Why fish overturn n die when there is no injury?

I found this at Fishing kaki forum and i think its very useful.

QUOTE:

How to Deflate a Fish

You’ve hauled in the fish from deep down. His eyes are bulging and his stomach is sticking out of his mouth. How can you get him back in the water to swim away? You can’t unless you vent his swim bladder gases.

The swim bladder is an organ filled with gases, mostly oxygen and nitrogen. This organ is in the abdominal cavity along the backbone beneath the first dorsal fin. Except for sharks and rays most marine fishes have well-developed swim bladders. The swim bladder helps keep the fish buoyant at whatever depth they normally live.

When fish are at rest, the pressure of the gases in the swim bladder equals the pressure exerted by the surrounding water. The gases are compressed and the swim bladder occupies a relatively small space. If you bring a fish rapidly to the surface from down below 30’, the outside pressure lessens and the gases in the swim bladder expand in direct proportion.

Say you catch a fish at a depth of 33 feet. By the time you get him to the surface the gases in his swim bladder will occupy twice their original volume. These bladders can only stretch so far before they burst. When they do, the gases escape into the fishes abdomen, where they continue to expand. This puts pressure on the stomach, the intestines and other internal organs. To return the fish safely, you must vent those gases.And you must do it without further injury to the fish. Don’t try to force the stomach or other organs back into the body cavity. DO NOT vent the gases by puncturing the protruding stomach or going through the anus. The fish will probably swim away but won’t live.

Most fish are hardy and can survive this trauma, if properly deflated, he will survive, get bigger and reproduce. Here is how you do it:

* Use a deflating tool (an atheletic ball inflation needle ground to a sharp point or a carpet needle).
* Insert it through the thin part of the body wall on the underside of the belly below the pectoral fin.
* Squeeze the fish gently to force the gases out through the tube or needle.
* Do this quickly and very soon after capture
* Release the fish holding him in the water until he can swim away.

Excerpts were taken from an article written by the Mote Marine Laboratory Newsletter