The festive fish is an item that is part of the 2015 Christmas event. They are caught during the final quest episode of the event, Quantum of Solstice, and can also be fished from the top of the Iceberg and turned in for experience in the Fishing skill after the quest is complete.
The amount of experience gained depends on the player's fishing level. At level 99 fishing, catching the festive fish grants 120 experience. Turning in the festive fish to nearby penguins grants an additional 26 experience, ultimately granting 146 experience per fish at level 99 fishing. Free-to-play players have a 50% experience penalty for this activity.
The consume effect of the Fury shark outfit works on these fish; however, they are lost and thus cannot be turned in for extra experience.