Development of gas bubble disease in juvenile Atlantic salmon exposed to water supersaturated with oxygen

Abstract

In intensive Atlantic salmon smolt rearing systems, oxygen supplementation of the water is required, but the importance of tight regulation of oxygen supply may not always be recognised. Consequently, there is a risk that fish may be exposed to water which is supersaturated with oxygen. This study was done to investigate the time course and exposure level associated with hyperoxia-induced gas bubble disease, through a combination of morphological and behavioural methods. Atlantic salmon pre-smolts were divided into three groups; one control group (no added oxygen), and two groups receiving gradually increasing oxygen saturations over three weeks, the high oxygen exposure group (130%, 160% and 220% O 2 saturation in week 1, 2 and 3, respectively) and the low oxygen exposure group (110%, 140% and 190% O 2 saturation in weeks 1, 2 and 3, respectively). The first signs of impaired water quality appeared 8 days after exposure start when a surface film of organic matter was observed in one of the tanks in the high exposure group receiving 160% oxygen. The first signs of subcutaneous gas bubbles appeared after 14 days, in the high exposure group at a level of 160% oxygen saturation. After 16 days, 50% of the low oxygen group and 77% of the high oxygen group had gas bubbles on major parts of the body, at exposure levels of 190% and 220% respectively. The bubbles appeared on most of the fins, along the lateral line, on the gills, and in the eyes. Gas bubbles were detected by visual observations and the diagnosis was supported by histology. Fish behaviour recordings revealed that during the first week, fish in the low oxygen group (110%) swam significantly more than both controls and high oxygen exposed fish. By week 3, at 190%, swimming activity was reduced. Fish exposed to 110% oxygen during week one also made more turns than the controls, and this difference persisted also in week 3 at 190% exposure. Fish in both high and low oxygen exposed groups displayed more panic episodes compared to the controls, indicating physiological stress and possibly pain.