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It could also be wound from one of those herons. They like to sit still in shallow water or on docks (both places LM hang out) and will take a 'stab' at any unsuspecting fish that swims by. I bet your knife did more damage than the turtle or bird did!!
I doubt if it was a heron. They like to grab the fish whole and I think you guys might be thinking it might have stabbed it with the beak. Believe it or not, I seen a heron up at Fletcher's Lake in Fulton County snatch up a largemouth probably as big as that one, maybe even a little bigger, and swallowed it whole. Craziest thing I ever seen. Thought I'd call the DNR and tell them the herons were grabbing bass under the legal limit LOL.
I've seen plenty of heron stab fish, frogs, etc with their beak then shake it off, grab it as it struggles to move, then swallow it whole. I think it depends on how easy of a target the prey is, and the size of it. I've also seen them go after bigger bass like that, just not always very successful!