In my view, you've just identified the key scientific question for northern cod, that is, is it truly one single unit or is it composed of multiple units? We have very good reason to believe that cod in some areas don't migrate offshore, that they remain in inshore waters throughout their lives, whereas other cod come in from the offshore waters to the inshore waters to feed in the summer months and then migrate back again.
I think it is a really key question in science, and we increasingly have genetic tools to do this. They're doing this in Norway right now. They can grab a cod in Norway and do what's called a “genomic analysis”, and they can determine whether it came from the North Sea or coastal Norway or some other location. I suspect that in five or 10 years we'll know a lot more.
My gut feeling, knowing what I know about the biology of cod, is that northern cod are likely made up of a number of different reproductive units on smaller scales than 2J3KL, but how many of those there are and how big they are remains an open question.