Any of the small, slender fishes of the family Osmeridæ, characterized by a large mouth, cycloid scales, a lateral line, an adipose fin, and no spines in the fins. They eat crustaceans. Represented in the North Country by the popular Rainbow Smelt ( Osmerus mordax), a small, ocean fish introduced into the Great Lakes early in the last century and arriving in Superior by 1930. Their annual spawning run up the streams of the North Shore is a major event.