Salmonidae is a family of ray-finned fish, the only living family currently placed in the order Salmoniformes. It includes salmon, trout, chars, freshwater whitefishes and graylings. The Atlantic salmon and trout of genus Salmo give the family and order their names.
Salmonids have a relatively primitive appearance among the teleost fish, with the pelvic fins being placed far back, and an adipose fin towards the rear of the back. They are slender fish, with rounded scales and a forked tail. Their mouths contain a single row of sharp teeth. Although the smallest species is just long as an adult, most are much larger, and the largest can reach .
All salmonids spawn in fresh water, but in many cases, the fish spend most of their life at sea, returning to the rivers only to reproduce. This type of life cycle is described as anadromous. They are predators, feeding on small crustaceans, aquatic insects, and smaller fish.
Current salmonids arose from three lineages: whitefish (Coregoninae), graylings (Thymallinae), and the char, trout and salmons (Salmoninae). Generally, it is accepted that all three lineages share a suite of derived traits indicating a monophyletic group.
Salmonidae first appear in the fossil record in the middle Eocene with the fossil Eosalmo driftwoodensis first described from fossils found at Driftwood Creek, central British Columbia. This genus shares traits found in the Salmoninae, whitefish and grayling lineages. Hence, E. driftwoodensis is an archaic salmonid, representing an important stage in salmonid evolution.
A gap appears in the salmonine fossil record after E. driftwoodensis; until the late Miocene (~7 m.y.a.) trout-like fossils appear in Idaho, in the Clarkia Lake beds. Several of these species appear to be Oncorhynchus—the current genus for Pacific salmon and some trout. The presence of these species so far inland established that Oncorhynchus was not only present in the Pacific drainages before the beginning of the Pliocene (~5–6 m.y.a.), but also that rainbow and cutthroat trout, and Pacific salmon lineages had diverged before the beginning of the Pliocene. Consequently, the split between Oncorhynchus and Salmo (Atlantic salmon) must have occurred well before the Pliocene. Suggestions have gone back as far as the early Miocene (~20 m.y.a.).
The Salmonidae (and Salmoniformes) are divided into three subfamilies and around ten genera: