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 acclimated 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 tensions of inspired water (P(i)o(2)) were reduced. Gill ventilation (V-G) increased significantly in response to hypoxia and resulted mainly in larger breath volume (V-S,V-R) whereas breathing frequency was hardly 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% during severe hypoxia (P(i)o(2) = 30 mmHg; 21% sat.). The results suggest that this species should not be exposed to long periods of low O-2 since portions of the metabolic energy are used by the ventilatory system to maintain O-2 uptake to the detriment of other metabolic functions such as weight gain and growth.