To release a fish, keep it in the water if you can. Handle the fish as little as possible and avoid holding with dry hands to prevent removal of protective slime coating.  Don't let the fish bounce on the boat deck, carpet or on shoreline rocks and gravel.

If it's a fish without sharp teeth like a bass, hold its lower lip between your thumb and index finger. Hold them vertically and support large fish with a hand under the belly.  Grasp toothy fish (such as walleye or northern pike) across the back of the head, with fingers and thumb holding gill plates closed. Watch out for sharp edges and if you must hold a fish by putting your hand through the gill opening, avoid touching delicate gill filaments. 

Don't keep a fish out of water longer than you can hold your breath and never tear a hook out. This can harm the fish and it may not live. If the fish is hooked deeply and you can't easily remove the hook, cut the line to release the fish. The hook will rust, dissolve, or become loose without harming the fish.

Make your own hook remover!

If you are using bait or lures that are frequently swallowed and deep hooking is likely to be a problem, use barbless hooks. Unhooking your catch with barbless hooks is faster, easier and healthier for the fish.

If a fish loses consciousness, try to revive it by gently moving it forward and backward so water moves through its gills. When the fish begins to struggle and can swim, let it go.

Learn what to do if you plan to keep your fish.

Source: Oklahoma Dept. of Wildlife Conservation