This huge family of saltwater fish has over a hundred varieties. The popular herring swims in gigantic schools and can be found in the cold waters of the North Atlantic and Pacific oceans. In the United States, two of the most popular members of this family are the American shad and the alewife, both of which are anadromous, meaning that they migrate from their saltwater habitat to spawn in fresh water. Herring are generally small (ranging between 1/4 and 1 pound) and silvery. The major exception to that rule is the American shad, which averages 3 to 5 pounds and is prized for its eggs — the delicacy known as shad roe. Young herring are frequently labeled and sold as sardines. Fresh herring are available during the spring on both the Pacific and Atlantic coasts. When fresh, the high-fat herring has a fine, soft texture that is suited for baking, sautéing and grilling. The herring's flesh becomes firm when cured by either pickling, salting, smoking or a combination of those techniques. There are many variations of cured herring. Bismarck herring are unskinned fillets that have been cured in a mixture of vinegar, sugar, salt and onions. Rollmops are Bismarck herring fillets wrapped around a piece of pickle or onion and preserved in spiced vinegar. Pickled herring (also called marinated herring) have been marinated in vinegar and spices before being bottled in either a sour-cream sauce or a wine sauce. The term can also refer to herring that have been dry-salted before being cured in brine. Kippered herring (also called kippers) are split, then cured by salting, drying and cold-smoking. Bloaters are larger than kippers but treated in a similar manner. They have a slightly milder flavor due to a lighter salting and shorter smoking period. Their name comes from their swollen appearance. Schmaltz herring are mature, higher-fat herring that are filleted and preserved in brine. The reddish Matjes herring are skinned and filleted before being cured in a spiced sugar-vinegar brine. See also fish.