How closely do sea trout relate to brown trout which remain in fresh water?

The broad species that we call brown trout occurs in a wide range of forms differing in average size, in colouration and spot pattern, diet and migratory behaviour. Some live only in streams; others migrate to find better space and feeding in lakes or larger rivers, in estuaries, or in the sea.  Sea trout become silvery smolts before or during migration to sea, but so do some strains of brown trout that migrate only as far as shallow freshwater lakes, for instance Loch Leven, Kinross-shire, in Scotland.