About 10 years ago I worked as a commercial diver and did some jobs for various fish farms.
Slightly awed at the site of a hundred thousand salmon in a net pen 100 feet square, I spoke at length with both their staff fish biologists and some government inspectors who were there to monitor operations, and they both told me that the salmon jump primarily to try to dislodge the sea lice, both in the pens and in the wild, except when spawning.
If you watch them long enough (and these guys had been watching them in 8 hour shifts, 5 days a week, for years) I'm told that you can differentiate the different types of jumping, whether it's a predator avoidance (you should see them move when there's a seal circling the pens), catching food like the trout after a chronomid hatch, the 'rolling humpback' of a fish navigating upriver, etc.