wow shadeblack you totally beat me to it with almost the exact same comment. You didn't necessarily kill all the big daddies in the first game, just all the ones in the areas you went to, if you desired. Maybe he was swimmin' in the ocean the whole time. Trust me, you would have known if you fought one that could use plasmids, hes a survivor that one.
But how can you be the 1st big daddy and it be a sequel and your in rapture? They show you waking up with ten talking to you. Where were you during the events of the 1st game?