The best way to discern between trout and salmon is the fact that the trout always have plenty of spots beneath the sideline, whereas the salmon only has a few spots beneath the lateral line. Also, the stem of trout’s tail is usually sturdy and the border to the base of the tail fin is unclear, whereas it is very clear in salmons, and the tail fin of the trout is almost straight and without notches, whereas there often is a notch in salmon’s tail fin.
The trout is a very variable fish. Traditionally, trout have been divided into three different races according to the habitat: sea, lake and brook trout. However, recent studies have completely thrown out the idea of such a strict division. It has been found that for example fry born out of the so called brook trout go on a sea or lake migration just like the offspring of lake or sea trout, provided that the passages are open. Part of the offspring of the large sea and lake trout always stay in the river or brook and live as a brook trout, that is, a brown trout.