Alan,
I will be interested to know more about this for many reasons.
The first would be historical.
When were Brown trout introduced into these rivers that are known to have a run of Sea Trout.
And from what source did they originally come.
I may be able to get these facts from the USFW, but may be not if this happened when it was the US Fish Commission.
Alan, there is a difference between the true genetic strain of Sea trout and Brown trout that venture into brackish and salt water, which all Brown trout are capable of, so are river Bows if they have time to adapt.
Which is also interesting as there are times that Sea trout run into rivers that run out of lake systems. They can become trapped in that system due to low water that does not let them return to the oceans, and they will survive.
Which is the same scenario that happened for Steelhead, which entered lake systems and remained there to become strains of Rainbow trout.
Much research has been done in the UK regarding Sea Trout.
Some are of the view that all Brown trout were at one time anadromous, as all rivers or lakes either have or would at some time a source that flows into the ocean.
As l already said here there were times that the route of water became cut off for one reason or the other which trapped those fish. Which may well have been the parr and smolts. They adapted to the new environment and became over time a genetic resident strain.
We also see in some of the natural lake systems in the UK and Ireland as many as 3 very distinct strains that have different habits. Some strains are pelagic while others chose to live at depth and become plankton, crustacean and mollusk feeders at depth, or other benthic organisms.
Other theories are that many of the rives that sea trout run are freestone and in some cases contain little food base, all be it there will be resident Browns trout of a strain that do not grow very large.
The theory being that in search of food many of these fish travelled to the oceans and found food. They then only returned to the rivers and lakes to spawn.
I know for a fact that this can be the case as l used to fish a river at its estuary to the ocean for shad and we would also catch Browns that were colored same as the river strain.
Sea Trout are mainly a silver fish with black markings. They show no other color as we see with stream fish.
Either way Browns trout have fascinated me since my childhood days, and particularly since they were introduced into North America, as they were also by the English to South America and the Falkland Islands.
Let me know what you find out Alan.
Davy.