What was fishy about the story of a trout in a glass case in a village side inn the author and one of his friends went to? (Three Men in a Boat)

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!