Re: An intriguing question.
With rainbow, brown, brook trout, and char, there are 'Coasters' and 'Salters', These are fresh water species that are able to assimilate between fresh, brackish, and saline waters and their metabolisms handles the variances of Ph and salinity. What I am writing is off the top of my head but with some research you will find that what I am saying is supported by biologic studies of many salomoids. Here in Alaska there are strains (watershed specific) of rainbow trout that have evolved by way of their migratory nature into both coasters and salters or sea run fish. When I say coaster this is the behavior of leaving the estuary area of a creek or river and actually spending a good deal of time in the brackish and saline waters that are found along the coastal areas and returning to the mouths or tidal lagoons of their home watershed. These fish produce a spring spawning run that is no less exciting as true spring steelhead returning to spawn. They are generally larger than any trout that has not taken to leaving the mid or upper reaches of the streams.
As for real 'sea run' fish other than Pacific steelhead, the char here do go to sea in certain watersheds and these fish attain respectable size by living this way. The char return often with summer salmon runs but they are fall spawner's so you can always expect some fresh fish in late fall depending on what river or creek you are focusing on.
When I lived in Pennsylvania I used to fish the streams that feed into the Finger Lakes for the spring run rainbow trout. Catherine's Creek which flows into Seneca Lake at Montour Falls is the most famous run but there are lesser known streams that see some big fish. These rainbows average 5 pound (at least they did in the 1980's) and fish of ten pound are not uncommon; same behavior as steelhead but they do not leave Seneca lake and go to sea.
I could go on with tales about where I targeted a stream or river because it flowed into a big lake or to sea for trout who were returning to spawn but I think you get the picture. The most famous traveling trout I have ever heard of was a rainbow tagged and stocked by the Pennsylvania Fish & Boat Commission in the late 1960's. The fish was released in the upper Allegheny River and was recovered some five years later by a commercial fishing vessel in the Gulf Of Mexico. Urban legend? I got that on good source and have always believed it.
When I catch a big trout that I have every reason to believe attained its size by traveling to the Ocean I call it a Steelhead. When I fished the Finger Lakes and caught lake run trout in the creeks I always thought of them as steelhead and never paid too much attention to actual taxonomy. Your fish is what you call it,