Phylum: Chordata - Class: Actinopterygii - Order: Salmoniformes - Family: Salmonidae
The sea trout - sewin is their Welsh name - and brown trout are biologically the same species, but sea trout have developed the habit, probably out of a need for richer feeding, of heading out to sea after a year or two as fry and parr.
In Wales a majority of sea trout are female, while the majority of resident brown trout in rivers are cock fish. Sea trout and brown trout can, and do, interbreed.
After typically two years as alevins, fry and then parr, the juvenile sea trout turn bright silver and head off to sea. These smolts, as the sea-going young sewin are called, grow rapidly and return usually within a year to run up river and spawn. At that stage they are called whitling. (In other parts of Britain names such as peal and herling are given to the small sea trout that return to their natal rivers in summer.)
Sea trout generally spawn each year of their adult lives and may return to the river many times. On each return migration they will have put on typically two pounds (1kg) or more. The fastest growing sea trout in Wales are believed to be those on the western rivers, including the Towy and the Teifi.
Fishing for sea trout is often done at night, when these shy fish will take the angler's fly.