Malformation of the swimbladder of farmed Atlantic salmon Salmo salar L. is an increasing problem in Norwegian aquaculture.
Affected fish exhibit a characteristic behaviour suggesting that balance and buoyancy are altered.
In affected fish, the swimbladder is shortened and dilated and the pneumatic duct opens at the normal point in the oesophagus, but it runs caudally along the ventral side of the swimbladder and enters the caudal pole of the organ. This is in contrast to normal fish, where the pneumatic duct is very short and enters the cranioventral part of the swimbladder. This malformation of the pneumatic duct may lead to altered filling of the swimbladder, which in turn may cause the abnormal shape and size of the organ.