While build up of lactic acid is a major cause and does contributes to fish mortality it's not the direct cause in most cases.
Before we get to the real bugger in killing fish, let's look at a few specifics that apply:
(1) First the amount of handling, fighting and time out of water varies tremendously from species to species. Some catfish can take several minutes of air time and survive quite nicely while a Rainbow might not fare well after just 30 seconds of air time as research shows that to be at nearly 30% mortality if everything else was perfect except for the fish being out of water for the 30 seconds.
(2) Water temps and the fish's normal range of preferred temperatures plays a huge part in fish kill.
If the fish is accustomed and prefers water at 58 degree F and you catch and release it within 30 secs, it will have a much better chance for survival than one caught in 68 degree water. In short, water that's a bit too warm, holds less oxygen and the fish suffer when held out of the water on top of fighting in that warm water.
(3) Hook and hooking mortality plays a huge part. Hooks that are larger cause more damage, multiple barb hooks cause more damage, hooks with barbs cause more damage, hooks or flies that lend themselves to swallowing cause more damage. A hook in the gill area causes severe damage.
In short use barbless hooks that are appropriately sized and don't allow fish to swallow the fly if you can help it.
(4) Predators can chase down and kill an exhausted fish quickly and do.
Play the fish quickly get it in, even put it in a live well, till the coast is clear and then release. This usually applies to saltwater fish but I suppose it happens in freshwater also.
(5) Handling fish improperly causes mortality to shoot upward.
Big heavy fish should only be handled with both hands, one on the head and one supporting the belly, as a one hand, jaw lift usually causes damage. Trout and others in the salmonid family should be handled as little as humanly possible as they lose slime easily to hands and are very susceptible to injures from fungus after the slime is gone and from pressure damage to internal organs as they are slippery and folks tend to squeeze a bit too much. Use a hook release if at all possible
and release in the water, and for trout the Runje's Releaser, available from Silver Bow in Spokane where it was invented is inexpensive, very simple, and the best I've found.
(S) to summarize the bad boy of the above bunch is suffocating the fish by keeping it out of water too long.
Warmer water demands a fast fight and quick, in water, release if the trout is to live. High mid summer temps in some trout streams demand no fishing if you want the fish to live and all of this talk about temps and even lactic acid above, directly relates to the way the fish takes in oxygen and then uses it in addition to how water hold oxygen and what part temperature plays there.
After all that please keep in mind that an automatic catch and release is not always best for the fish and fishery. I prefer the Selective Release that the In Fisherman crowd coined. Too many brookies in a western stream--catch and keep! Browns or rainboews threatening an eastern Brook Trout stream, catch and keep! Ample fish and fish survival such as often is found with bluegills and with some runs of salmon -- catch and keep! When the fish is an invasive species threatening native species it edges upward on the catch and keep ladder! Prolific and under fished species such as Bluegill, Perch, Sunfish, Perch, and Crappie in many places, catch and keep! In short Selective Release demands a better than average knowledge of the fish and the fishery and lends itself to conservation!