Investigations of the toxic mechanisms of ammonia to fish–gas exchange in rainbow trout (Salmo gairdneri) exposed to acutely lethal concentrations

Exposure to acutely lethal un‐ionised ammonia concentrations resulted in a 3.3‐fold increase in oxygen consumption. This was associated with increases in ventilation volume, respiratory frequency and amplitude. ‘Cough’ frequency was not significantly affected. Heart rate and dorsal aortic blood pressure were increased and cardiac output was estimated to have more than doubled. Dorsal aortic blood Po 2 decreased but no significant changes in erythrocyte number, haematocrit, haemoglobin concentration, blood pH or P50 were observed. Possible toxic actions of ammonia were discussed and it was speculated that, as in mammals, ammonia may act by impairing cerebral energy metabolism.