Well, the common wisdom in fisheries assessment is that we only control fishing mortality, really. Therefore, there is very little we can do, other than making sure that at least enough cod grows up to spawn, and then hope for the best that they will become juveniles, and so on. Of course, if you have large predators and stuff like that, like the seals or the minke whales, and so on, that's kind of a political minefield that I won't go into.
I don't think there is much one can do to really enhance things, other than simply allowing them to grow and spawn.