From Sonic Retro
The Lure upgrades are a series of items that can be found by Big the Cat in Sonic Adventure. There are four of these upgrades, and each one will allow him to catch bigger fish. The order of which they are obtained does not seem to matter. Sonic Adventure is the only game where Big fishes, and so this upgrade has not appeared anywhere since.
- Lure upgrade 1 location: Station Square Adventure Field. Near where Tails finds his Jet Anklet. It is sitting in the water just under the same opening.
- Lure upgrade 2 location: Mystic Ruins Adventure Field. In the jungle area with Big's house and the ruins of the Echidna temple. There will be a secret room hidden near Big's house. It is much easier to find in Sonic Adventure: DX thanks to the new map function on the menu screen.
- Lure upgrade 3 location: Ice Cap Action Stage. This is the only upgrade to be found during an action stage in the whole game. Big must climb up the ice stairs until he finds a small pool that will lead to another area. In that area, there will be dinosaur bones on the ice. There should be a spot of thin ice that Big can break through. Dive down and you will see a dinosaur head at the bottom. The lure will be right in the dinosaur's eye.
- Lure upgrade 4 location: Egg Carrier Adventure Field. In the holding cell area where Amy was taken. It will be in the cell closest to the door.