Short answer
An endangered fish is not one hidden species. On the third island, use the pink Standard Lure and inspect every catch before you sell, cook, or eat it. Keep a fish only when its information shows the Endangered Species label, then give that fish to the swimmer in yellow floaties.
The swimmer gives you a Carrot in return. Equip the Carrot as bait to summon the Pufferfish boss.
- Reach the third island and speak to the swimmer.
- Buy the new Fishing Rod and pink Standard Lure.
- Inspect every catch and look for the Endangered Species label.
- Trade the labeled fish for a Carrot, then use it as Pufferfish bait.
What counts as an endangered fish?
The label matters more than the species name or appearance. A rare-looking catch or special variant does not automatically complete the request. Inspect the fish and confirm that the endangered label is actually present.
Treat a valid catch like a quest item. If you sell or consume it first, you will need to keep fishing for another labeled catch.
If the swimmer will not accept the fish
Check the catch again and make sure it carries the Endangered Species label. Then confirm that you started the request with the swimmer and that the fish is still intact in your possession.
If you lose the Pufferfish fight, repeat the exchange to obtain another Carrot before trying again.
Sources and update notes
Game details can change after patches. These sources were checked for the date shown above; use current in-game labels and official announcements when they differ.
- PC Gamer — How to Fish Pufferfish guide ↗Independent launch-week walkthrough used to cross-check the endangered-fish exchange and Pufferfish bait flow.
- Steam — How to Fish announcements ↗Official source for release announcements and patch posts; check it before relying on older route details.
- Steam — How to Fish ↗Official source for the game description and release information; check current announcements for details that may change after patches.