Campbell of Airds wrote, in his wonderful article on West Highland Heraldry:
"Even today, the salmon is a Royal Fish and Salmon fishing rights are retained by and dispensed by The Crown
Clearly it is a powerful symbol. To my mind there is a pagan feel to it; I do not think it is the txoua of early Christianity and wonder if it is not a reference to the Old Religion of the Celts, or more accurately, to a person or family connected with it.
On a different note, I have been told by Professor Per Andraesson that although the salmon is a rarity in Norse heraldry he has seen a coin dating back to Jutland in the 8th century which has both a salmon and a galley on it. And even today its use is clearly important to status. When MacCailein Mor goes to the Oban Ball he is clad in a doublet decorated with silver salmon as is Maclean of Duart.