Can’t speak to mortality well after release but fly caught, non-netted released coho do pretty well imo. Can’t remember any floaters or damaged fish, and remember seeing floaters around gear anglers, especially in the early days of wild coho release offshore. Netting, bringing fish on board and double hook rigs imo are the three worst things you can do if you are planning on releasing any fish.
I remain quite curious about how high "delayed mortality" rates might be for released coho. I started beach fishing for coho in 2014-2015, and have caught quite a number of them since then. For direct immediate mortality in released adult fish, something I remember when I see it, I am pretty sure I have had 3, and maybe a 4th, die on me upon release, all unclipped/wild fish that had to be released, all deep hooked in the gills and obviously bleeding as they came in. 2 died as I watched, one/two swam away but I doubt it survived long. If that's roughly accurate for non netted, reasonably well handled fish, thats ~1% or less immediate mortality in fly caught adults I have touched, I think.
Would love to know what the story is for any additional delayed mortality, due to stress or some sort of slower acting damage that happens during the fight. I havent netted many fish, but agree that nets cause more scale loss, even for knotless rubber nets. I see some occasional scale loss from line wrap, sometimes some scale loss from too much beach contact. Very curious to know if light beaching up on their side to dehook them (always wet,always fast as possible) is better than the risks of a slightly longer fight in shin deep water where you can grab and dehook them in the water, causes any substantial delayed mortality in the next few days, or prevents them from getting into the river while healthy.
If delayed mortality was even 5-10% in fish that seemed well, seemed intact, and seemed to swim away strongly, I'd probably have to rethink my fishing operations. It's a blood sport, but the degree of bloody does matter.
Shakers are another story, the number of damaged and likely died are solidly higher, but I have no idea how high.