Fish that are rapidly retrieved from depths greater than 10 metres do not have time to adjust to water pressure changes, resulting in the expansion of gases in the swim bladder and other organs.
These rapid pressure changes can cause fish to suffer a range of injuries, known as barotrauma.
Physical effects of barotraumas include inflated swim bladder and intestines, bulging eyes and stomach pushed out through the mouth or gill cover. The physical effects and severity increase with depth of capture, and susceptibility varies between species.
The symptoms of barotrauma may not be evident in fish caught from very deep water as the swim bladder can burst.
Learn more about other factors that can reduce survival
Methods used to treat barotrauma depend on the severity of the symptoms.
It is recommended that anglers use a release weight to return excessively buoyant fish to depth. A release weight can be made easily by securing a barbless hook to a heavy weight or large sinker.
Use of venting needles to deflate the swim bladder should be used only as a last resort (there is a chance of damaging internal organs if done incorrectly).