During spring in Fields of Mistria, you'll be able to catch fish you wouldn't normally find in the other seasons. You'll get different types of fish depending on the size of its shadow and the body of water you're fishing in (i.e: lake, river, ocean). For some fish, you'll also need to keep an eye on the weather conditions, too. The Walleye is one of these. Currently, you don't need the Walleye for any side quests in Fields of Mistria as of the 0.12 update. You'll still want to catch one if you intend to collect all the rewards from the museum.