What lies were told about the trout in the glass case? What exposed the liers in the end?

Various locals walk into the room to claim capture of the great fish. Jerome and George know they can't all be the person who caught it so they think they are merely boasting. Fishermen, Jerome had remarked, have a penchant for exaggerated stories about their fishing prowess,

"I should never make anything of a fisherman. I had not got sufficient imagination"

The fish turns out to be a model!