In traditional Celtic myth, an ordinary salmon ate nine sacred hazel nuts which fell into the pool where it lived and so was magically transformed into the Salmon of Wisdom.
I prefer the re-visioning of this myth by Patricia Monaghan in her book, The Red-Haired Girl from the Bog (2003):
I understand why the ancient Irish saw wisdom as a salmon . . . . Who is wiser than one who knows the way home?
What natural phenomenon is more mysterious and compelling than salmon who return from the ocean, years later and against impossible odds, to spawn and die in the very same stream where they were born?
As Rabbi Seymour Siegel wrote:
In everyone's heart stirs a great homesickness . . . .
We are all called by the deepest yearning of our hearts to be Salmon of Wisdom.