“The Fish Filcher” is a weird Octopath Traveler 2 side story, because the solution is so unexpected. Fish are being stolen at Conning Creek. You’re supposed to help. But, it’s not like any NPC in town has fish in their inventory. To complete this quest, you need to think outside of the box.
To start “The Fish Filcher,” talk to the Head Fisherman at Conning Creek: Harbor during the day. Then, change to night and wait. I actually stumbled upon this accidentally. I swapped the time to see if anyone was there, headed to the main part of town to do some stealing and information gathering with Osvald and Throne, then headed back to the harbor. When I did, I saw a Fish Thief standing there.
Knocking out the fish thief with Throne’s Ambush or Castti’s Soothe will complete the quest. A scene will then play out once you’ve done that. You’ll then get 6,000 leaves and a Fortifying Nut (M) for completing “The Fish Filcher” in Octopath Traveler 2. If you want, you can also then steal from the new NPC standing there to get an Enlightening Earring and Fool’s Leaf.
Octopath Traveler 2 is available for the Switch, PS4, PS5, and PC. A demo is available.