Fish and saltwater mammals were a major source of food and were, throughout regions with enough waterways, more important in diet than hunted game or gathered plants. Often concentrated in large numbers and apparently available endlessly, seafood was the chief food for some tribes and was second only to agriculture for others. In all, seafood was the major source of protein and oil for tribes living on the shores of the Atlantic and Pacific Oceans, the Gulf of Mexico, Chesapeake and San Francisco bays, and the lower reaches of the Mississippi and Hudson Rivers. Whales, the largest of the sea animals, were highly prized. Although beached or stranded whales were easiest to obtain, tribes of the coastal East and Northwest mastered hunting techniques with harpoons and bows and arrows. Seals and porpoises were harpooned. (Harpooning skills made many Native Americans valuable to the Anglo-American whaling industry.) Oil was a major contribution of these mammals, but meat and sometimes skins were important as well.