Short answer
You need three Leeches. They are small ground pickups on the second island, also called the Forest Island; you do not catch them with a fishing rod.
Give all three to the quest NPC near the central shack. She turns them into Modified Leech bait, which you equip before casting into the lake to summon the Giant Piranha.
Where to look on Forest Island
Walk the ground around the island instead of searching the water. Leeches look like tiny dark lines and are easy to miss in tall grass, so sweep your view from side to side and watch for the pickup prompt.
Check the island perimeter, the grass around the middle area, and the dirt path near the central cabin. When one appears, look nearby because the pickups can be close together.
- Search the ground, not the lake.
- Move slowly through grass and watch for the Leech prompt.
- Check nearby ground after finding the first pickup.
- Confirm that all three are in your possession before returning to the NPC.
Use the Modified Leech bait
After the handoff, equip the Modified Leech on your fishing rod and cast into the Forest Island lake. Slowly reel it in until the Giant Piranha takes the bait and starts the boss fight.
The bait is consumed. If the fight is lost and the boss must be summoned again, collect another set of three Leeches and repeat the exchange.
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.
- GamesRadar+ — How to get Leeches ↗Independent launch-week walkthrough confirming the three ground pickups, NPC handoff, and Giant Piranha bait loop.
- Dexerto — Leech bait reference ↗Second independent reference confirming the Forest Island location and three-Leech requirement.
- Steam — How to Fish announcements ↗Official source for release announcements and patch posts; check it before relying on older route details.