Double Cod

The reason to hoard these is Ammo Reservation Potion. If you shoot anything at all, that buff quietly saves you a stack of ammo across a long boss attempt, and this fish is the gate on brewing it.

Because it only bites in one biome, it is worth a dedicated fishing session before a mechanical boss rather than something you pick up one at a time while doing other quests. The cooking use is real but forgettable. A Seafood Dinner is a fine thing to eat when you have spares, though you will always want the potions more.