Sea trout runs aren't "regenerating" over much of the west coast because they are being hammered by lice from fish farms.
I can think of several places that even in my fishing lifetime have seen a collapse and return of sea trout runs over ten years or so. Several East Devon rivers went from mediocre to excellent to dire and back to just about OK for no obvious (to me or anyone else) reason. South Uist sea trout runs went from pretty good to as near as makes no difference totally failed over a few years. Then from nowhere they suddenly returned one year in numbers way above those that were around when I first went there.
It seems to me the pattern is almost always the same. For some reason the finnock (whitling/peal/herling) numbers suddenly drop one year but the mature fish are still there in good numbers. The next year the finnock are still missing and naturally the mature fish start to thin out, the 2lb'ers go missing but the big ones are still there. The year after there are still no finnock to speak of and now you are starting to see less and less mature fish (they are dying off naturally and not being replaced by last years finnock). And so it goes on until all you are left with is a few old big mature fish. Then just as you are about to throw the towel in one year out of the blue the finnock reappear in great numbers. Thereafter the stocks rebuild and everyone is happy until the finnock vanish again! Obviously this doesn't happen everywhere but it a pattern I've seen enough times first hand and also looking back through the record books of some West County fishing hotels (when such things still existed!).
What causes these cycles? I don't know but it does show that sea trout runs are quite capable of "regenerating" from a very low base very quickly if the conditions suit them.
We often refer to 'sea trout stocks', but when you look at how the species operates, there is no such thing as 'sea trout stocks'. Sea trout are the fish we associate with the 'furthest outpost' of the migration of a population of Salmo trutta. In any year the proportion of the population that are sent to the furthest outpost is subject to many influences - genetic, and environmental. So, it is prone to large fluctuations when these influences are in a state of flux. That could be why you can get these boom and bust years out of seemingly nowhere... ???
The only 'stock' is the entire parent population of Salmo trutta. It is up to them how many sea trout they choose to give us in any year... Just a thought...