RELATIONSHIPS BETWEEN OXYGEN AVAILABILITY AND METABOLIC COST OF BREATHING IN NILE TILAPIA (OREOCHROMIS-NILOTICUS) - AQUACULTURAL CONSEQUENCES

Oxygen uptake (Vo(2)) and ventilatory parameters were measured during graded hypoxia in Nile tilapia (Oreochromis niloticus) previously accl imated to normoxia (oO(2) greater than or equal to 125 mmHg; 90% sat.) at 25 +/- 1 degrees C. Vo(2) was slightly increased as the oxygen ten sions of inspired water (P(i)o(2)) were reduced. Gill ventilation (V-G ) increased significantly in response to hypoxia and resulted mainly i n larger breath volume (V-S,V-R) whereas breathing frequency was hardl y changed. The increase in V-G was also accompanied by a reduction in oxygen extraction efficiency (Eo(2)-%). The metabolic cost of V-G was calculated as 3% of total Vo(2) in normoxia but increased to 18% durin g severe hypoxia (P(i)o(2) = 30 mmHg; 21% sat.). The results suggest t hat this species should not be exposed to long periods of low O-2 sinc e portions of the metabolic energy are used by the ventilatory system to maintain O-2 uptake to the detriment of other metabolic functions s uch as weight gain and growth.