We don't, or at least I don't, think much of fish in my dogs' diets.
Probably because here in CO there isn't any low cost source for fish that I'm aware of other than catching carp or something similar myself.
Surely would be a good thing for them I would think and I should probably add a bit in.
They sure go nuts for cooked trout skin, that's for sure!
For a hunting dog, I'm not sure I would want too much fish or fish oil in their diet during hunting season as it thins the blood and cuts bleed more than normal - same in humans.

I had my dogs into the vet the other day. They are at their slimmest this time of year and my vet told me to add cooked pasta into their diet if I wanted them a bit heavier.
Once the cool weather is here they will fill out again naturally but maybe I'll do that just to see what happens.
One surgeon I know here puts her malnourished patients on an ice cream diet - meaning lots of ice cream - before performing surgery. Helps with wound healing and pain management apparently.