That's a complicated question, because there are multiple reasons for the decline. But if I were to list them.... I'm sorry that I don't know them beyond the British Columbia circumstances, but here it's a matter of protecting our wild salmon streams from agriculture and logging near streamsides. It's a matter of ensuring there aren't major pollution sources that are detrimental to wild juvenile salmon. It's about making sure that fisheries are sustainable: commercial fisheries, recreational fisheries, and first nations fisheries. Finally, it's a matter of making sure that there aren't impacts from salmon farms in the British Columbia context.
I wish I had answers for the east coast, but that's not my experience.