That's partly it.
The thing we need to remember is that, just like the Atlantic salmon itself, rivers, freshwater environments, are very dynamic. In some rivers you can actually count the fish by doing snorkel dives, for lack of a better term. Biologists will go to the top of a river, float down, and actually count the fish, because the water is as clear as what's in my glass here. In other rivers where the water is dark, you can't do that.
In some rivers we are lucky enough to have fish passages, fishways. We can count each fish that comes through individually. In other rivers, there is no possibility of counting fish because of their sheer size, so a lot of times, in the past, they were basing some of their decisions on angling success.
But we all know that the number of fish caught is usually a lot less than the number of fish that are actually in the river.