Grandpa has hit it on the head...

It is interesting to note that s good number of the sound fish will be caught in the straight in the winter months. As Grandpa noted, the majority of the bait will head north- so do the fish. This is why you'll often hear of "great" catches up north during these months. Also, a lot of folks will assume these mighty salmon are Springers. Is there a way to tell? A Blackmouth will have small scales compared to a mature fish with larger scales. Experience on the water will show you a Springer of 15# will have larger scales than a 15# Blackmouth)

On a side note, there are two main reasons why the bait moves north: (1) spawning and (2) the abundance of krill (or euphasids) to feed on. Not all herring migrate north. It depends on the species and the amount feed in the water. Many will linger in the mid/south sound...

As far as wild or not, you and I will never know being out on a boat. (I imagine there some subtle way of telling though.)

I would not want to speculate, but I would say the numbers of actual wild blackmouth in existence is pretty darn low. I do know that most go through the chute unmarked- probably because of time constraints.