rough handling (removing trout from the water without a net or destroying the slime coat) and high summer temps.

Generally, If the hook is any where in the mouth, you can safely unhook and release a trout. If you are gentle and get the fish into the water in under a minute, 99% survive. But don't drop the fish back in. Make sure he is swimming and not belly up. Trout are not bass.

If the line goes back into the throat and you can't see the hook or fly, but you need to release the fish, gently cut the line. You will damage too many internal organs trying to get the hook out if it is deep and it takes too long in warm weather.

Even when fly fishing, don't even stop for a photo in summer. I mean that. You have caught nice fish before and you will again. Get the blasted trout back in the water. I can't begin to tell you how many nice trout I have watched go belly up after being released by fly fishermen. It was not from injuries, it was from exhaustion. So don't "play" the fish too much in warm weather. Get 'em in. Use a C & R net. Send them home fast. Be gentle. Trout are a renewable resource.

Oh yeah, and NEVER use a treble hook. I have replaced trebles on all of my lures and still catch loads of fish. Never use trebles for bait. Powerbait works just fine on a standard hook. IMO, trebles murder fish. I once reeled in a nice trout I wanted to release and the treble hooked into both sides of the trout's mouth. Tore it up pretty bad.

Never release a bleeding trout unless mandated by law. It is usually severe damage to the gills.

I love all types of fishing and fish 25+ days a year.