Prepare the bait
Catch an endangered fish such as Bowlfish with Standard Lure, inspect it with F, and give an eligible catch to the Tourist.
Take Carrot [E] when the yellow-floaties Tourist offers it; select it on the rod to begin the boss encounter.

Respond to its changing size
Pufferfish moves quickly in a straight line and grows as it takes damage. A shotgun or SMG lets you keep firing while changing direction.
| Visible situation | Action |
|---|---|
| Small and fast | Keep turning while firing. |
| Larger body closes the gap | Use the trees to make room. |
| Purple clouds ahead | Leave that patch and shoot from clear ground. |

Complete the Fin exchange
If you need another Carrot, repeat the endangered-fish exchange.
- Pick up Pufferfish Fin [E] after the kill.
- Carry it to the Tourist and collect the offered island 4 Flash Drive.
Sources and further reading
Third island Swimsuit guy quest (One of these endangered creatures would be cool) - How to Fish Game