Many fish have a gas-filled swim bladder to adjust their bouyancy. If they are brought to the surface too quickly, the excess gas cannot escape quickly enough as it expands. This will cause the swim bladder to swell, sometimes forcing the guts out of the mouth or the anus. In extreme cases the swim bladder can burst.
I used to work in an aquarium that collected fish from the sea with the help of local fishing boats. When the fish were brought aboard, an aquarist would use a hypodermic needle to relieve the pressure in the swim bladder without causing any long term damage to the fish.