You should not have a problem with any Salmon on 8lb line... question I have is, what size/type of rod are you using? A standard 7 ft. medium rod is not going to cut it! You need a very long rod and PATIENCE! I've seen guys fight salmon for hours on light line! They will beat the fish, if they have the patience.
As Bob said, set your drag, pull up a chair and fight it out!
Your obviously doing something right, your getting hook ups!
Also, as suggested, check your line frequently... zebra mussels will make quick order of your line, with even the slightest rub on the bottom. And mono is far more forgiving then no stretch on those long runs.
HH