Effects of graded hypoxia on Atlantic salmon infected with amoebic gill disease

Atlantic salmon Salmo salar with amoebic gill disease (AGD) were exposed to a graded hypoxia (135-40 mmHg water PO2) and blood samples analysed for re spiratory gases and pH at 119, 79.5 and 40 mmHg water PO2. There were no di fferences in the rate of oxygen uptake between infected and control fish. H owever, arterial PO2, and pH were significantly lower in the infected fish whereas PCO2 was significantly higher in infected fish compared with contro ls prior to hypoxia and at 119 mmHg water PO2. At 79.5 and 40 mmHg water PO 2 saturation, there were no significant differences in blood PO2 or pH alth ough blood PCO2 was elevated in AGD affected fish at 50% hypoxia (79.5 mmHg water PO2). The elevated levels of PCO2 in fish affected by AGD resulted i n a persistent respiratory acidosis even during hypoxic challenge. These da ta suggest that even though the fish were severely affected by AGD, the pre sence of AGD while impairing gas transfer under normoxic conditions, did no t contribute to respiratory failure during hypoxia. (C) 2000 The Fisheries Society of the British Isles.