Barotrauma occurs when a fish is pulled up from deeper water (> 10m) to near the surface where the pressure is less.
In such case the air within the bladder of the fish expands causing dammage to its organs.   It is a similar situation as when divers rise too quickly from deeper water, contracting the "bends".
Snapper and Jewfish are species susceptible to Barotrauma.

Barotraumas are easy to spot : the fish will have either ahard stomach, a stomach that has pushed out through the gills, or bulging eyes.