Barotrauma is a pressure (baro) related injury (trauma) that fish experience when reeled up from depth. Barotrauma typically occurs in depths of 50 feet or greater, but can occur in shallower depths depending on the species caught. As fish are reeled up from deep water, gases in the body cavity and internal organs expands, often displacing the organs and leaving the fish severely bloated.
The major organ responsible for this expansion of gas is the swim bladder, which fish use to regulate their position in the water column. If you throw bloated fish back in the water, they will likely float on the surface unable to swim back to depth. This alone could lead to death or leave them more susceptible to predation.
Discard mortality, or death after release, kills millions of reef fish each year. Adopting best release practices can significantly improve the survival of released reef fish and reduce waste in the fishery.