Fish are peaceful creatures that occasionally swim in the Void. They do not attack but simply swim peacefully in groups.
They are the main source of seeds, leather, and fat in the Void. You can catch them using your hands, a revolver, or a grappling hook to gather these materials.
If you kill one of them, the entire group will become frightened and swim very quickly and chaotically in the Void for a while, but eventually, they will calm down, and you can continue fishing.
Sharks also feed on them to restore their HP.