From V Rising Wiki
Fish
Fish in V Rising are items that can be used as an ingredient to recipes. Main way to obtain Fish is by fishing. Fishing is an activity accomplished by using a Fishing Pole at a body of liquid that has bubbles. While near the body of liquid, pressing Attack button at a point over the bubble will cast a line into the liquid. Pressing the button again when the bobber splashes making a different sound reel in the line and an item will come up with the line.
Fishes is crucial for crafting potions, feeding your prisoner and finding some Hats. It is also possible to obtain some fish through looting chests and breaking barrels at Silverlight Hills near the dock area. There are currently 9 different Fishes in the game.