You will need to craft a fishing rod or harpoons at your workshop (you will need to upgrade the workshop to be able to craft these tools). Now, send out an expedition and make sure to equip the fishing rod or harpoons with one of the characters on the expedition. Head to the northwest part of the map to find the river. Move/Search around this area until a fishing event triggers and keep choosing to fish until you catch something.
See the video below for more information.