In aquaponics we salt to 3ppt and it helps repair the slime coat and those fungus and ick just fall off. But that's a lot of salt, it turns out to be about 2.5 lbs of salt per 100 gallons of water and i am sure there's a lot of reasons why you wouldnt want to do that to your pond.
When we do that the tradeoff is that some plants dont like it, it can kill strawberries and tomatoes may struggle, etc. So mostof us only do it when fish look sick or as cecil says during transportation.
I believe that the long cold could be enough to cause it.
In my nonaquaponic systems I find that 2 ppt is sufficient to reduce stress. I always thought you needed at least 5 ppt to do what your saying but I could be wrong.
Just my observations of course and yours could very well be different.
