The only way to determine the difference between this and other trout is to look at the spots. There are numerous spots along the flanks, down to the pelvic fin. Brown trout have red spots, while sea trout have dark spots.
How to identify a Sea Trout
Sea trout is the common name usually applied to anadromous (or sea-run) forms of brown trout (Salmo trutta), and is often referred to as Salmo trutta morpha trutta. In terms of their shape, sea-run brown trout are identical to brown trout, but a sea trout has a much more silvery appearance.
However, the color of an adult sea trout may darken as it journeys through freshwater, at which point it can be challenging to distinguish a sea trout from a brown trout. The one way to determine the difference is to look at the spots. There are numerous spots along the flanks, down to a line that is level with the base of the pelvic fin. Brown trout have red spots, while sea trout have dark spots.
Where to catch Sea Trout
Anadromous brown trout (sea trout) are widely distributed in Europe along the Atlantic and Baltic coasts, the United Kingdom and off the coasts of Iceland. They are found in the Black and Caspian Seas and as far north as the Barents and Kara Seas in the Arctic Ocean. In Argentina, there are populations that migrate up and down the Rio Grande, Rio Gallegos and Rio Irigoyen.
In the U.S., saltwater sea trout are found in the Columbia River and its tributaries in the Pacific Northwest, and in rivers on the Atlantic coast from New Jersey to Maine. Sea trout fishing has become particularly popular in the state of Connecticut, where this species is stocked on an annual basis.