Remember those dead fish are just those that died at the site. Additional delayed mortalities can be expected. Mortalities can also be an issue with the fish not bought to the weigh-in - either immediately released or "culled" during the day.
Yes, "Fizzing" is one of the methods used to deflate "bloated" fish. For those not familar with the method the gas is relieved by puncturing the air bladder with sharp object. The mostly commonly used (best?) tool has been a hollow needle. The idea is to poke a hole in the bladder to relieve the gases. Several things to be careful with this mehtod.
One is that you are stick the insides wtih a foreign object so infections can be an issue - be sure that the needle is sterile (sterilize before treating every fish).
There is a high potential for hitting critical organs if not done with care and knowledge. Approaching through the mouth is a way to avoid some of that potential hit critical organs though of course you are puncturing the esophagus. The idea here is that as the bladder expands it will push the other organs out of the way. Of course for that to be the case the bladder will have expanded and there is potential for some damage to have occurred prior to the "fizzing".
In addition to the damage done by the expanded bladder there is also the issue of gas bubbles forming in the blood of the fish (much like the bends for divers). "Fizzing" the fish will not address that problem and the deeper the fish caught the more likey there will a problem with bubbles in the blood. To give you an idea the role of the depth. Feather and Knable in 1983 subject largemouth bass to pressure changes in the lab and found that a change in pressue equavalent to bring the fish to the surface from 30 feet resulted in a 25% mortality while a change equavalent to bring the fish to the surface from 50 feet resulted in 42% mortality.
For fish with expanded bladders several alternate methods of treatment other than "fizzing" has been developed all of which are generally preferred. However each requires returning the fish immediately to the depths thus de-pressurize the fish. Of course doing so negates holding the fish in live wells and having weight-ins.
This issue is also more than just a "tournament issue". Some every day practices that anglers often do routinely also may result in additional mortalities of such fish. Holding fish for later pictures or weighting, hold caught fish until done fishing a spot, etc can be a negative for the released fish.
As always successful catch and release of any fish requires anglers be informed of the issues. Knowledge of such techniques and problems and being aware of potential trade offs are key to the successful practice of CnR.
Tight lines
Curt