A professional Scottish salmon fishing guide's 'career based' explanations as to why salmon jump in the rivers of Scotland.
Atlantic salmon that populate the Northern Hemisphere rivers will jump for various different reasons once they're back in the river of their birth. Here's a few of the main reasons why a fish that was named by the Romans as 'The Leaper' two thousand years ago will ocassionally launch itself clear out of the water.
The first reason salmon will jump in the fresh water rivers of Scotland is to dislodge the little tadpole like parasites called sea-lice which they often pick up on their backs and flanks while in the sea. There's no doubt that the irritation of these parasites (that soon fall off salmon in fresh water) are a reason why a salmon will jump.
Another reason why salmon will often jump is just because they're in a state of excitement about being back in fresh water or they smell fresh water from recent rainfall that again has made them exciteable. Often, when a run of salmon enters a pool that has older resident fish lying in it the resident fish will be stirred up sufficiently and will often jump. Alternatively, if a pool has quite a few resident salmon lying up in it there can be territorial posturing going on under the water which will make salmon jump which again is just a basic in-river type of salmon social interaction.
Later in the year salmon can become host to a small maggot type of parasite that lodges in the salmon's gills which can irritate the fish and will cause it to periodically jump. Nearer the spawing season when salmon are pushing on up a river they will leap up over waterfalls or weirs or any other obstacle that posses as a barrier to their upstream migration. Even on larger scale waterfalls that appear unnavigable a salmon will soon work out how to align its nose with the main flow descending the waterfall and will power its way up which is truly an amazing feat to observe.
The best way however to force a salmon to jump is to temporarily place a carefully tied salmon fly in its mouth as then you're almost guaranteed to see a few jumps from the fish as it attempts to dislodge the fly. To gain assistance in observing this kind of salmon jump behaviour follow this link to book a professional salmon guide who will assist with that process.