A. The Atlantic salmon, Salmo salar, is the closest native relative of the brown trout, Salmo trutta. There are some differences in the spots on the two species, but these are not always easy to distinguish. The best distinguishing characteristics are the shape of the tail and the length of the upper jaw. The brown generally has a square or even slightly rounded tail; the Atlantic salmon's tail is forked. The maxilla, or upper jaw, of the brown trout extends well behind the eye; that of the Atlantic salmon terminates below the center of the eye.

One other feature found only in the brown trout is two zigzgag rows of well-developed teeth on the vomer, the raised shaft of bone in the center of the roof of the mouth.