Gas bubble disease in Atlantic salmon (Salmo salar L.) induced by simulated well boat operations

Decompression-induced gas bubble disease (GBD) has been studied across various species and contexts, but its occurrence in well boat operations in Atlantic salmon aquaculture has not been investigated. In the present study, Atlantic salmon were exposed to pressure conditions designed to mimic the outer operational boundaries of well boat loading (0.4 atmospheres absolute (ata)) and unloading (2.6 ata) in the Norwegian aquaculture industry. The simulated loading, starting from atmospheric pressure (1 ata), with rapid decompression to 0.4 ata resulted in altered positioning, behavioural abnormalities, gas bubbles in blood detected by ultrasound, macroscopically visible gas bubbles in gills and fins, and histopathological lesions. A time-dependent increase in mortality was observed, reaching 100 % after 113 min, with the first mortality recorded at 65 min. Although dissolved oxygen of 100 % and 150 % of air saturation level prior to decompression resulted in different partial pressure ratios (N 2 +Ar):O 2 upon exposure to 0.4 ata, the clinical findings were similar. The sub-lethal decompression-induced symptoms of GBD appeared to be reversible. Compression and exposure to 2.6 ata resulted in upward-tilted swimming, likely representing a behavioural response to maintain buoyancy; however, no other notable behavioural or GBD related signs were observed. This study clearly demonstrates that a pressure profile representative of certain well boats op- erations in Atlantic salmon aquaculture can induce acute GBD.