Casket Fishing is a brand new style of fishing for sponsors+ and it gives you Rainbow Fish and a chance at awesome new rewards. To fish these you need to go to Skilling Tab > Fishing & Cooking > Casket Fishing The only requirements you need is a Fly Fishing Rod and Stripy Feathers and a fishing level of 99.
Casket Fishing Rewards
Norma Caskets (1-25 DSCoins)
Dead Sea Scroll - New EXP Reward
Dead Sea Chest - Gives (1-2500 DSCoins)
Davy Jones' Locket (1-5000 DSCoins)