Respiratory responses of <i>Oreochromis niloticus</i> (Pisces, Cichlidae) to environmental hypoxia under different thermal conditions

Abstract

The oxygen uptake ( V O 2 ), breathing frequency ( f R ), breath volume ( V S.R ), gill ventilation ( V G) and oxygen extraction (%) from the ventilatory current of four groups of Oreochromis niloticus during graded hypoxia were measured under the following acclimation temperatures: 20. 25. 30 and 35°C. The critical oxygen tensions ( P O 2 ), determined from V O 2 v. P O 2 of inspired water at each experimental temperature were, respectively. 19±1±3±1. 18±0±4±9, 29±7± 4±1 and 30±2± 0.6 mmHg. The f R remained nearly constant during the reductions of O 2 at all the temperatures studied. V G increased discretely from normoxic levels until the P O 2 was reached, below which it assumed extremely high values (17‐fold higher or more). The increases observed in V G resulted, at all the acclimation temperatures, in an elevation in V S.R rather than in f R . The extraction of O 2 decreased gradually from normoxia until the P O 2 was reached, below which an abrupt reduction of extraction was recorded, except at 35°C when fish showed a gradual reduction in extraction just below the tension of 80 mmHg.