Hypoxia tolerance of Amazon fish Respirometry and energy metabolism of the cichlid Astronotus ocellatus

As a result of regular flood pulses, the Amazon basin exhibits large annual changes in its chemical and physical parameters. Ecology and distribution of fish communities seem to be directed by seasonal and diurnal oxygen level oscillations. Amazon fish have developed strategies to thrive under these varying conditions. Astronotus ocellatus normally survives large fluctuations in oxygen availability in varzea lakes. Respirometric and metabolic changes in A. ocellatus were studied during exposure to stepwise declining oxygen levels. Respiration rates were continuously recorded. Haematologic and metabolic parameters (lactate, glucose, cortisol and free fatty acids (FFA)) were determined. A. ocellatus was found to be hypoxia tolerant; it survives more than 16 h of severe hypoxia (pO(2) less than or equal to 0.4 mg l(-1)) and even 4 h of complete anoxia at 28 degrees C. Its routine metabolic rate is 10.8 mg O-2 h(-1) per 100 g fresh weight. A significant decrease in standard metabolic rate (SMR) starts at 20% air saturation, whereas a significant change of blood lactate does not start until 6% air saturation in water. We suggest therefore that A. ocellatus responses to environmental hypoxia are based mainly on suppressed metabolic rate, whereas under deep hypoxia or anoxia partial compensation is obtained from anaerobic glycolysis. (C) 1998 Elsevier Science Inc. All rights reserved.