More on Mute as a Fish from Infoplease:

• Mute as a Fish - Mute as a Fish Quite silent. Some fish make noises, but these are mechanical, not organic. Source: ...
• Fish - Fish The French have a remarkable locution respecting fish as a food: Aprés poisson, lait ...
• Dictionary of Phrase and Fable: M - Definitions, origins, and illustrative excerpts for words, phases, and literary allusions starting with "M"