Short answer
"Mysterious fish" is the last stage of the Fisherman's quest Fish for the Fisher, and the hand-in checks for 5 Mysterious fish fillets, not whole fish. Cut them at a Kitchen Table (Farming mastery 6) from Goldfish, Silverfin or Swamp Ghost. Swamp Ghost is the fastest: 3 fillets each, 6 with Fish Filleter. The reward is 3 Town Gratitude and the Pro Rod.
The Town harbour at dusk
View full-size image ↗Official promotional screenshot · © Lazy Bear Games / tinyBuild · Official media kit · Material credits
Why your fish is not accepted
The quest text says "mysterious fish", but the Fisherman only takes Mysterious fish fillets.
Fillets need the Mysterious fish dishes technology, which is the reward for the quest's previous stage (sea fish).
So finish the stages in order: 5 River fish fillets, then 5 Sea fish fillets, then the mysterious stage.
Which fish make fillets
Goldfish: 1 fillet (2 with Fish Filleter). Caught at the Port or in the Forest with Insect Bait.
Silverfin: 2 fillets (4 with Fish Filleter). Caught in the Swamp with Breadcrumb Bait.
Swamp Ghost: 3 fillets (6 with Fish Filleter). Caught in the Swamp with Insect Bait.
Two Swamp Ghosts cover the whole hand-in, so the Swamp is the place to go.
Cutting the fillets
Use a Kitchen Table I or II. Each batch uses one whole fish, takes 7 ticks at 2 energy per tick and needs Farming mastery 6.
Fish Filleter is a Farming perk (tier 4, after Fisherman) that doubles fillet output.
Each craft also pays +1 green tech point, and studying a fillet gives +10 green and +10 blue.
The keeper's home and garden from the official media kit, where kitchen stations sit. Not a fishing or Kitchen Table screen.
View full-size image ↗Official promotional screenshot · © Lazy Bear Games / tinyBuild · Official media kit · Material credits
Bait and stock
Insect Bait is made from 4 Fireflies; Breadcrumb Bait from 1 Bread.
Each fishing spot holds 1 to 5 of each fish and restocks one at a time, so a spot can run dry if you fish it hard.
The Eel, the most valuable fish to sell (1 silver 92 bronze at the Fish Stand), is not one of the fillet fish.
The reward
Handing in the fillets pays 3 Town Gratitude and the Pro Rod, the best rod in the game (+6 line bonus).
Across all three stages the quest pays 6 Town Gratitude plus the Sea fish dishes and Mysterious fish dishes technologies.
Leftover fillets go into Fisherman's pie.
Sources
- communityFextralife — Mysterious Fish
- communitywikily.gg — Fish for the Fisher
- communitywikily.gg — Mysterious fish fillet
- communitywikily.gg — Fishing
