The Stranglethorn Fishing Extravaganza was removed, because of CRZ issues. With Patch 7.1.5 it is back as a world quest, offering the same rewards, but now it's more forgiving as more players per region to complete the quest are considered winners. This is a good change especially for realms with high population.
At BlizzCon they talked about 50 winners, but based on live, numbers seem to be higher.
Winners will be awarded the Master Angler of AzerothMaster Angler of Azeroth achievement, which is required for Accomplished AnglerAccomplished Angler and the "Salty" title.
Rewards that all winners can choose from are
Arcanite Fishing PoleArcanite Fishing Pole
Boots of the BayBoots of the Bay
Dread Pirate RingDread Pirate Ring
High Test Eternium Fishing LineHigh Test Eternium Fishing Line
Hook of the Master AnglerHook of the Master Angler
Stranglethorn Fishing Extravaganza can now be found as a world quest in Booty Bay (similar to how quests appear on the map during the Call of Scarab micro-holiday).
Winner screen after completion
Pictures are a courtesy of redditor Bal0rCIM (Source)