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  The sea trout (salmo trutta f. trutta) spawns in the upper reaches of our rivers and streams. The young grow in a freshwater environment, and after one to three years can migrate to the sea as trout smolt. It has however been shown that smolts from the local brook trout population also migrate to the sea. So both sea trout and brook trout are – at least in part – propagating populations.
By contrast with the salmon, on reaching the sea the sea trout tend to make for waters close to the coast and frequently swim upriver. As a parallel phenomenon, some trout probably set off in search of food by going from streams into larger rivers and estuary waters. So it does not necessarily follow that a large trout migrating upriver has actually been in the sea.
The different forms of migratory trout very likely exhibit fluid transitions. Even at times of serious water pollution, large-sized migratory trout have been found in the Rhine system, as stock has been reinforced by local populations of brook trout in the upper reaches (as well as large numbers of wild trout on the move). So by contrast with salmon, migratory trout never completely disappeared from Rhine waters.

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