Ventilation, gill perfusion and blood gases in dourado, Salminus maxillosus Valenciennes (Teleostei, Characidae), exposed to graded hypoxia

The dourado, Salminus maxillosus, is an active and migratory teleost found in lotic waters of Southern Brazil. We have studied the relationships of ga s transport in dourado to the specific ecophysiology of this-species. Measu rements were performed of blood gases, O-2 uptake, gill ventilation and per fusion at normoxia and various levels of hypoxia. Thus, the study aimed at a detailed assessment of the causes Of O-2 transport failure, using recent models for gas transport in vertebrates. Oxygen uptake was maintained down to a critical water partial O-2 pressure of 42 mmHg, below which it markedl y decreased. This could be explained based on ventilatory and cardiovascula r responses: Ventilation increased sufficiently to match decreases of water O-2 partial pressure during moderate hypoxia (partial pressure Of O-2 > 42 mmHg) but failed to meet O-2 demands below this value. Likewise, the cardi ovascular responses were insufficient to maintain an adequate transport bel ow moderatelevels of hypoxia. Thus, combined failure of ventilation and blo od gas transport account for the abrupt decreases Of O-2 transport. The spe cies proved highly vulnerable to hypoxia, which is consistent with the norm ally well-aerated habitat and the active mode of life of the species.