Frequently I have watched novice anglers try to gaff a fish, and it often looks as if they are chopping wood. That is definitely not the way to do it.
The first rule of the gaff is never use one when a net will do. But when it comes to big fish (or ones with dangerously sharp teeth) that are destined for the table, a gaff is often the best means for transferring your catch from the water to the fish box. And surprisingly, there are many situations when a gaff is a better tool than a landing net for releasing fish.
A sure way to lose a fish is to try to gaff it before it is ready, but even a tired fish often reacts violently to any sudden moves it perceives as threatening. The best way to gaff a fish you plan to keep is to reach smoothly across its back with the gaff, keeping the point facing down, on the underside of the handle. Next, pull the point smartly toward the boat so that it penetrates the back of the fish under the dorsal fin. Once the gaff is set, continue the pulling movement, now at an increasingly upward angle as the fish nears the side of the boat. If done properly, the gaff goes home and the fish moves from the water to the fish box in a single, smooth motion.
Do not gaff a fish from beneath. More often than not, the fish will feel the gaff before it can penetrate properly and will lunge upward and forward. Often it will be injured badly enough to die if it manages to escape. Besides, the stomach area is soft, and the gaff can tear out far more easily than when it is firmly planted in the solid meat of the back.
It is always best to release a fish without removing it from the water, and a gaff around the lower jaw is often the best way to control a large saltwater fish at boatside for hook removal. The procedure is simple enough. Once the fish has been subdued by the angler to the point where it is tired and quiet in the water, the gaff point is inserted into the mouth and smoothly passed through the thin membrane just behind the jawbone. This allows the fish's head to be held tightly against the side of the boat, where its movements are restricted.
Once the fish is immobilized, the hook can be removed with pliers or a long-handled hookout. I use a Baker Hookout for toothy species like sharks and barracuda; it is also much safer to use a hookout with a long reach if treble hooks are involved.
The membrane behind the jawbone is essentially avascular and bleeds very little, if at all, and although I have no doubt that this procedure causes some minor discomfort for the fish, keep in mind these creatures routinely eat very spiny baitfish, so they are used to puncture wounds inside the mouth. I've also noticed that if the gaff is inserted very gently, and if the head of the fish is not violently yanked out of the water (so skip that macho demonstration), there is often very little reaction. Just be careful to avoid the gills, which isn't a problem if the gaff is centered right at the front of the mouth.
I do not think it is a good idea to lift large fish out of the water by the lower jaw for a photograph, whether using a rope or a gaff, because the heavy strain can injure the fish's jaw, although some smaller species can tolerate this better than others. For example, it probably does no damage to a 10- or 15-pound snook or barracuda, but a 100-pound tarpon is another matter. And never lift a Pacific salmon out of the water with a gaff around the lower jaw; these fish have soft jawbones that split very easily.
Some years ago, I quit hauling big silver kings into the boat and have since photographed them in the water alongside. This sure makes cleaning the decks a lot easier afterward since I don't have to deal with copious amounts of incredibly slippery, stinky slime all over the place.
Short-handled and rope gaffs work bestt for releasing fish. My own preference is simply a large, barbless hook with a loop of rope for a handle (a big shark hook with the barb flattened is perfect). The idea is that the rope acts as a hinge if the fish suddenly starts flopping violently.
The loop should be just long enough for you to put your thumb through it and then wrap the rope across the back of your hand and, finally, across the palm. That way, if the fish gets the better of you, it's just a matter of opening your fist and letting the whole rig go. Otherwise you run the risk of being yanked overboard because you put the loop of rope around your wrist.
If you prefer a stick gaff to release fish (a good idea for fish with sharp teeth), the handle should be at least long enough to grab securely with two hands but not so long it becomes a club if the fish becomes violent. Two to 3 feet is about the right length for a release-gaff handle, and it will still work just fine for grocery duty.
Skip the one-handed gaff with a very short handle. I've had them yanked out of my hand a few times, and many years ago one even hit my companion in the head, fortunately handle-first or the consequences could have been serious. I've never owned one since.
There are also instances where a long-handled gaff works best for releasing fish. Just like a shepherd's crook, an 8-footer can be used to snare the bill of a sailfish or marlin so that the fish can be gently pulled to boatside and a gloved hand applied to the bill. To avoid accidentally sticking the fish during this procedure, many anglers keep a small piece of clear vinyl tubing over the point of the gaff and remove it only for taking fish aboard.