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knock them on a rock and get it over with. I can't believe I am even answering this
Bonk and pop a gill to bleed.
I've always head bonked trout. In Alaska, most folks I've known with experience will rip a gill of the salmon to bleed it. "Makes for better fillets" was the claim.
Gill cut is the quickest way, they are dead within seconds of bleeding out. Bled out trout/salmon taste better than not.