Sea trout are the sea-running form of the brown trout (Salmo trutta). Scottish rivers were colonised by sea trout at the end of the last Ice Age, and their descendents form the populations of brown trout and sea trout we know today (both Salmo trutta). It is believed that genetics and environmental issues (such as habitat and food availability) play an important part in whether or not a trout migrates to sea :) | Ness District Salmon Fishery Board