In Sherlock Holmes - A Game of Shadows Sherlock replaces Moriarty's bank book with a similar one containing a fish animation while he's being tortured with the hook.
However this is the first time the trout analogy is used (who is the fisherman who is the trout?). If this is the case when did Sherlock do the drawing? Did he expect the analogy to come up? Did he draw it while dangling from the hook? Did he switch books at a different time?