In Fields of Mistria, you'll notice that the shadows of fish in the water are different sizes. You'll want to cast your line on every shadow if you plan to fill out the Fish Wing in the museum. One of the larger shadows will net you an Alligator Gar, a winter-exclusive fish. The size of the fish's shadow doens't determine its difficulty when catching it, but it does determine its rarity. The Alligator Gar is a rare winter fish in Fields of Mistria that you'll want to nab before the season ends.