RESUMO
The dourado, Salminus maxillosus, is an active and migratory teleost found in lotic waters of Southern Brazil. We have studied the relationships of gas transport in dourado to the specific ecophysiology of this-species. Measurements were performed of blood gases, O2 uptake, gill ventilation and perfusion at normoxia and various levels of hypoxia. Thus, the study aimed at a detailed assessment of the causes of O2 transport failure, using recent models for gas transport in vertebrates. Oxygen uptake was maintained down to a critical water partial O2 pressure of 42 mmHg, below which it markedly decreased. This could be explained based on ventilatory and cardiovascular responses: Ventilation increased sufficiently to match decreases of water O2 partial pressure during moderate hypoxia (partial pressure of O2 > 42 mmHg) but failed to meet O2 demands below this value. Likewise, the cardiovascular responses were insufficient to maintain an adequate transport below moderatelevels of hypoxia. Thus, combined failure of ventilation and blood gas transport account for the abrupt decreases of O2 transport. The species proved highly vulnerable to hypoxia, which is consistent with the normally well-aerated habitat and the active mode of life of the species.