I will copy and paste Parker's ifish advice for proper release of native salmon and steelhead (for trout and other fish too). ....
I have released well over 1500 steelhead and did spend 3 years working in the UW Fish pond with salmons. Here's what I can tell you about C&R.
Play the fish hard and bring it in fast. Do *not* overplay the fish to tire it out. That's probably the absolute worst thing you can do. That's almost a certain Kiss-of-Death for that fish. I forget the biological reason, but it has something to do with white blood cell counts. The more you fight the fish, the worse these counts get.
In shallow waters, Wet your hand, wade out to your shins and tail the fish. Remove the hook. At this point, if you are like me and want your picture taken, wet your other hand and gently cradle it's belly. When the fish is calm, lift the fish out of the water, head slightly pointed down. Smile, and gently put the fish in the water with it's head pointed up river. At this point, the fish should vigorously slap it's tail and get you all wet as it darts back to deeper water for safety. If not, gently keep your hand on it's tail and move the tail back and forth in a psudo-swimming motion. In a few seconds, a health steelhead will again, slap the water and get you all wet in a "thank you", as it swims away. Any fish that lazily swims away, is not a good sign. That fish will probably not survive - not a sure thing, but it's really not a good sign.
In deeper waters, where you are stuck in your boat, use a net to contain the fish. Ideally, we all have catch-and-release rubber nets. If you do not, then why are you catch and releasing fish? Go out and spend the dollars if you care about good C&R practices. If you don't have the rubber C&R net, a good old net will work. Once the fish is in the net, keep the fish submerged in the water. DO NOT bring the fish and net in to the boat. Tail the fish and remove the hook ASAP. You do run the real risk of having a trashing fish tangle itself in the net and hook. With the hook removed, and the fish tailed, gently cradle the fish by the belly with the head pointed down. Again, repeate the above process for releasing it.
Some personal notes:
Holding the fish by the tail with one hand, cradling it's belly with the other, and slightly pointing the fish in a heads-down position will calm any fish down for short periods of time. Don't know why, but it does work.
Tailing a fish with wet bare hands is recommended. Tailing a fish with a glove will remove the natural slime off of the fish, thereby increasing the risk of infections and other nasties. Yes, you will get a better grip on the fish with a glove, but you will do more damage.
Chinook are easy to tail as they have a pronounced forked tail that does not collapse.
Steelhead are a little more tricky, as their tails are not nearly as forked and somewhat collapse when tailing.
Coho are almost impossible to tail without a glove. Their tails will completely collapse. Not impossible, but a real pain.
I don't even bother tailing Chums. Yuk! Don't want them nasties in net, either!
That's all I can say for now. There's really nothing to it and if done correctly, very little harm is ever done to a tailed fish.
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Thanks Parker.