Well the better question is was it a public land or private, was it your land or someone you know in which case you can be perfectly justified in doing so. and with public land just as much, as nobody "owns" a fish let alone tied and unattended. "Moral of the story if you are not by it, possession transfers to the one who is" my dad taught me that long ago when I would tie up my stringer too close to him. On a side note depending on where you found it I would have left a note and left a bluegill or something as tradeIt wasn't your fish to make that decision.
it is not your place or mine to take it upon ourselves to play "save the fish" just because we don't like people keeping them.or for any other reason,unless you have reason to believe a law was broken.in which case,the right approach would be to contact law enforcement.unless you left something out,you have no clue as to why the fish was there,or who caught it,etc.and i always get po'd when people keep those things
that's the most asinine reason of all.i always get po'd when people keep those things so i drove it back to the river