Yes, it moves (or, migrates) to a different part of the river system at different times.
Salmon are anadromous fish, not freshwater fish. They are born in fresh water, mature at sea, then return to fresh water to spawn. If they did the reverse, lived in freshwater and spawned in the sea, they would be catadromous fish. If they moved freely between fresh and salt water they would be euryhaline fish.
A freshwater fish lives and spawns in fresh water. A migratory fish moves to different areas for different reasons (food, spawning, etc) but not necessarily between fresh and salt water. You do not find freshwater fish, of any size, at sea - unless they got very, very lost. At least, not living ones.
As I previously said, the surprising bit is its size. This is because fish that live in rivers have evolved to be smaller (in general), due to space/ability to manoeuvre/food supply/etc, than fish that live at sea. They were surprised to find A FISH (any fish) of that size outwith the sea, not surprised to find a fish that size of THAT PARTICULAR SPECIES outwith the sea.
Unless the article you linked to is wrong and it's not a freshwater fish but actually an anadromous fish, then all of what I have said applies, and you're talking pish.
But, anyway, I'm really out of this conversation now.