A big bass is an abnormally large bass caught while big net fishing at any big net/harpoon fishing spot. Since players can only get it while fishing for bass, it requires 46 Fishing to catch. Players have a 1/1000 chance per bass caught to instead receive a big bass. Catching one prompts the message "You catch an enormous bass!" instead of the usual "You catch a bass."
It can be mounted in a Skill Hall in a player-owned house. Before it can be mounted, it must be stuffed by the Taxidermist in Canifis, where she will charge 1,000 coins and return a stuffed big bass to the player. They can then mount the fish using 2 oak planks. When players take it to the Taxidermist, she will exclaim "That's a mighty fine sea bass you've caught there."
A big bass may be cooked into a regular bass for the standard cooking experience.
Rada's Blessing does not allow you to catch double big bass.
For an exhaustive list of all known sources for this item, see here