I'm guessing this is probably a brown trout,
Salmo trutta. I'm not sure of the ecotype or subspecies.
This fish was kept in a puplic aquarium, but in the wild the different ecotypes of brown trout can migrate during their lifecycle.
There are several different subspecies and local populations.
Some only migrate in rivers. Some migrate between rivers and lakes.
Some migrate between freshwater, brackish water and marine saltwater.
Humans have introduced brown trout in many locations where they were originally not native.
Brown trout have spread from where they were introduced.
Sometimes different populations and subspecies may have been mixed together.
Humans have also built many dams with electric power stations (hydro electric power plants) that regulate waterflow in rivers.
This further complicates the migration and habitats of brown trout and many other species of fish.