- A white, seawater fish of the genus Alosa, belonging to the herring family, which migrates from the sea to rivers to spawn. There are several varieties, e.g. Allis shad, Alosa alosa, and the slightly smaller twaite shad, Alosa fallax, ranging from 30 cm to 1 m in length. Its oily delicate flesh is often marinated in acid or cooked with acidic leaves to loosen or dissolve the bones. It is generally grilled, fried or baked, and is also used raw in sushi.