EFFECTS OF THERMAL-STRESS ON RESPIRATORY RESPONSES TO HYPOXIA OF A SOUTH-AMERICAN PROCHILODONTID FISH, PROCHILODUS-SCROFA

Oxygen consumption (VO2) and respiratory variables were measured in th e Prochilodontid fish, Prochilodus scrofa exposed to graded hypoxia af ter changes in temperature. The measurements were performed on fish ac climated to 25 degrees C and in four further groups also acclimated to 25 degrees C and then changed to 15, 20, 30 and 35 degrees C. An incr ease in VO2, occurred with rising temperature, but at each temperature VO2, was kept constant over a wide range of O-2 tensions of inspired water (PiO(2)). The critical oxygen tensions (PcO(2)) were PiO(2) = 22 mmHg for 25 degrees C acclimated specimens and after transfer from 25 degrees C to 15, 20, 30 and 35 degrees C the PcO(2) changed to PiO(2) = 28, 22, 24 and 45 mmHg, respectively. Gill ventilation (V-G) increa sed or decreased following the changes in VO2 as the temperature chang ed and was the result of an accentuated increase in breath frequency. During hypoxia the increases in V-G were characterized by larger incre ases in breath volume. Oxygen extraction was kept almost constant at a bout 63% regardless of temperature and ambient oxygen tensions in norm oxia and moderate hypoxia (PO2 similar to 70 mmHg). P. scrofa showed h igh tolerance to hypoxia after abrupt changes in temperature although its survival upon transfer to 35 degrees C could become limited by the capacity of ventilatory mechanisms to alleviate hypoxic stress.