There are three genera of Salmonidae: Salmo, Oncorhynchus, and Salvelinus. Unfortunately, the common names of the species do not correspond to the natural divisions. The true, or black-spotted, trout is actually a Salmo, and the speckled, or brook, trout of the E United States is a Salvelinus and should more properly be called a char, as similar fishes in Europe are.

The American species of Salmo were originally split by the Mississippi basin, and were represented in the east by the Atlantic salmon and in the west by the rainbow and cutthroat trouts. The Atlantic salmon was a plentiful source of food for the Native Americans and the colonists, but its populations have declined. This salmon is a large fish (15 lb/6.8 kg average) found along the Atlantic coast of NE America, in Greenland, and in Europe. When in the sea it feeds on crustaceans, but as it approaches the the large rivers to spawn, it changes its diet to small fish. A landlocked species, the Sebago salmon, is found in Maine. Of the many races of cutthroat trout, some are now extinct; the greenback trout of the Colorado Rockies was recently rediscovered. The steelhead trout is believed to be the silvery saltwater phase of the colorful rainbow trout. Rainbows and cutthroats are known to hybridize, and a new species, the Gila trout, combining characteristics of both, has been discovered in New Mexico. The brown trout, introduced from Europe in 1883, requires warmer waters than the native species and is important in fish-management programs.

The genus Oncorhynchus is comprised of the five species of Pacific salmon, found from S California to Alaska. These fish are the most important commercial species. Canning centers are located on the Columbia River and on Puget Sound and in British Columbia, Siberia, and N Japan. The largest and commercially most important of the Pacific salmon is the chinook (or quinnat or king) salmon, which averages 20 lb (9 kg) and may reach 100 lb (45 kg). It is found from the Bering Sea to Japan and S California and is marketed fresh, smoked, and canned. The white-fleshed fish of this normally red-fleshed species have become highly prized in the restaurant trade. The blueback salmon (called sockeye in Oregon and redfish in Alaska) has firm reddish flesh and forms the bulk of the canned salmon. Also of economic importance are the humpback, or pink, salmon, the smallest of the group; and the silver, or coho, salmon, important in the fall catch because of its late spawning season. The meat of the dog salmon is palatable when fresh or smoked.

The genus Salvelinus includes the various European chars; the common brook, or speckled, trout, a popular game fish of E North America, introduced in the West; and the Dolly Varden, or bull, trout, a similar western form. A fourth genus, Cristivomer, contains one species, the common lake trout, and one subspecies, the siscowet, or fat trout. These are deepwater fishes of North American lakes, more sluggish, less migratory, and bulkier than the other Salmonidae; individuals have been recorded at 100 lb (45 kg). A fish called the splake has been produced by crossing the speckled trout and the lake trout.

The basic life pattern of the Salmonidae begins when, within the first year or two of life, the fish travels downstream to the sea, where it grows to its full size. After reaching maturity (one to nine years) it returns to its hatching site to spawn. The Pacific salmon are famed for their grueling journeys of hundreds of miles to their headwater breeding grounds. When they begin this trip they are in prime condition, but they cease eating when they leave the sea and arrive months later, exhausted and battered by their fight upstream against swift currents and over falls. Those that survive the trip and escape fishermen and predatory animals spawn with their last strength and then die. These salmon are taken at the mouths of large rivers, as they begin their upstream migration. The Atlantic salmon and the trouts spawn more than once. Most trouts migrate to the sea if there is a cold-water connection, but also will sometimes live and reproduce if landlocked.

Because of such human activities as overfishing, development, dam building, logging, and farm irrigation, Pacific salmon populations have greatly declined, and many species are now listed as rare and endangered. The United States and Canada negotiated a conservation agreement in 1999 that includes setting catch limits based upon ongoing scientific assessments of salmon population levels. In addition, multiple-approach conservation efforts are under way in Washington and Oregon states to restore the salmon runs. For reasons less well understood, and despite international conservation measures, Atlantic salmon populations have also sharply declined. The desirability of salmon as food fish has led to their being raised in aquaculture.

Salmon are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Clupeiformes, family Salmonidae.