Back in April, we were all trying to make some sense of the datamined cooking and fishing items -- fish, as a base item, didn't appear to be used in cooking any longer. Instead, the flesh of the fish was used. For example, Blackwater Whiptail Flesh. Now that the alpha is available, and more items have been populated, the process has become a little more clear. Using the Blackwater Whiptail example, the following fish can be caught: its own form of bait. It's too early in the alpha to say whether those forms of bait are required to catch that particular form of fish, or if it will simply make the process easier. It may be that without bait, the best you can catch is a small or standard fish, and with the bait, catching an enormous fish becomes more likely.
While it all seems far more complex than fishing used to be, the Warlords model of fishing makes a lot of sense when viewed through the context of profession catch-up systems. It's no different than leveling herbalism in Mists of Pandaria -- you can pick Pandaren herbs at a low level of herbalism, but at a much smaller yield.