiratory responses to hypoxia of a South American Prochilodontid fish, Prochilodus scrofa

Oxygen consumption (V̇o2) and respiratory variables were measured in the Prochilodontid fish, Prochilodus scrofa exposed to graded hypoxia after changes in temperature. The measurements were performed on fish acclimated to 25°C and in four further groups also acclimated to 25°C and then changed to 15, 20, 30 and 35°C. An increase in V̇o2 occurred with rising temperature, but at each temperature V̇o2 was kept constant over a wide range of O2 tensions of inspired water (Pio2). The critical oxygen tensions (Pco2) were Pio2= 22 mmHg for 25°C acclimated specimens and after transfer from 25°C to 15, 20, 30 and 35°C the Pco2 changed to Pio2= 28, 22, 24 and 45 mmHg, respectively. Gill ventilation (V̇G) increased or decreased following the changes in V̇o2 as the temperature changed and was the result of an accentuated increase in breath frequency. During hypoxia the increases in V̇G were characterized by larger increases in breath volume. Oxygen extraction was kept almost constant at about 63% regardless of temperature and ambient oxygen tensions in normoxia and moderate hypoxia (Po2∼70 mmHg). P. scrofa showed high tolerance to hypoxia after abrupt changes in temperature although its survival upon transfer to 35°C could become limited by the capacity of ventilatory mechanisms to alleviate hypoxic stress.