It’s what happens when they hold on to farts ….

Thanks MB that theory does make sense although Trevs don't seem to show symptoms of barotrauma like snapper do there still could be internal harm going on.
In this instance I trimmed the affected area out as it had a hard feel to it

I've had it heaps in Snapper lately. Makes filleting more of a challenge.

Thanks strx7, goodto have a name for it, I got some understanding when I googled it.
Sounds like the cavities are connected to the swim bladder and form from pressure shock. Nothing I read was sure what caused the pressure trauma or like MB said barotrauma. We caught a few trevs that day and looking down onto the back of the fish they were rounded and fat which indicated good condition.