Fishing in Assassin's Creed Valhalla is unlocked by building the Fishing Shack.
As I mentioned earlier, Assassin's Creed Valhalla Fishing Hut becomes available when your colony is Tier 2, and it costs 600 supplies and 45 raw materials to build. Once your fishing hut is up and running, talk to the little kid again to earn your fishing line and access his fishing task checklist. These tasks are simple in concept, but will require some serious angling. Each will require a fixed amount of a specific fish or four, and you will be rewarded for your efforts (the reward is displayed to the right of the interface). You can return the fish at any time, so you don't have to wait to get them all.
The fishing itself is simple. Venture to any body of water (preferably a place where you can see fish swimming and jumping), then click the D-Pad to bring up the Quick Radial menu. Select your fishing line from here (the fish icon) to equip your fishing line, then hold the left trigger to aim and the right trigger to cast. After throwing you can release the left trigger, but keep your finger on the right trigger primed. The controller with vibrator and you will hear a loud lapping sound when a fish bites, and you will have to pull the trigger to hook the fish.
From there you will have to mash the A / Cross button to coil the fish, while using the left stick to pull the fish towards you. The line will turn mocking and red as you stress it out, and you'll want to stop both pulling and spinning to keep it from breaking. Once the fish is near Eivor, it will descend and add it to your inventory. You can either return the fish to the fishing hut or sell it for massive cash. If you want to know where specific fish are hiding, be sure to purchase the “Fantastic Fish and Where to Find Them” notes from the vendors as they appear. These won't mark your map, but reading them will tell you where some fish like to hide.
