A big swordfish is an abnormally large swordfish caught, while fishing for swordfish, at any cage/harpoon fishing spot. It is obtainable by both barbarian fishing and by normally fishing swordfish using a harpoon. Since players can only get it while fishing for swordfish, it requires 50 Fishing to catch. Players have a 1/2,500 chance per swordfish caught to instead receive a big swordfish.[1] Catching one prompts the message You catch an enormous swordfish! instead of the usual You catch a swordfish.. If fishing with a fish barrel, a big swordfish caught will be placed into your inventory.[2]
A big swordfish may be cooked into a regular swordfish for the standard Cooking experience. When attempting to cook it, there is a confirmation dialogue stating "You could get that stuffed instead." with the options to "Cook it anyway." or "Don't cook it."
The benefits of Rada's blessing and spirit flakes will cause two big swordfishes to be caught at once if either item activates.[3]
The benefits of Rada's blessing and spirit flakes now allow players to receive two big fish at once on activation.
References
^Jagex. Mod Ash's Twitter account. 13 April 2015. Archived from the original on 26 March 2019. Mod Ash: "Bass = 1/1000. Swordfish = 1/2500. Shark = 1/5000."
^Jagex. Mod Ash's Twitter account. 8 June 2022. Archived from the original on 9 July 2022. Mod Ash: "You can still receive the big fish, and it'd have to go to your inventory."
^Jagex. Mod Ash's Twitter account. 2 March 2023. Archived from the original on 2 March 2023. Mod Ash: "[Does the fishing boost from both Rada's Blessing 4 and Spirit Flakes apply towards the chance to get a Big Fish (Shark/Swordfish/Tuna) while fishing?] I believe they can double the big fish these days. Originally they wouldn't, but there was a rewrite since then that applied the bonus more widely."
^Jagex. Mod Ash's Twitter account. 21 August 2023. Archived from the original on 29 August 2023. Mod Ash: "Yeah, the answer to that question changed since March 2020 - I think we did a rewrite of the core fishing code during the installation of a League." The earliest occurence of a player reporting a catch of two big fish took place on 23 September 2021.[4] The rewrite in question likely dates from the implemention of Trailblazer League.
50 
Harpoon
