Most Helpful Girl
A fish isn't caught, as a thief is caught, for doing something wrong. A fish dies, because it made the mistake of, literally, opening its mouth and allowing itself to be hooked.
Persons who open their mouths--speak inappropriately--may not be arrested, but they betray themselves in other ways, as illiterate, as uninformed, as foolish, as uncouth, as a gossip... whatever.
It should be noted, that for every fish that is caught for opening its mouth, hundreds are caught--in nets--for being in the wrong company. But that is another metaphor.0