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:
Each size of fish yields a differing amount of flesh. The larger the fish, the more flesh.