Salmonids are teleosts belonging to the order Salmoniformes, which includes marine and freshwater fish originally from the northern hemisphere, and introduced by man into cold-temperate waters around the world for sporting and farming purposes. Within the order, they belong to the family Salmonidae, characterized by a primitive appearance compared to other teleosts, as the pelvic fins are placed far back and there is a dorsal adipose fin. The most widely known genera in the family are Salmo, Salvelinus and Oncorhynchus. Some salmonid species spend their entire lives in fresh water, e.g. Salvelinus fontinalis (brook trout) or S. namaycush (landlocked salmon). Others spend most of their lives in the sea and swim up rivers and streams to spawn (anadromous), e.g. Oncorhynchus kisutch (coho salmon), O. gorbuscha (pink salmon), O. keta (chum salmon), O. tshawytscha (Pacific or Chinook salmon), O. nerka (sockeye salmon) or Salmo salar (Atlantic salmon). There are also species that usually live in fresh water but may spend some time in the sea, such as Oncorhynchus mykiss (rainbow trout and steelhead) and Salmo trutta (brown trout and sea trout). All salmonids are excellent swimmers. Preferentially benthopelagic, they are diurnal predators that feed on different types of prey depending on their size and species. They live in clear water with high levels of dissolved oxygen and temperatures ranging from 2 to 25ºC. The rainbow trout is the most rustic of the salmonid species. Salmonids may be as long as 150 cm, weigh as much as 62 kg, and live up to 50 years. While these are the maximum values recorded for trout and salmon, the latter are usually larger, heavier and live longer. All salmonid species are important resources for fishing (sport and commercial) or production (aquaculture), with Atlantic salmon and rainbow trout supplying the greatest volume to the world market through aquaculture.