Simple answer - they're not. Some browns spend their whole life in the river, only moving up and down to spawn and feed. Others move out into the estuary, others move out into the sea for months at a time. On the river Blyth the trout used to be landlocked, in that they wanted to get down to the sea but couldn't because the lower reaches were permanently polluted (hope that's changed now).Up this high (the Upper Wye is 100 miles+ from the sea, the trout are resident and don't move down to the coast. I did catch a 3lb+ sea-trout at Builth Wells some years ago - Builth is only a few miles from where I fish now, and that fish had been down to the sea and returned. That was a rare enough occurrence to get a mention in the Catch Reports of Trout and Salmon. Just to confuse the issue further, it is common to catch Wye trout which are quite silvery (and look like sea-trout) after prolonged high water. Wild brown trout in lakes and lochs of course are landlocked.