Not to play devil's advocate, but that doesn't look like a second mouth to me. It looks like the fleshy area betwen the lower jaw has been pulled down as is often done when cleaning a trout. That looks like the fish's tongue to me -- like this trout was injured and healed this way.

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Joe C.
"Drift: If you saw a
piece of steak moving
unnaturally on your
plate, would you eat it?"
- Nick Lyons