Anadromous brown trout are a silvery color with faint black spots. However, once they return to freshwater they quickly take on the normal coloration of resident brown trout in preparation for spawning. Sea trout kelts (post-spawn) return to their silvery stage as they migrate back to saltwater. Adult brown trout are between 35 to 60 centimeters (14 to 24 in) long and can weigh from 0.5 to 2.4 kilograms (1.1 to 5.3 lb).
Breeding males will develop a hook-like, upward-facing protrusion on the lower jaw called a kype. In freshwater, the top of the trout is an olive color with brown and black spots with the ventral side being tan to yellow. The sides have many orange and red spots ringed with light blue.
Their average length is 60 cm, but they can grow up to 130 cm in length and weigh up to 20 kg under favorable habitat conditions.
Their most striking feature is the long, elongated, torpedo-shaped body. They have silver-grey sides and grey-green backs. The belly is white. Like all trout species, the sea trout has an adipose fin.
Sea trout feed mainly on fish, small crabs, shrimps, and prawns.
It is an anadromous, migratory fish, which closely resembles the Atlantic salmon in its form and lifestyle. In the sea, it makes long journeys and also swims upstream into small rivers in order to spawn. Spawning occurs in winter on gravelly river beds in the grayling zone to the barbel zone. Their eggs are laid in troughs or redds.
The young fish remain in freshwater for one to five years and then make their way to the sea. The "inner clock" signals to the fish when they need to make their return journey to the sea. During this migration, they can cover up to 40 km per day.
Fish that are ready for spawning are usually lean and have to eat a lot of food in order to increase their energy levels as quickly as possible. After completion of the spawning process, the fish return to the sea. The mass mortality after spawning that is common in some species of salmon is not usual for brown trout. Once back in the sea, the fish regain their weight and lose their brown spawning coloring.
The surviving young of sea trout will generally migrate back to the sea, to feed in estuaries and coastal waters. However, it is also known that adult brown trout, which may have spent some years entirely in a river, can for whatever reason decide to migrate to sea, to return next year as a much larger (sea) trout, with beautiful silver coloration.
Sea trout fishing might be had in most parts of Scotland. Most salmon rivers have a run of sea trout. Indeed, most rivers that hold brown trout will see some of them migrating to sea as sea trout, except where the upstream passage of migratory fish is barred by impassable obstacles. In many salmon rivers, the sea trout, although present in good numbers, often play second fiddle to the salmon - as in the Spey and Dee, which are among the top sea trout rivers in Scotland.