There is no fishing puzzle in the Puzzles & Milestones section. There is one with a fish picture, but it's not fishing. Fishing comes from having farming tech level 3. The fish in the puzzles is something else. Rather than give away the secret outright... I'll give you this hint:

Check places that may have fish in them.

If you really must know, check this Strategy Guide for a general set of hints, or the Detailed Spoilers.