Swimbladder healing in Atlantic cod (Gadus morhua), after decompression and rupture in capture-based aquaculture

During rapid decompression in the course of fish capture, cod (Gadus morhua L.) is unable to reabsorb excess gas in the swimbladder sufficiently rapidly, and the bladder ruptures as the gas expands. In capture-based aquaculture (CBA), knowledge of the severity of the rupture stress is required. The objective of this study was to describe the swimbladder rupture and healing processes. Cod captured by Danish seine at depths of between 130 and 200 m were stored in a netpen. At 0, 24, 72 and 384 h post-capture, randomly selected fish (n = 20) were dissected. All fish had ruptured the swimbladder, most close to the dorsal point of pinbones; these holes were sealed and the bladders were inflated. On testing, all the bladders withstood a minimum of 20 mmHg above atmospheric pressure, a strength that increased with time and inversely with the size of holes. As the overinflated bladder deflates after rupture, the two layers of the bladder wall slide relative to each other, closing the open gas passage in the bladder wall. This repair mechanism explains the high survival rates and vitality of cod, in which ruptures have acute but reversible effects on function and welfare.