If you can draw out the shape and size of the walleye on some paper, it'll make it easier for you to carve the cake into shape. Just cut out the shape from the paper, and then layout enough cake onto your cake board and start taking away small amounts of cake until you get the basic shape. You don't want to make too many small details, just get the general shape defined. Then cover it with a thin layer of royal icing and allow it to chill. After the cake has chilled, you can take it out and start decorating. I'm not sure what you had in mind (since you said you don't use fondant), and whether or not you want to use fondant or just icing; but I can tell you from experience (making a fish shaped cake), that using a paper template will make it a lot easier to shape.