Fishing in Fields of Mistria is fairly easy, but if you're planning on completing the Fish Wing in the Museum, you'll need to fish in pretty much every area. The fish that appear will be influenced by the type of water you're fishing in, the fish's shadow, and the weather. You should be able to get every seasonal fish in Mistria, but if you're going for the Legendary Fish, you'll want to work on leveling up your Fishing Skill. There are a total of four Legendary Fish, one for each season.