A released trout is not a survived trout. Fish swim away from bad handling and die an hour later, out of sight, of a lost slime coat, oxygen debt, or an infection that sets in days after. The good news is that the things that kill released fish are known, and all of them are under your control from the moment you hook up.
Air exposure
A trout out of water is a trout not breathing. Its gills collapse together without water to hold the filaments apart, and every second in air adds to the oxygen debt from the fight. The single most effective change most anglers can make is to keep the fish in the water until the hook is out, and then lift it only for a photo, only briefly, and only if it has recovered enough to hold itself upright.
A practical target is to count. If a fish is out of the water for longer than it takes you to take a breath and let it out, that is long enough. Ten seconds is a long time to a trout that just sprinted. Two lifts of a few seconds each are better than one long one.
The fight matters too. A long fight on light tippet is not sporting; it exhausts the fish past recovery. Use the heaviest tippet the fishing allows, put the rod low and to the side to apply real pressure, and get the fish in the net before it has burned everything it has. A 16 inch trout on 5X should not take five minutes.
Wet hands and the slime coat
A trout's skin is covered in a mucus layer that is its immune system and its barrier against fungus and bacteria. Dry hands, cotton gloves, shirts, grass, and the bottom of a boat all strip it. Wet your hands before touching a fish, and touch it as little as possible. Never lay a fish on rocks or dry sand for a photo.
Hold a trout gently around the wrist of the tail with one hand and cradle it under the pectoral fins with the other. Do not squeeze the belly, which is where the organs are, and do not hold a fish vertically by the jaw; its skeleton is not built to hang from its head and the weight can damage the jaw and internal organs.
Barbless hooks
Pinch the barb down with your forceps before you tie the fly on. A barbless hook comes out in a second, often without lifting the fish, and it is the difference between a fish that is released in ten seconds and one that is held for a minute while you work a barb out of cartilage. You may lose a few more fish on the way in; you will land more than you think because you will keep the line tight, and every one you lose was going to be released anyway. Some waters require barbless; check the state agency. The rest of the time it is simply better.
If a fish is hooked deep, cut the tippet as close to the hook as you can and let it go. A hook rusts out; a torn gill does not heal.
Water temperature
This is where the science is clearest and least followed. Trout are cold-water fish. As water warms, it holds less dissolved oxygen, and at the same time a trout's metabolism speeds up and it needs more. A fish fought hard in warm water builds an oxygen debt it may not be able to repay, and lactic acid it may not be able to clear. It can swim off looking fine and die later.
The practical response is simple. Carry a thermometer. As water temperature climbs through the sixties, fish stress rises sharply, and many anglers stop fishing for trout when their thermometer reads in the high sixties, fishing early mornings only or switching to warmwater species. This is a judgment about whether a fish you release will live, not usually a regulation, though some states and some waters do impose closures during heat; check the agency for the water you are on.
Rivers that warm are well known. The Firehole in July and August, the lower Green in Browns Park in late summer, the Big Hole in drought years, the lower Yakima in August. Tailwaters near the dam, cold spring creeks, and high-elevation freestones like the Uinta stay cool when valley rivers do not. The river pages here note where a gauge reports temperature; use it.
Rubber nets and photos
A net lets you control a fish without gripping it, keep it in the water while you unhook it, and photograph it from above without lifting it. Knotted nylon nets split fins and strip slime; rubber and rubber-coated bags do not. If your net is the old string kind, replace the bag. Keep the fish in the net, in the water, with its head upstream, while you get the camera out.
Set up the shot before you lift the fish. Camera out, framed, photographer ready. Lift the fish a few inches, take the picture, put it back. Holding a trout at arm's length over dry rocks while someone finds their phone is the way most photo fish die. Better photos anyway: the fish half in the water, the fish in the net, the fish being released. A fish held low over the water, with water dripping off it, is the picture that says you did it right.
The release
Face the fish into gentle current and hold it loosely until it swims out of your hand on its own. Do not pump it back and forth; water flowing backward over gills does nothing useful. If the current is strong, find a slack pocket. A fish that turns on its side needs more time. A fish that swims off strongly and then floats up needs to be recovered and held again. Stay with it until it holds itself upright and goes.
The order of operations, every fish
Barb pinched before the cast. Fight short. Net in the water. Hook out with forceps while the fish stays wet. Photo ready before the lift, lift for a breath, back in the net. Revive in current until it leaves on its own. On warm afternoons, put the thermometer in first and decide whether to fish at all.