In Assassin's Creed Valhalla, you can find Offering Altars. On this page of the guide, you will learn where to find the altars. This will make it easier for you to max out the Sciropescire region.
You will find the offering altar in the middle of the map, right next to the synchronization points.
To complete this side activity you need 5 small brown trouts. You will catch them in the nearby river.