As you explore each island, you’ll uncover one portal that, instead of taking you to a traditional speed-based stage, leads you to a familiar face within Cyber Space. Big the Cat’s fishing portals may seem like time-wasters at first, but it quickly becomes apparent when you look at the rewards Big offers. Filling out the Fish-o-pedia is fine, but the real reward comes in trading the tokens Big gives you for each catch.
Those tokens are as valuable as anything in the game – seriously! You can use them to purchase all of the upgrade collectibles (Seeds of Power, Seeds of Defense, Lost Koco, Skill Points), game-progression collectibles (Chaos Emerald Vault Keys, Portal Keys, side-character Memory Tokens), rings, and even Egg Memos (voice logs from Eggman that contain additional lore). You can find coins that allow you to fish throughout the open zones (they’re the purple ones), but you can really stack them high every few nights when meteor showers occur. These showers resurrect the enemies on the island, but if you pick up the meteorite pieces, you get a spin on the fishing coin slot machine.
Source: www.gameinformer.com