The rainbow trout (Oncorhynchus mykiss), also called the redband trout, is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America as well as much of the central, western, eastern, and especially the northern portions of the United States. The ocean going (anadromous) form (including those returning for spawning) are known as steelhead, or ocean trout (Australia). The species has been introduced for food or sport to at least 45 countries, and every continent except Antarctica. In some of these locations, such as Australia and South America, they have had very serious negative impacts on upland native fish species, either by eating them, outcompeting them or transmitting contagious diseases. In some cases, they have been responsible for the extinction of native fish populations.