Sea trout also known as brown trout live complicated lives. Like salmon, they begin their lives in a river hatched from eggs that were laid in a small nest called redds. As the young fish mature they go through several development stages, all of which involve substantial physiological changes.
The most important stage from this perspective is when the fish becomes a smolt after about two years. Entering the smolt stage is like being given a passport out to sea, because the physiological changes allow the fish to tolerate salt water.
If you would like to read the full article please visit The Norwegian University of Science and Technology or how about my previous post about Fish Behaviour