Dubbed "pool migrations" by El, the new fishing event spawns multiple pools in one of eight different areas across Pandaria. Seven of these locations are marked on the minimap by a red x, shown on the map above. Each spot has a multitude of rapid-spawning pools that contain the same fish as usual, but occasionally you'll encounter one very, very large pool, home to Krakkanon, a giant sea monster that can be engaged and defeated for rare rewards and a multitude of fish.
El has also caught one of the four new carp pets from these rapid-spawning pools, which indicates the others are likely scattered around the rest of Azeroth. Also of note, each pool migration area also has one or two Anglers hanging around, including El. At the moment this appears to simply be for flavor purposes, as none of the NPCs offer any quests or rewards. They're just in it for the fish!
Honestly, I have to say I'm pretty happy with the addition of this event as opposed to yet another fishing tourney. I'd much rather fish pools without the added stress of being the first to nab some rare fish -- and I like the implementation of new fish pets into the mix, too. For more information on the new event, including spawn locations and more info about the NPCs, head on over El's Extreme Anglin'.